This is a dictionary of names and concepts that one comes across in Physics, put together to help students of class XI, XII/Junior College in India, for appreciating, remembering and recalling the Physics they learn.



α – Alpha particle:Emitted from the nucleus of radioactive elements. It has two protons and two neutrons. (ionized helium atom). Emission of α particle is always accompanied by the emission of gamma rays from the same nucleus.


Aberration:Defects noticed in the images formedby optical elements (lenses/ curved mirrors). They are Spherical aberration and Astigmatismobserved whenthe light is monochromatic, or polychromatic (white light)and Chromatic aberration when it is white light.


Absolute Zero: (0o K),the lowest temperature one can reach. It is the lower fixed point of the absolute (Kelvin) scale of temperature –273oC below the melting point of ice.


Acceleration: rate of change of velocity,ms-2) a vector.Acceleration due to gravity (g): the acceleration of a freely falling body on the earth. It is maximum at the poles and minimum along the equator. Due to rotational motion of the earth about its axis objects on the earth surface experience an outward force, which is a maximum at the equator, reducing the gravitational attraction.


Accelerator: used for accelerating charged particles (electrons, protons, ions etc.)to high speeds (energies) –Linearaccelerator, Cyclotron, Betatron and Van de Graff generator.


Acoustics:The science of Sound. Acoustics of buildings relate to science/engineering in the design/construction of auditoria/halls for good sound effects (no echo, reverberation etc.)


Action: is force. Action and reaction are equal and opposite (Newton’s third Law)


Adiabatic:change/process iscarried out with no exchange of heat with the surrounding. Adiabatic compression of a gas causes heating while adiabatic expansion causes cooling.Adiabaticdemagnetization is used to reach temperatures near 0o K.


AEET: Atomic Energy Establishment, Trombay, Mumbai.Several of the including Apsara, the first Atomic reactors built in India, for study and research are located here.


Albert Einstein: gave the theory of relativity (Special and General). The results of theSpecial Theory of Relativityinclude(1) mass and energy are related (E= mc2) and showed that the mass of an object increases as it moves,(2)length of an object decreases in the direction of motion and (3) time dilates (events get slowed down, take longer time)all in a platform that moves. The changes are significant when the speed is comparable to the speed of light.Though well known for his theories of Relativity he received the Nobel Prize in Physics for his work on Photo electric effect.

Alfred Nobel:created the foundation that awards Noble prizes in Physics, Chemistry, Biology, Medicine d Literature every year since 1901.


Ampere (A): unit of electric current. A flow ofone coulomb of electric charge per second is one ampere. It isalso defined using the force of attraction between two long conductors carrying the same current. Another definition involves the mass of silver deposited on the cathode in a silver voltammeter. Ampere-hour is used to specify the current capacity of a battery.


Aneroid barometer:  an instrument for measurement of atmospheric pressure withoutmercury.Altimeteris a modified aneroid barometer used to measure the altitude by measuring theatmospheric pressure that decreases with altitude.


Angle of contact: When a liquid surface meets a solid surface, there is an angle between the tangents to the two surfaces at points of contact. This is the angle of contact between them.  It is usually an acute angle (almost zero) for most liquids and glass but is a large angle (obtuse angle) for mercury and glass.


Angular velocity (ω), acceleration (α): when a body is moving in a curve or is revolving about an axis it has an angular velocity.The radius vector connecting the body and the point (or axis) about which it is rotating,sweeps an angle that changes with time.The angle is measured in radian unit, 3600 (degrees) = 2πc(radians). The angle swept per second isangular velocity (ωor Ω) is a vector quantity.The direction of angular velocity is perpendicular to the plane of motion. If one imagines a right-hand cork screw to rotate as the object, the direction in which the tip of the screw advances is the direction of the angular velocity vector. v= rxω. Angular accelerationis therate of change of angular velocity. (α).Angularmomentumis the product of moment of inertia and angular velocity. Rate of change of angular momentum is the torque(couple) acting on the body.


Anodeisthe positive electrode (used in electrolysis, in gas discharge tubes and vacuum tubes, The P side of solid state device (PN diode) is also referred to as anode.


Appleton layer: is one or more layers of charged particles surrounding the earth, reflects electromagnetic waves (radio waves). Helps in radio communication across the world.


Apsara:theswimming pool reactor, the first experimental atomic reactor built in India.It uses ordinary water as moderator and coolant.


Archimedes Principle: A body fully or partially immersed in a fluid suffers an apparent loss in weight (due to up thrust) equal to the weight of the fluid displaced.

Artificial radioactivity: Certain elements formed in nuclear reactions exhibit radioactivity. They decay through beta decay giving out electrons or positrons and gamma rays.Radiations from artificial radioactive elements are used inmedicine for diagnosis and cure.


Aston Mass spectrometer: Used to compare the masses of nuclei of atoms. Used in the estimation of the masses of isotopes and their relative abundance.


Astigmatism:One of the defects in the images formed by optical elements because of the wide aperture (opening/ width). Rays of light that are close to the axis (paraxial rays) and that are close to the edge (peripheral rays) do not converge to the same point in the image. Sharp image is not formed.  Astigmatism is removed using parabolic mirrors in case of reflecting telescope and bylimiting the aperture in telescope with objective lens.A defect in human vision is also called Astigmatism. This is corrected using spectacle lenses with cylindrical power.


Atom isthe smallest entity that has all the properties of an element.  92 different elements exist in nature (Hydrogen to Uranium). They have beenarranged in a table known as periodic tableconsidering similarities and gradual progression in their properties.Atomic weight of an element denotes how heavy an element is compared to hydrogen. Atomic number of an element is the number of protons in the nucleus of the element.


Atom bomb: (fission bomb) is a destructive device built toreleases a large quantity of energy bynuclear fission (breaking of heavynuclei). Two such devices brought the Second World War to an end.


Atomic energy:  Energy released when an atom undergoes fission (split). This is accompanied by a loss in mass. The energy released is given by the equation E = mc2


Atomic reactor:Devicebuilt to generate energy for other applicationsbycontrolled nuclear fission. Also used for the production of artificial radioactive elements.


Atomic mass unit (a.m.u) is (1/12th) of the mas of an atom of carbon-12. It is 1.67 x 10-27kg.


Atmospheric pressure: pressure of atmospheric air at a place is equal to the pressure at the bottom of a column of mercury of height 760 mm (= 1.01X105 N/m2). Standard atmospheric pressure is the pressure at the bottom of that column ofmercurycolumnat sea levelat 45o latitude.


Avogadro law: Equal volume of all gases under the conditions of same pressure and temperature contain the same number of molecules.Avogadro number:  the number ofatoms/molecules in a mole (atomic/molecular weight expressed in grams) of a gas. 6.022 x 1023.

This pocket book is a tribute to Samuel Johnson, who produced the first dictionary of English words.


Balance:called physical balance is used to determine the masses of objects by comparison with standard masses from a weight box.


BARC: Bhabha Atomic Research Center, Trombay, Mumbai. Many atomic reactors are located here. A training school for trainees joining BARC, TIFR and other similar research institutes of Government of India is also located there.


Barometer: an instrument used to measure atmospheric pressure, Fortin’s barometeris widely used mercury in glass barometer.


Battery: device for storage of electrical energy for portable usage. Its capacity is measured in ampere-hour (AH) the product of the current and the time before it gets fully used up.


Betaparticle, Beta decay, Beta rays (β): The disintegration (decay) of radioactive element releasing an electron (or positron – positive electron) is beta decay. In natural radioactivity beta decay is accompanied with emission of electron only, whilein artificial (manmade) radioactive element the beta decay maygive out electron or positron. A continuous stream of beta particles isbetaradiation. Betatron is a device for accelerating electrons.


Bernoulli’s theorem/principle: In a streamlined flow of a fluidwhen there is constriction in the path and the speed of flow increases thefluid pressure decreases. (Principlethat keepsair planes flying)


Bhatnagar awards:Awarded by Government of India every year. The highest awards for science research in India.  Named after Prof. Shanti Swaroop Bhatnagar, the father of research laboratories in India


Blackbody: A concept in the study of heat radiation. A black body absorbsradiation of all wavelengths falling on it. It also radiates in all wave lengths. The amount of radiation given out is directly proportional to the fourth power of absolute temperature of the body. (Stefan’s law)


Black hole:  Place of very intense gravity in the cosmos. Nothing (not even a photon) can get out of it. It is a singularity in the four-dimensional space time continuum of general relativity.


Bohr (Niels Bohr): Considered the father of Atomic Physics. Postulated the Bohr’s atomic theory and explained the origin of spectral lines. To him goes the credit of saving the lives some German (Jewish)Physicist like Einstein in Europe from the Nazi massacres. He had helped them to migrate to USA.


Bohr’s atom model (Bohr’s atomic theory): Electrons go around the nucleus in different circular/elliptical orbits like planets going round the sun. The energy of electrons is different in different orbits. The atom emits radiation (give out its spectrum) when electrons moving in one orbit jump to orbits of lower energy.


Boilingis the large-scaleconversion of a liquid into its vapor at a fixed temperaturecalled boiling point of the liquid. Boiling is accompanied by bubblingin the body of the liquid.Latent heat is absorbed.

Boyle’s law:For a given mass of a gas, at constant temperature the pressure (P) and volume (V) are inversely related. (PV = constant).


Bose S.N: known for Bose Einstein statistics in the study of energy distribution in a system of indistinguishable particles (bosons). It is a tribute thatan elementary particle Higgs Boson,(also known as God particle),is given his name.


Brownian motion: the zigzag motion of fine particles (pollen grains) in water (or in any liquid). This was observed by a Botanist, Robert Brown. It was studied by Einstein. This gives the proof of molecular motion and the kinetic theory of liquid and gases.Particles of smoke in air also execute Brownianmotion.


Bulk Modulus of a solid is the ratio of pressure (force per unit area, bulk stress) to the bulk strain (change in volume per unit volume). The reciprocal of bulk modulus is compressibility.


Buoyancy: the up thrust (upward force) experienced by a body partially or fully immersed in a fluid (liquid or gas). The weight of the fluid displaced equals the weight of the body (Archimedes Principle)





c: the symbol for speed of light. (3 x 108 m/s2)


Capacitoralso calledcondenser is a circuit element in electrical/electronic circuits. It performs by storing and releasing electric charges during its operation.Has two conductors (plates) with an insulator (dielectric) separating them. The insulator could be air.


Capacity/Capacitance: ofa condenser/capacitoris ratio of the charge stored in it to the potential difference across the terminals. If 1 coulomb of charge creates a potential difference of 1 volt between the plates, the capacity is 1 farad(F). The capacitors commonly used in electronic circuits are in the range of microfarad (µF = 10-6 F) or picofarad (pF = 10-12F).


Capillary rise: When a capillary tube is dipped vertically in a liquid the liquidlevels are not the same inside and outside the tube. Surface tension of the liquid determines the difference in levels. The angle of contact of the liquid determines increase or decrease in level inside the tube. Water exhibits capillary rise and mercury exhibits capillary depression in a glass tube.


Carnot Engine is an imaginary heat engine working between two temperatures of a source and sink and using a perfect gas as the working substance. The cycle operations of the engine consisting of a pair of alternate isothermal and adiabatic processes is known as the Carnot cycle. The work done by the engine in a full cycle is equal to the area of the cycle in a p-v diagram.


Carnot theorem: Of all the heat engines working between two fixed temperatures of the source and the sink the efficiency of the Carnot engine is the maximum.


Cathode:the negative electrode ina discharge tube (vacuum/gas filled tube)and in electrolytic tanks. The N side of the PN junction device (diode) is also referred to as the cathode.


Cathode ray: stream of negatively charged particles (electrons) moving from cathode to anode in a discharge tube.


Cavendish determined the value of G, the gravitational constant in the laboratorynamed after him in Cambridge.


Celsius (centigrade) scale is usedin the measurement of temperature. On this scale, there are 100 equal intervals (degrees) between the two fixed points (melting point of ice and the boiling point of water under normal atmospheric pressure).


CERN:European organization for nuclear research located in Geneva part of the laboratories going into France through underground tunnels. India is an associated member of CERN. Higgs Boson was discovered in laboratories in CERN.


Chandrasekhar limit: Stars have a progression in their lives. They contract as they get aged. Stars of mass greater than 1.4 times the mass of our sun would finally explode forming a super nova leaving behind a neutron star. This 1.4 is known as the Chandrasekhar limit.


Charge an attribute (property) of elementary particles like electrons, protons etc., as a result they exert force on one another.They are of two types, positive (+) and negative (-). Like charges repel while unlike charges attract. In an electric field charges move in line with the field while in a magnetic field they move at right angles to the field.


Charles laws: The pressure of a gas at constant volume is directly proportional to its temperature in absolute (Kelvin) scale. Similarly, the volume of a gas at constant pressure is directly proportional to its temperature in Kelvin scale.


Chromatic aberration is the defectin images formed with lenses because of which the image is colored with colors not present in the object arises because of differences in refractive indices of the medium for light of different colors (wave lengths).


Coefficient of thermal expansion in length (area or volume) is the ratio of increase in length (area, or volume) per unit length (area or volume) per degreeCelsius increase in temperature. (known as coefficient of linear, areal, cubical expansion respectively) This is a positive quantity except for volume behavior of water over a small range of temperature. Between 00C and 40 C water contracts on heating (thuswater has maximum density at 40C, a property of great consequence for marine life).


Coefficient of friction (µ) is the ratio of the force of friction to normal reaction between two surfaces in contact when there is relative motion between them.


Coefficient of viscosity is tangential force experienced per unit area of a fluid surface moving steadily over a solid surface under unit velocity gradient. Unit: Poise. (N /m2/under unit velocity gradient)


Coherenceis the property of a set of waves in the same phase as they travel (laser beam).They will have the same wavelength (monochromatic). The phase difference between them will remain the same as the waves travel. The two beams of biprism set up of Young or the light from the object slit and its reflected light in Lloyd’s mirror set for demonstrating interference of light were the early examples of coherent beams.


Compressibility: the ratio of change in volume per unit volume per unit change in pressure. The reciprocal of bulk modulus. For gases, the bulk modulus is equal to its pressure.


Conical pendulum: When the bob of a simple pendulum is set into motion in a circle in horizontal plane the string of the pendulum describes a cone. This is conical pendulum. As the bob moves faster the height of the cone decreases. The period of a conical pendulum (time taken for complete circle) is the same as that a simple pendulum of length equal to the height of the cone (length of equivalent simple pendulum).


Cosmic radiation (cosmic ray) is particles (charged or neutral) and radiation (like X-rays,gamma rays) that bombard the earth from outside (cosmos) at all times.


Cosmosis the entire space surrounding the earth.


Coulombis the unit of electric charge. When one ampere current flows for one second a charge of one coulomb flows in that conductor.


Couple: Two equal forces acting on a body and not along the same line cause rotary motion. The pair constitute a couple.The product of the force and the perpendicular distance between them is a measure of the turning effect and is known as couple (torque).


Critical angle:When light travels from a denser medium (medium of high refractive index) to a rarer medium (medium of low refractive index) and when the angle of incidence is more than a certain angle, the light is not refracted in to the rarer medium but totally and internally reflected into the denser medium. This angle is the critical angle.


Critical constants (TcPc Vc): the temperature above which a gas cannot be liquefied(converted into liquid) by pressure alone is the critical temperature. The minimum pressure required for condensing the gas (vapor) into liquid is the critical pressure. The volume of the gas (vapor) at critical pressure and temperature is the critical volume. (RTc/PcVc) = 8/3 (for gases that obey Van der Waal equation).


Current (Electric current): is the flow of electrons in a conductor. Electrons have negative charge hence the direction of electric current is opposite to the direction of flow of electrons.


Cyclotron: used for accelerating charged particles to high energies using alternating electric field and a powerful magnetic field in a perpendicular directions.


Cylindrical lens: lenses in which oneside is cylindrical while the other surface is plane or spherical. They have focal lines on either side, unlike regular (spherical lenses) that have focal points.Cylindrical lenses are used to rectify a common defect in vision called astigmatism.




Dalton’s atomic atomic theory matter is made of molecules and molecules are made of atoms and there are only a limited number of atoms (92 in number) that constitute all matter.


Dalton’s Law of partial pressures: The pressure of a mixture of chemically non-reacting gases in a container is the sum of pressures exerted by the individual gases when they alone present occupy the container.


Daniel Cellis one of the early sources of electricity for use in laboratories. Its EMF (electro motive force) is 1.08V


de Broglie wave (matter wave): Louis de Broglie in his Ph.D thesis had proposed that matter exhibits waves nature as well. The wave length of matter wave is = h/p. h is Planck’s constant and p is the momentum.


Decibel (db) is logarithmic unit(one tenth of bel) for comparison of sound power levels. Thedifference in power of two soundsof power levels P1 and P2 is (1/10) log (P1/P2) db.


Delta(δ, d or ∆) used to denote small quantities as the prefix.


Demagnetization: Removing (destroying) magnetism in a magnetized material.


Demodulation (opposite of modulation)is extraction of signal (information) from a modulated electromagnetic wavecarryingsignal.


Deviation in the path of a ray of light occurs in refraction when light enters from one medium into another.However when light is incident at right angles to the surface separating two media it precedeswithout any deviation.


Dielectric constant: The force between two electric charges q1 and q2 in a medium is q1q2/4πεoεd2, here ε is the dielectric constant of the medium.εo is the dielectric constant (permittivity) of free space (vacuum)


Diffraction of light: Light waves travel in a straight line path. When it encounters obstacle in its path it is bent and deviated in many directions from its straight line path. This is diffraction.


Diffraction grating is a device with series of slits (openings) separated by opaque spaces.Composite light passing through a grating gets split into its spectrum by diffraction. The slits are usually large in number and are very close that they appear as lines. The number of lines per centimeter is a parameter specifying a grating. Gratings can be plane or spherical. Spherical gratings have their lines on a spherical reflecting mirror and it forms a part of the optics of the instrument using the grating.


Dimensional analysis: Length,Mass and Time (L,M,T) are independent physical quantities. All other physical quantities are derived from them and can be expressed using them. Example: Area L2,Velocity LT-1, Force MLT-2, Pressure ML-1T-2, etc. This notation is used to check validity of expressions/equation and units of physical quantities. This is dimensional analysis.


Dip: A magnetic needle held to move freely in a vertical plane, when kept in the magnetic meridian (north south plane), remains dipped. The angle which the needle makes with the horizontal is known as the dip in that place. Dip is positive in northern hemisphere and negative in the southern hemisphere. It is zero along the magnetic equator and 900 at the poles


Discharge tube: A glass or quartz tube sealed at both ends, provided with two metal electrodes, filled with required gas at a low pressure. A potential difference (voltage) between the electrodes causes a flow of current (discharge through the gas in the tube.


Dispersion is spreading light beam revealing the different colors in the beam. This arises because waves of light colors have different wavelengths and travel with different velocities in some medium.(Violet light travels faster thanred light and the progression is in the order VIBGYOR)


Displacement: change of position. Itis a vector.(Requires both magnitude and direction).


Doping: Properties of very pure semiconductors (elements/alloys) used in solid state electronic devices are altered as desired with the addition of controlled amount of certain elements (impurities). This isdoping.Arsenic and Antimony are dopes for pureSilicon and Germanium to get P and N type semiconductors respectively.


Double refraction: when a light beam enters certain crystals, it gets refracted as two beams. This is double refraction. The beams are known as ordinary and extraordinary beams and travel with different velocities, with different refractive indices. They are polarizedin planes perpendicular to each other.


Dry cell:Leclanchecell in which the electrolyte is a paste and remains so for a good duration. The cell offers an e.m.f of 1.54 V. Such cells either alone or in packs are used in portable electronic devices




Earthis a concept in electric and electronic circuits it is atzero volt.All voltages (signals) in the circuit are measuredtaking the earth as reference.


Echo: Reflected sound waves from obstacles. This is used in Sonar(used in the measurement of distances and depths). Also used in modern medicine in (Sonography) forsome measurements and examination ofcertainpartsinsidehuman body.


Elastic limit: Bodies subject to stress get strained and return to their original size and shape when the strain is removed. This happens only when within a limit known as elastic limit.


Electrolysis is separation of constituents in molten chemical compoundsorsolutions of compounds using electric current, used in extraction of certain and purification of elements and also in electroplating of metals.


Electrolytic capacitor (condenser): A capacitor in which the dielectric is replaced by suitable electrolyte (a chemical compound to get capacitors of high capacity required in certain circuits. Electrolytic capacitors are polarized. (they have +ve and -ve poles). If they are not properly connected theyexplode.


Electromagnetic Wave: A wide range of waves (radiation, energy) extending from Radio waves, Micro waves, Infra-Red radiation, Visible light, Ultra violet rays, X-Rays, γ-rays are known as electromagnetic waves. They all have common properties. Travel in vacuum (and air) with the velocity of light (3×108 m/s). They are transverse waves with electric and magnetic fields (vectors) oscillating at a high frequency and remaining perpendicular to the direction of propagation.


Electro motive force (EMF)the force that cause flow of electric charges (current) from sources of electricity. Unit: volt (V)

Electron:One of the elementary particles of nature. It carries one unit of negative electric charge. Present in all atoms orbiting the nucleus, emitted as beta particle in radioactivity. Heated metals emit electrons (thermions). Light shining on certain metals liberate electrons (photo electrons).


Energy: Capacity to do work measured in joule.


Entropy: This is aconcept in thermodynamics. It is a measure of the available energy in a system. When a system does work, energy is spent and entropy increases. One statement of second law of thermodynamics is “entropy of the universe always increases”.


Evaporation: The change of stateliquid to its vapor (gas) is evaporation. It takes place at all temperature but the rate of evaporation is high at higher temperatures. It takes place only from the surface of the liquid. Evaporation also requires absorption of latent heat.




Faraday’s laws of electromagnetic induction: When electric flux through a coil of wire changes an electro motive force is induced across the ends of the coil. The induced e.m.f is proportional to the rate of change of flux.


Faraday’s Laws of electrolysis: When an electric current pass through an electrolyte certain chemical elements are deposited, released or dissolved at the electrodes. The mass of element deposited, released or dissolved by the same current is directly proportional to the respective chemical equivalent weight.


Fahrenheit is the British system of temperature measurement in which there are 180 equal intervals between the melting point of ice and boiling point of water. This system is still in use in human health care. The normal temperature of human body is 98.4o F. Melting point of ice is 0oF and the boiling point of water is 212oF.


Femto is 10-15– femtometer, femtosecond


Fleming’s left hand rule: for force on a conductor carrying current placed in a magnetic field, stretch the fore finger, the middle finger and the thumb of the left hand so that they point three directions mutually perpendicular to one another. If the fore finger gives the direction of the magnetic field and middle finger the direction of electric current the thumb will indicate the direction of the force on the conductor carrying current. (Principle of galvanometers and electric motors)


Fleming’s right hand rule: for induced current, if the fingers of the right hand are stretched as above and if the fore finger indicates the direction of magnetic field, thumb the direction of motion then the middle finger indicates the direction of induced electric current. (Principle of dynamo/generator of electricity


Force: causes the change in state of rest of a body or its uniform motion in a straight line (Newton’s first law). It is equal to the rate of change of momentum of the body.(Newton’s Second law). Unit – newton (N).


Fortin’s barometer is the standard mercury in glass barometer used to measure the atmospheric pressure.


Focus(Principal focus) of (1) curved mirror: A parallel beam of light parallel to and close to the principal axis of a curved mirror converges to or appears to diverge from a point on the principal axis. This point is known as the principal focus. The distance of the point from the center (pole) of the mirror is the focal length of the mirror. (2) Lens: A parallel beam of light parallel to and close to the principal axis of the lens after refraction passes through a point on the principal axis on the other side (converging lens) or appears to diverge from a point on the same side (diverging lens) of the lens. There are two foci one on each side. Their distance from the optic center of the lens is the focal length.


Fraunhoffer diffraction: This is diffraction ofa plane wave (parallel beam) of light when it encounters an obstacle or an aperture. The diffraction patternisobserved on a screen at a distance or using a telescope.


Frequency: it is the number of times per second an event repeats itself. It could be the vibration of an object or the periodic variation in a physical quantity like electric current, voltage or field strength, expressed in Hertz (Hz).KHz (kilohertz,103Hz) or MHz (megahertz, 106Hz).


Fresnel Diffraction: This is the diffraction observed when the light source is close by and the light wave is spherical/cylindrical. (divergentbeam). The diffraction is observed on a screen or using a telescope.


Frictionis always present when there is relative motion between two objects in contact. The force of friction opposes the relative motion; it is proportional to the normal reaction (the force exerted by one of the bodies on the other). The force of starting friction (the friction to start the motion) is always more than the friction while there is motion. If an objects rolls over another, the force of rolling friction is less the force of sliding friction. Friction is necessary one cannot walk on a frictionless surface. Nails used in carpentry holds pieces together by friction. Friction causes resistance and heat requiring lubrication. The ratio of force of friction to normal reaction is the coefficient of friction. (µ).


Fulcrum: a fixed point in a simple machine about which there is rotation to get work done with the machine.




Galaxy is a group ofstars that stay together as a group as the universe expands. There are several galaxies in the cosmos. Galaxies are identifiable just like individual stars and constellations in the sky. Sun and most of the stars we see in the night sky are part of a galaxyknown as the Milky Way.


Galileo: Is the father of experimental Physics. Discovered the laws of simple pendulum using his pulserate as timer. He invented telescope (Galilean Telescope with just a positive and a negative lens) and discovered four of the satellites of Jupiter. (Galilean satellites)


Gamma(γ) ray:  Electromagnetic wave of very short wavelength released in radioactive decay, in nuclear fission and nuclear reactions. Present incosmic rays. Some stars are known to give outγ-rays.


GMRT: Giant Meter Wave Radio Telescopelocated in Pune, is an array of parabolic antenna to receive radio signals as part of radio astronomy studies/research.


Gravity/Gravitation:one of theinteractions (forces) of nature.Isaac Newton noticed it for the first time as a force.Einstein’s General Theory of relativity says that it is because of the curvature of space (four-dimensionalspace-time continuum) near any object that makesother objects move suggesting the presence of a force called gravity.


Gravitational Constant (G): the gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. F = G(m1m2)/d2. The constant of proportionality G is the Gravitational constant.


Ground: as described earlier for earlier for Earthit is a concept in electricity/electronics. It is at zero volt.




Heat engine converts heat energy into mechanical work.


Hook’s Law: Within the limits of perfect elasticity the strain in a body is directly proportional to the stress causing it. Stress is force per unit area. Strain could be (1)linear strain – change in length to original length, or (2)volume(bulk) strain – change in volume to original volume or (3) shearing strain -the angle in radians through which the plane in the body perpendicular to the stress is rotated.


Hydrostatics:Study of Physics of fluids at rest. The pressure at a point in a fluid (liquid/gas) is the same in all direction. The pressure at different points in the same horizontal level and connected by the same liquid is the same.Hydrodynamics: Study of fluids in motion.


Hysteresis:There is always a lag behind in effect and what cause it. Effect does not follow cause. This is hysteresis.Elastic hysteresis: A load causes a change in length/shape of a wire/object. The removal of the load does not restore them to original length/shape immediately. Magnetic Hysteresis: Magnetic fields cause magnetization in magnetic materials. Removal/ reduction of the field donot decreasemagnetization by the same extent.


Hydrogen Bomb:When nuclei of two heavy hydrogen atoms (deuterons) fuse to form the nucleus of helium atom a loss of mass and release of a large amount of energy occurs. This isnuclear fusion, the principle of Hydrogen bomb that has been tested by a few countries.




Image:The image that we see while using optical elements (lenses/mirrors) or Optical instruments (microscopes/telescopes) are generally a replica of the object. Any deviation we notice in the image is aberration.


Internal energyis the energy of any physical system(solid, liquid or gas) due to motion, vibration and rotation of atoms and molecules of the system. It depends on temperature.


Ionosphereconsists of layers of charged particles (electrons/ions) surrounding the earth. They help in reflecting electromagnetic waves back to earth and assist in radio broadcast. (Sky wave propagation)


Isothermal process: take place at constant temperature. Isobaric process: take place under at constant pressure. Isochoric process: take place at constant volume.Isentropic process(adiabatic process): takes place with no change in entropy.


Isotopes/Isobars: Elements of the same atomic number but of different atomic weights are Isotopes of the element. They have same number of protons but different number of neutrons. Similarly, elements that have the same atomic weight but of different atomic numbers are Isobars. They are different elements.


ISRO: Indian Space Research Organization. It has its laboratories in Kovalam (Kerala), Manavalakuruchi (Tamilnadu), Bengaluru (Karnataka) and Sriharikota (Andhra Pradesh). It undertakesrocket and space research and launching of artificial earthsatellites and space missions of India and othercountries.Chandrayan the mission to mars is major achievement.


IUCAA: Inter University Center for Astronomy and Astrophysics, a research center supported by the UGC (University Grants Commission of India) located in Pune. J V Narlikarthe,Founder Director IUCCA had also created a few otherAstrophysics centers in India.



Joule effect: There are a few effects called by this name. (1) The heat produced in a conductor due to electric current (I2R) is also called Joule heating. (2) Cooling of a gas due to sudden expansion (reduction in pressure) due to change in internal energy. (3) Magnetostriction, or change in dimensions of a ferroelectric specimen due to magnetization is Joule effect


Joule-Kelvin effect change in temperature of a gas that is throttled (made to expand through fine apertures). The gas expands at the same time does work pushing the air in front. Depending on the initial conditions of pressure and temperature the gas gets heated or cooled. Thecooling effect is used in liquefaction of gases.




Kelvin scale of temperature: The temperature interval is the same as in Celsius (centigrade) scale but the lowest temperature is -273oC which is the lowest temperature that one can expect to reach.


Kepler’s laws of planetarymotion:  (1) Planets move in elliptical orbits with sun at one of the foci of the ellipse. (2) The line joining the planet and the sun (the radius vector) sweeps equal areas in equal intervals of time. (3) The square of the period of revolution is directly proportional to the cube of the semi major axis of the ellipse.


Kinetic energy: Energy possessed by a body due to its motion. It is (1/2)mv2 for a body of massm moving with a velocity v and (1/2)Iω2 for a body of moment of inertia I rotating with an angular velocity of ω.


Kirchhoff’s Laws: (1) Algebraic sum of the current at any node in an electric circuit is zero (2) In a closed network algebraic sum of the products of the resistance in each branch and the current in that branch is equal to algebraicsum of theelectro motive force in the network.




Latent heat: Heat energy absorbed per unit mass for change of state. J/kg. Latent heat offusion for conversion of solid into liquid, latent heat of vaporizationot boiling for conversion of liquid to vapor, latent heat of sublimation for direct conversion of solid to vapor.


Leclanche cellis one of the early sources of electricity. Used extensively in telephony in the early days. The most common dry cells in use these days are Leclanche cells. (EMF 1.54 V)


Lens: A transparent medium enclosed between two curved surfaces or between one curved surface and a plane surface is a lens. They are used in optical instruments. A lens that converges a parallel beam of light incident on it is converging (convex) lens. Similarly, a lens that diverges the parallel beam of light incident on it is the diverging (concave) lens. When of the surfaces is plane, the lens is known as planoconvex or planoconcave lens. Transparent liquid trapped between two curved surfaces or a curved surface and a plane surface exhibit lens action and are known asliquid lens.


Lepton: collective name of elementary particles electron, positronand mesons.


Leyden jar(capacitors) used for storage of electric charges in static electricity experiments.


Linear accelerator:Charged particles get accelerated to high energies while passing through a series of co-axial tubes of progressively increase in lengths arranged in straight line with suitable high frequency alternating voltage between successive tubes.


Logic gates: These are basic digital electronics circuits for carrying out logic operations – NOT, AND, NAND, OR, NOR etc., of Boolean algebra.


Longitudinal wavehas the vibrationsof the medium in the same line as the direction of propagation of the wave. (Sound waves in air/solid mass)




Meson: elementary particle like electron and positronand lighter than proton. There are of two types π and µ, π mesons are heavier than µ mesons. They carry positive or negative charge or may also be neutral. They are found in cosmic rays and are bye product in certain nuclearreactions. Electron, positron and mesons are collectively known as leptons.


Michelson Morley experiment:Huygens proposed that light that light is a wave.Theexistence of the medium called ‘ether’ that pervades all space was proposed. This historic experiment was performed tocheck the existence of ether. This interferometer was however used later to determine the speed of light in the laboratory.The electromagnetic theory of light removed the need for ‘ether’


Moderator: Neutrons required to sustain nuclear fusion should be at thermal (low) energies. The neutrons released in nuclear fission are fast neutrons. For chain reaction to happen the speed of neutrons are reduced using moderators. Graphite and heavy water (water with heavy hydrogen) are the most common moderators. When there are too many neutrons, to control nuclear fission excess neutrons are absorbed using Cobalt known as arrestor.


Modulation: Electromagnetic waves are used in the propagation of radio, telephone and television signals (programs). This is done(1) by amplitude modulation by modifying the amplitude of the carrier electromagnetic wave or (2) byfrequency modulation in which the carrier wave frequency is changed or (3) by phase modulation in which the phase of the carrier wave is modified or by (4) pulse modulation in which the carrier wave is in the form of pulses and their frequency (number of pulses per second) is changed, using the signal.


Modulus of elasticity: Young’s modulus, Bulk modulus and Rigidity modulus are the three moduli of elasticity of rigid bodies.


Momentum: the product of mass and velocity (mv) for linear motion, and the product of moment of inertia and angular velocity of a revolving object in angular motion. (Iω).




Neutron:One of theelementary particles of nature. Present in all atoms except Hydrogen. It carries nocharge (neutral). Causes nuclear fission. Used in the production of artificial radioactive elements. Present in cosmic rays. Neutron star is the collapsed state of a star atleast 1.4 times heavier than our Sun during the course of its long life.


Neutrino: One of the elementary particles of nature. Emitted along with electrons/positrons in beta decay in radioactivity. High energy Neutrinos are prt of cosmic rays. They arett highly penetrating. A project known as INO(International Neutrino Obseratory) is ecpected to come up soon in Theni District, Tamilnadu.


Newton: Sir Isaac Newton – Physicist, noticed Gravity, gave the three laws of motion, proposed the corpuscular theory of light and discovered calculus, the author of the five volumes of great Principia, earliest texts in Physics. Demonstrated theNewton’s rings in optics which confirmed wave theory of light.


Newton’s law of cooling: Rateof cooling of a hot body is directly proportional to the difference in temperature of the body and its surroundings.


Normalto a surface at a point is a straight line perpendicular to all straight lines on the surface passing through that point.


NPL: National Physical Laboratory, New Delhi. Premier Physics Research Laboratory in India founded before the independence of our country. Founder Director Prof. K.S. Krishnan.


Nuclear fission/fusion: The nuclei of certain heavy elements when hit by neutrons break forming nuclei of lighter elements. This is nuclear fission. During this process, there is a loss of mass that results in the release of large quantity of energy. (E=mc2). Principle of atom bomb.Similarly, under favorable conditions two light nuclei like heavy hydrogen 1H2(deuterium) coalesce forming a helium nucleus 2He4accompanied by a similar loss of mass and release of large amount ofenergy. This is nuclear fusion.Principle of hydrogen bomb. This is the source of energy generated by the sun.


Nuclear reactions: Nuclei of atoms bombarded with missiles like alpha particles, neutrons etc., undergo changes that are describes like chemical equations. For example: Aluminum bombarded with alpha particles gives an isotope of Phosphorus that is radioactive.

. Here  are positron and neutrino.




Oil drop experiment: Performed by Robert Millikan to determine the charge carried by an electron. He was awarded Nobel Prize for this experiment.


Optic axis is a straight line related to an optical element or an optical instrumentsuch that any ray of light passing in that direction will go undeviated


Optic center: A point associated with a lens. All rays of light passing through that point areundeviated


Optical elements: lenses, mirrors, prisms used in optical instruments.


Optical instrumentsare Telescopes, Microscopes, Projectors Binoculars, Cameras, etc. that use visible light. Similar instruments that use infrared light (radiation) are also in use for night vision equipment.


Optical fiber is fiber of glass or of some transparent plastic with a suitable cladding. It used for transmitting light beams/signals over long distances. This happens by total internal reflection of light at the wall/cladding.


ORT: The Ooty Radio Telescope project is located on the slope of a hill in Ooty, Nilgiris, Tamilnadu. It is part of NCRA (National Center for Radio Astronomy) ofTIFR.




Pascal law: The pressure at a point in a fluid at rest is the same in al directions. When the pressure at the point is increased or decreases by a certain amount, the pressure at all other points in the fluid connected by the same fluid is also increased or decreased by the same amount in all directions.


Pettier effect: when a current flows in a thermocouple (formed by joining the ends of two different metals (or alloys) to form a loop, heat is either generated or absorbed at the junctions. This is Peltier effect. If the direction of current is reversed heat liberated or absorbed is also reversed. An example easy to remember: with wires ofof two metals antimony (A) and bismuth (B), when current flows from A to B the junction gets cold (C).


Photon is a packet of electromagnetic energy. Electromagnetic energy can exist as a wave also. It has dual nature according to quantum theory. If the wavelength of the energy is λ and the frequency ν, the velocity is c = ν λ. The energy E = hν. (h is Planck’s constant)


Positron is an elementary particle of naturelike electron but with an equal positive charge. They are released in beta decay of some artificial radioactive elements. An electron and positron coming togetherannihilate each other andrelease energy as gamma ray. Likewise, a gamma ray photons may produce electron-positron pairs. This observed in cosmic rays.


Proton: An elementary particle of nature. Present in the nucleus of all atoms, carries one unit of positive electric charge. Proton and neutron are Baryons.


Planck’s constant (h): The energy of a photon is E = hν, here h is Planck’s constantand ν the frequency. The wave length of matter waves also involves h, λ = h/p where p is the momentum.


Planets: Objects that go aroundthe sun with regular periodicity they are Mercury, Venus, Mars, Earth, Jupiter, Saturn, Uranus, Neptune and Pluto. Planets have been observed in a few stars also.


Platinum Resistance Thermometer: Thermometer that uses a coil of platinum to measure temperature. Electrical resistance changes with temperature.


Polarization (plane, circular and elliptical): This is a property of light andof all electromagnetic waves. Generally they areunpolarised. The variations of the electric field (and the magnetic field) vectors that constitutes the wave is confined in a plane perpendicular to the direction of propagation. If the electric (and magnetic field) vectors are in afixed direction in that plane the wave is plane polarized. If the electric (and magnetic) vectors revolve without change in strength in a plane perpendicular to the direction of propagation the wave is circularly polarized. If during the rotation the length of the vectors keep changing between a minimum and maximum strength the wave is elliptically polarized.


Pressure is force per unit area (N/m2). Pressure at a point in a fluid (liquid or gas) is the same in all directions. Unit:(Pascal, Pa).


Power is the rate at which work is done (power is consumed). Unit: Watt (W).Kilowatt (KW = 103W), Megawatt (MW = 106 W), milliwatt (mW =10-3W)) are also in use


Power (focal power) of a lens is the reciprocal of its focal length in meter. It is +ve for converging lens and -ve for diverging lens. Unit: Diopter(D)


Principal axis (1) of spherical mirror: is the line joining the center of the mirror and the geometric center of the spherical mirror of which the mirror is a part.(2) of a lens is the line joining the centers of curvature of the two curved surfaces forming the lens. (3) of an optical instrument is an imaginary line through the instrument, a ray of light passing through that line goes is not deviated.




Q factor (quality): The sharpness in the tuning of a tuning circuit is its quality factor = (ωL/R). L is the inductance and Ris the resistance of the coil used and ω the angular frequency (2π f).


Quantumis a pocket of electromagnetic energy in quantum theory. The energyof a quantum E = hν. (h is Planck’s constant and ν the frequency of the wave associated with the quantum).


Quinck’s drop is a drop of mercury on a glass plate.From measurements of the height of the flat top of the drop and the height of the circle of most pointing points of the drop above the glass plate one can calculate the surface tension of mercury and its angle of contact with glass.


Quantum Theory / Quantum Mechanics is a branch of Physics based on the proposition that matter exhibits dual nature (particle and wave nature) at the same time.




Radianis the unit of angle in circular measure. 1800 (degrees) = πc (radians)


Radio: receiver of signals (speech, music or codes) sent as electromagnetic (radio) waves.


Radioactivity: Certain elements change(decay) by themselvesto another element spontaneously bythemselves with the emission of charged particles alpha (α)or beta (β) particles. This is also accompanied by the emission of high frequency radiation known asgamma (γ) radiation. Those elements are said to be radioactive.This property found naturally in heavyelements (natural radioactivity). It can be induced in lighter atoms by nuclear reactions.(artificial radioactivity)


Rainbowis a colorful spectacle seen when rain drizzles. It is seenon evenings in the eastern sky in mornings in the western skywhen the sun shines. The sunlight gets refracted in the water drops get dispersed and also get reflected within the drops and reaches the observers eyes giving themulticolored rainbow. It should be remembered that what one sees is personalized. Sometimes two bows are seen primary and secondary bows. The secondary bow is at a higher angle wider than the primary and less bright formed after two reflections of light within the drop. If the conditions are special one can see a paler tertiary bow also but on the same side as the sun. Rainbow is also seen at water fountains with sun and the observer located suitably.


Raman Effect: When light waves of discrete wavelengths get scattered in a medium, waves of wavelength longer and shorter than the original wave are generated. This is Raman Effect. Seenin a spectroscope besides the spectral line corresponding to the original light wave, linescorresponding to scattered waves (Raman lines) are also seen. Lines of longerwavelength are called Stokes lines and lines shorter wavelengths are anti-Stoke lines. The wave length separation between the parent line and Raman lines are related to the structure of the molecule. This is used in the study of molecular structure.


Ray of light is a concept in geometrical optics. When light travels in a direction its path is represented by a straight line called the ray of light. A bundle of such rays is known as a beam of light. The rays in a beam could be diverging, converging or may remain parallel


Reflection is the property of all waves, mechanical (sound) and electromagnetic. They all obey the well-known Snell’s laws of reflection (1) The incident ray the reflected ray and the normal to the surface at the point of reflection are in the same plane. (2) The angle of incidence, the angle between the incident ray and the normal is equal to the angle of reflection between the reflected ray and the normal. The reflection gives rise to an image which is as far behind the reflecting surface as the object is in front of it. When the reflecting surface is rotated through an angle the reflected ray for the same incident ray is turned through double that angle. When the reflecting surface is translated (moved without rotation) the image of a given object moves twice the distance.Reflectoris a surface that reflects waves. Smooth glass surface or of polished metal surfaces reflect light. They are also known as mirrors. Mirrors may be plane or curved (spherical or parabolic) used in optical instruments


Refraction: This is also a property of all waves (mechanical/electromagnetic). When a wave crosses the surface separating two media its path is deviated at the surface of separation. This is refraction. However, when the wave travels in a direction at right angles to the surface of separation there is no deviation. The speed of the waves differs in different media. For light waves the speed in maximum in vacuum and is almost the same in air. The ratio of the speed of light in air/vacuum to its speed in a medium is the refractive index of the medium. It is greater than 1 since the speed of light in vacuum/air is more than the speed in any other medium.Snell’sLaws of refraction of light: (1) the incident ray, the refracted ray and the normal to the refracting surface at the point of incidence are in the same plane. (2) the ratio of the sine of the angle in incidence to the sine of the angle of refraction is a constant for a given pair of media for a given color. Violet is deviated more than red light. The refractive index of violet  is more than the refractive index of red and that of other colors fitting in between (VIBGYOR) in that the order.


Refrigerator:  Takes heat away from a cold body, rejects heat it to a body at a higher temperature and requires work to be done on it. (air conditioner)


Rigidity Modulus: When body is subject to shearing strain (a change in shape without change in volume) the ratio of stress to shearing strain (angle in radians) is the rigidity modulus


Rocket:Devices that are propelled by Newton’s third law of motion.Used for launching artificial satellites and in space studies and research.


Rontgen: Discovered X-rays, also known as Rontgen rays. He received the first Nobel Prize in Physics in the year 1901 for the discovery.


RRI: Raman Research Institute, Bangalore. Founder Sir C V Raman.




Satellite are objects that go round planets. They are noticed by the light of the sun. There could be one or more satellites for a planet. Artificial (man-made) satellites have been launched to go round the earth. They are used in electronic communication, study of global weather and space research.


Saturated vapor pressure (SVP)is the maximum pressure exerted by a vapor. Any increase in pressure and/or decrease in temperature will result in the vapor condensing into liquid.


Seebeck effectis a thermoelectric effect. Wires of two different metals (or alloys) are joined at ends with the junctions are at different temperatures, get an e.m.f in the loop and some current flows. This is thermocouple and the currentthermoelectric current. Used in the measurement of temperatures. (Thermoelectric thermometer)


S I System of Units: unit of length (meter), mass (kilogram), time (second), electric current (ampere), temperature (Kelvin), amount of substance (mole) and luminous intensity (candela) are the seven basic units of theSystem International (SI) units currently used in science all over the world. This supersedes all earlier systems the CGS, FPS and MKSA systems used earlier.


Simple machines aredevices that enables us to get work done more easily. (Simple levers, scissors, cutters, pliers etc.). Load is what the machine tackles, effort is the force applied to tackle the load. The ratio of load to effort is themechanical advantage. The point around which the load and effort revolve is the fulcrum.


Spring balance: Compression or extension in a spring is directly proportional to the load. Spring balance uses this principle to measures the weight of objects.


Stefan’s Law: Total energy radiated by a perfect black body is directly proportional to the fourth power of its absolute temperature (E = σT4). σ is Stefan’s constant.


Solar cell converts solar energy to electrical energy. Widely used as a source of alternate energy these days.


Solar constant: The total energy in all wavelengths received per second from the sun on unit area of earth surface outside the earth’s atmosphere. It is 1.362 W/m2. This information is used to calculate the surface temperature of the sun.


Sonar (SOundNavigation And Ranging): Sound waves are used to locate objects when the visibility is low using echo. It is used in ships for navigation in the seas to avoid collision and in estimating the depth of the sea bed.Also, used in battle tanks. Some SONAR devices use ultrasonic waves


Spherical aberration is the defect observed in the images formed in optical instruments because of large aperture (opening) of the optics (mirror and lens). In reflecting telescopes it is removed completely by replacing the spherical mirror by parabolic mirror. With lenses it can only be reduced using lenses of suitable curvatures or by limiting only paraxial rays to form the image.


Stars: These are innumerable number of bright objects present around us in the sky all times but visible when the bright sun light if not there. Sun is also a star, our nearest star.Some of the bright objects could themselves be groups of stars (galaxy). The twinkling of stars is due to the medium in between and not a property of stars (forget the nursery rhyme)


Sublimation: it is the change of state from solid to gaseous (vapor) state without going through liquid state. Solid carbon-di-oxide used extensively inpacking and transporting of ice cream and certain living samples for biological studies and medical applications, is a good example. It is also known as dry ice as it evaporates directly from solid to gaseous state.


Super conductivity: At liquid helium temperatures (below 4oK) the electrical resistance of mercury drops to zero. This property of zero resistance is superconductivity. Few more metals and alloys also exhibit superconductivity at such low temperatures. Some alloys are known to exhibit super conductivity at higher temperatures (liquid air temperatures). They are high temperature super conductors. Electromagnets that produce very high magnetic field use coils of super conducting wires. Superconductors are used in large scale transmission of electrical energy with reduced transmission loss.


Surface tension is a property of liquids. The free surface of liquids behaves like stretched elastic sheet and always tend to contract.The level of liquid in a capillary inserted in a liquid is different from the level outside because of surface tension. (it is capillary rise for all liquids but capillary depression for mercury). The surface energy per unit area of liquid is equal to surface tension.Surface tension does not depend on the surface area. There are many associated properties. The pressure insideliquid drop and bubble (like soap bubble) is always more than the pressure outside. The arches and domes of ancient buildings use the same principle to support load.




Temperature indicates how hot or cold a body is. It is determined by the motion or vibration or rotation or combinations of any of theminatoms/moleculesforming the body.


Tesla: unit of magnetic field. One coulomb of charge mowing with a velocity of 1m/s in a direction perpendicular tofield of 1 Tesla experiences a force of 1 Newton


TIFR:  Tata Institute of Fundamental Research, Mumbai, a premier research institute. This has a few centers in other parts of the country.


Thermionic emission: Emission of electrons from heated metallic surfaces.Electronic devices like thermionic valves, Cathode Ray tubes (CRT) and X-Ray tubes use thermionic emission for generating electrons needed for their functioning.


Thermocouple/ Thermoelectric effect:A pair of wires of different metals (or alloys) joined at their ends is thermocouple. With the two junctions at different temperatures an e.m.f is developed across the junctions (thermoe.m.f) and current flows in the wires.


Thermodynamics is the study of interaction of heat (thermal) and mechanical energy. First law of thermodynamics: Energy is conserved. A definite amount of work is obtained from definite amount of heat and vice-versa. This law is also stated as an equation dQ = dU + dW. Where dQ is heat energy supplied or withdrawn, dU change in internal energy and dW is the work done. (Implying there is no loss). Second Law of Thermodynamics: Unaided by any external agencyheat cannot by itself flow from a body at a lower temperature to a body at a higher temperature. When two bodies A and B are in thermal equilibrium separately with a third body C, then A and B will be in thermal equilibrium between themselves. This is the Zeroth Law of thermodynamics. The temperature absolute zero cannot be reached. This is the Third law of thermodynamics.


Thermometer is an instrument formeasuring the temperature. The common thermometer is mercury in glass thermometer. Clinical thermometer is mercury in glass thermometer designed for a limited range of temperatures and with an additional feature (a narrow constriction in the capillary that does notpermit the mercury to fall back easily in to the bulb after the thermometer is taken away from where itwas used to record the temperature. This makes taking the reading more convenient). Other thermometers in use are: resistance thermometers that uses the property of change in electrical resistance as temperature changes, thethermoelectric thermometer in which the voltage changes with temperature. Presently electronic temperature measuring devices with digital display are also available using different transducers.


Thermos flask: a double walled container of glass or stainless steel suitably designed to ensure minimum exchange of heat between what it contains and the surroundings. Precautions are taken to reduce (avoid) transfer of heat byconduction, convection and radiation. Such thermos flasks used in science laboratories are also known as Leyden jars.


Thomson effect: when a temperature gradient is maintained along the length of a conductor an e.m.f. (a potential difference) exists along the length. It could be positive (voltage decreasing from hot end) or negative (voltage increasing as distance form hot end increases). The samples are said to have positive or negative Thomson coefficient (σ), the voltage difference per unit per degree Celsius (Kelvin).


TIFR: Tata Institute of Fundamental Research,  the premier science research laboratory of Government of India, located in Mumbai and has its activities spread in different parts of the country.


Torque: Cause rotatory motion.


Torsion/Torsional strainis caused insolids by shearing (tangential) stress resulting in change in shape but not in size.  When such a strain is released a torsional wave is created in the solid.


Total Internal Reflection:When light is directed to ente from a denser medium (higher refractive index) into a rarer medium (lower refractive index), for angles of incidence greater than a certain value (critical angle) the light is not refracted into the second medium but totally and internally reflected into the first medium. This is total internal reflection.It is used to build reflecting prismsused in optical instruments. Images formed in optical instruments suffer up-down inversion or left right (lateral) inversion. Such inversions are corrected using total reflecting prisms.


Transuranic elements are elements of atomic number greater than 92 that are produced in laboratories. They get fitted in the periodic table of elements beyond Uranium in the same period.


Transverse wave is one in which theoscillations (vibration) constituting the wavease in a direction perpendicular to the direction of propagation of the wave.Electromagnetic waves are transverse waves. Sound waves in solid medium can be transverse. Torsion waves are transverse waves. Transverse waves can be polarized.


Triple point: It is a unique temperature and pressure at which a solid, liquid and vapor are in equilibrium. For water, ice and water vapor it is 273.16K (0.16oC and 611.2 Pa).




Ultrasonic: Sound of very high frequency that human ears cannot hear. It isused in medical diagnosis and examination of certain parts of human body and also in some treatment procedures like the breaking of kidney stones.

Ultraviolet light: Electromagnetic radiation of wave length range between the visible (violet) light and (soft) X-rays.


Universal gas constant: In the gas equation PV = RT, for one mole of a gas the constant R is the same for all gases and hence universal.


Universe: In thermodynamics everything outside the system under study is referred to as the Universe. In astronomy the planets, stars, galaxies that surround the earth forms the Universe.


Uranium: Heaviest among the 92 naturally occurring chemical elements. It is radioactive, has a few isotopes, of them U235 is one that undergoes fission more readily. U235 is the first choice for bombs and atomic reactors.




Van Allen belts:High energy charged particles from solar winds,cosmic rays and ions in the upper atmosphere of the earthare held around the earthby its magnetic field. Earth has two such belts. The other planets also have similar belts.


Van de Graff generator is used to accelerate charged particles to high energies. Electrostatic induction is used to get electric charge stored in huge spherical condenser to build up the high voltage.


VanderWaal equation connects the pressure, volume and temperature of a real gas taking into account correction for the size of the molecules and the intermolecular attraction.


Variable capacitor: In certain electronic circuits for special applications like tuning, capacitors of variable capacity are needed. One such capacitor of this type will have a set of vanes that could move into another thus changing the area of overlap and hence the capacity.


Vapor is the gaseous state of matter that can be convertedback into liquid (or solid) state by compression (decrease in volume) or cooling (decrease in temperature) or both.


Vector: a physical quantity that requires both magnitude and direction for specification.


Vector product A X B is a vector C having a magnitude = AB Sin θ, here A and B are the magnitudes of the vectors A and B and θ is the angle between them. The direction of C is the direction in which a right hand cork screw will advance when it is turned from A to B


Velocity:the rate of displacement (change in displacement in unit time), a vector quantity. The direction of velocity is the same as the direction of displacement. The magnitude of velocity is speed (a scalar quantity).


Velocity ratio: In simple machines with the effort isapplied and the load ismoved. These happen at different speeds. The ratio of the speed of the load to the speed of the effort is the velocity ratio. This is the reciprocal of the mechanical advantage of the simple machine.


VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange and Red – the sequence of colors in rainbow. White light dispersed in a prism or using a diffraction grating is dispersed in the same fashion.


Vibration is to and fro motion. The number of vibrations per second is frequency.The unit, one vibration per secondis hertz (Hz). Multiples of this unit in common use are Kilohertz (KHz = 103 Hz), Megahertz (MHz = 106 Hz), Gigahertz (GHz = 109Hz)


Viscosity is a property of liquids and gases. Their flowin a tube and movement over solid surfaces and also the movement of solid bodies through them are resisted by viscosity. It is also referred to as fluid (liquid) friction.


Volt: Unit of electromotive force (EMF) (the force causing electric current / movement of electrons). KV (kilovolt, 103 V), MV (megavolt,106 V), mV (millivolt 10-3 V), microvolt (µV, 10-6 V) are multiples or sub multiples of volt. If the work done in moving 1 coulomb of charge from one point to another is 1 joule the potential difference (EMF) causing it is 1 volt.


Voltaic cell: (Galvanic cell) earliest device to generate electric current using suitable metal electrodes and electrolytes (dilute acid/salt solution) Daniel cell, Leclanche cell.




Water equivalent is a term used in in calorimetry.  When heated the temperature of a body increases. The mass of water that requires the same amount of heat for the same rise in temperature as the body is its water equivalent. It is equal to the product of the mass and the specific heat of the material.

Watt: Unit of power. Work done at the rate of 1 joule persecond is 1 watt.Kilo watt (1000 W), Megawatt (106 W) are higher multiples in use. Power dissipated in electronic components are specified in mW (milliwatt, 10-3W) orµW (microwatt, 10-6W)

Watt houris the unit energy spent (work done). When 1 watt of energy is spent in 1 hour the work done (energy spent) is 1 WH (watt hour). In commercial distribution of electricity I KWH is referred to as 1 Unit. Other practical units are KWH (kilo watt hour, 103WH) and MWH (megawatt hour 106WH.

Wave/Wave motion transmission of a periodic change in displacement (mechanical wave) or of a physical quantity like pressure (sound wave) or of electric/magnetic field strength (electro-magnetic wave, light wave). The periodic up-down movement of a stretched string and waves on the surface of a liquid are known as transverse waves. Alternate compression and rarefaction in the medium as sound waves get propagated are longitudinal wave. The number of times the displacements gets repeated per second or the number of compressions or rarefactions passing through per second is known as the frequency (n).

Wave length (λ) is the distance between two successive points in a wave in the same state (phase) of vibration.The product of wave length (λ) and the frequency (n) the number of vibrations in a second) is the velocity of the wave(= n λ)

Wave number(ν) is the reciprocal of wavelength.The number of waves in one meter(m-1).

Weak interaction is the weakest of three interactions (forces)we come across in nature. The other two are Electromagnetic and Nuclear (strong)interactions. The beta decay in radioactivity is due to weak interaction. As a result, neutron within the nucleus gets changed to a proton emitting an election or a proton gets changed to neutron emitting a positron(positively charged electron). These emissions are also accompanied by the emission of amass less particles known as neutrino.It should be remembered that the macroscopic Gravitational interaction is much weaker than this weak interaction.

Weber is unit of magnetic flex. A flux density of 1 wb/m2 is Tesla (unit of magnetic field)

Weight of an object is a measure of the gravitational pull on the object. It is mg newton, where m is the mass and g the acceleration due to gravity. It is a widespread practice to express the weight in mass unit (kg). It should however be remembered because g is not a constant on the earth, the weight of an object is not the same at all points on the earth while mass is. The spring balance measures weight.

Wheatstone’s bridge (network) is a circuit with four arms very common in electric, electronics circuits. A typical bridge used in the measurement of resistance has four resistances R1,R2,R3 andR4 connected between four points A, B,C and D as in figure. When (R1/R2) = (R3/R4) the points B and D will be at the same potential and no current flows through a galvanometer connected between B and D. The bridge is said to be balanced.

Wien’s law: The electromagnetic radiation from a black body spreads over a range of wavelengths. The graph showing the energy distribution among wave lengths is a curve (bell shaped). The product of the wavelength corresponding to maximum radiation (λm) and the absolute temperature of the body is a constant. (λmT = constant) This law is also known as the Wien’s displacement law. When the temperature of a black body is increased, bulk of the energy radiation from it is displaced towards to lower wave lengths. The color of heated iron turning from dull red to bright red to yellow is a good visible illustration of the law.

Work is done when the point of application of a force moves the point of application of the force in the direction of the force.A force of one newton (N) moves the point of application by one meter (m) in the direction of the force the work done is one joule (J).




X- ray is electromagnetic radiation of short wavelength. When fast moving electrons strike a metal target X-ray is generated. X-ray is used in the study of crystal structure. X-rays of shorter wavelength (higher energy) are called hard X-rays. Those of longer wavelength (lesser energy) are soft X-rays.Soft X-ray is used in diagnosis, while hard X-ray is used in treatment of cancer.  X-rays are present in Cosmic rays also. Some stars/galaxies radiate X-rays.




Yield point. When a body is strained,with in certain limit (elastic limit) the strain is proportional to the stress. The body returns to its original shape/size when the stress is removed. Beyond a certain limit (yield point)the strain increases with time and body brakes.

Young: Thomas Young is an ophthalmologist and a physicist. His theory of color vision and color blindness is well known in Ophthalmology. His experiments in interference in light and elasticity are known.Young’s double slit experiment confirmed the wave theory of light.

Young’s modulus is ratio of linear stress (force per unit area) to the linear strain (change length per unit length) of a solid. Unit: N/m2




Zeeman effect: When a discharge tube is placed between the pole pieces of a strong magnet and the radiation (light) is examined in a spectroscope, along with spectral lines of the gas in the tube, lines of wavelengths greater than and less than the original lines are seen. The additional  lines are known as Zeeman lines.  This ‘splitting’ of spectral lines into extra lines is the Normal Zeeman Effect. The same effect with a weaker magnetic field discovered subsequently is known as Anomalous Zeeman Effect. The Zeeman lines are polarized. The magnetic fields in celestial bodied (stars/planets) are estimated from the observed Zeeman splits of identifiable spectral lines of light from them.

 Zero error: Measuring instruments develop zero error as they get used. The initial reading in the instrument should be zero. Sometimes it could above or below zero (positive or negative). After making the measurement with the instrument the zero error is algebraically subtracted to get the correct measurement.

Zinc is the cathode in all voltaic cells including the dry cell.

Zone plate is a device with series of concentric alternate dark and transparent rings on glass or plastic sheet. The device acts like a convex lens with several foci. Used in overhead projectors.


This document put together in this form by Prof. V. Srinivasan, Professor of Physics (Retired), The American College, Madurai, India, a project undertaken to remain busy in retired life.

Physics Dictionary
In this photo he is in conversation with Sir C. V. Raman, at RRI in 1969.

Prof. Dr. J. Sethuraman my Physics friend and a valued colleague at American College, Madurai has gone through the manuscript and had suggested improvements. His role is acknowledged.

Physics Dictionary
Prof. Dr. J. Sethuraman my Physics friend

As this first venture gets circulated, feedback from users is requested. The feedback will be used for improving this compilation.

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