Class 12 · Physics · English medium · Part 2
Class 12 Physics Part 2 companion
An original companion to Physics, NCERT Class 12 Physics (2026–27 edition). Official textbook ↗ ncert.nic.in
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Chapters
- ENHinglish
Chapter 9: Ray Optics and Optical Instruments
Light can be treated as rays because its wavelength is tiny. - ENHinglish
Chapter 10: Wave Optics
Wave optics treats light as a wave. - ENHinglish
Chapter 11: Dual Nature of Radiation and Matter
Cathode rays are electrons (, ). - ENHinglish
Chapter 12: Atoms
Thomson's model spread an atom's positive charge through its volume. - ENHinglish
Chapter 13: Nuclei
The nucleus is about across, holds Z protons and N neutrons (A = Z + N) and nearly all the atom's mass. - ENHinglish
Chapter 14: Semiconductor Electronics: Materials, Devices and Simple Circuits
By resistivity and bands: metals (overlapping bands), insulators ( eV), semiconductors (Si 1.1 eV, Ge 0.7 eV).
Also in Hinglish
- Chapter 9: Ray Optics aur Optical Instruments (Hinglish)
- Chapter 10: Wave Optics (Hinglish)
- Chapter 11: Dual Nature of Radiation and Matter (Hinglish)
- Chapter 12: Atoms (Hinglish)
- Chapter 13: Nuclei (Hinglish)
- Chapter 14: Semiconductor Electronics: Materials, Devices aur Simple Circuits (Hinglish)
A
absorption spectrumअवशोषण स्पेक्ट्रमA continuous spectrum with dark lines where a cool gas has absorbed its own characteristic wavelengths.Ch 12 · Atomsacceptorग्राहीA trivalent impurity that takes an electron from the lattice, creating a hole, and becomes a fixed negative ion.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsangle of minimum deviationन्यूनतम विचलन कोणThe smallest deviation Dm a prism can produce, reached when the ray passes symmetrically (i = e).Ch 9 · Ray Optics and Optical Instrumentsatomic mass unitपरमाणु द्रव्यमान मात्रकOne-twelfth of the mass of a carbon-12 atom: 1 u = 1.6605×10⁻²⁷ kg, equivalent to 931.5 MeV.Ch 13 · NucleiB
barrier potentialरोधिका विभवThe potential difference V0 across the depletion region of an unbiased junction (n-side higher).Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsbinding energyबंधन ऊर्जाThe energy needed to separate a nucleus into free nucleons, equal to the energy released when it forms: Eb = Δ Mc².Ch 13 · Nucleibinding energy per nucleonप्रति न्यूक्लिऑन बंधन ऊर्जाEb/A, the average binding of one nucleon; about 8 MeV for most nuclei and highest (8.8 MeV) near iron.Ch 13 · NucleiBohr radiusबोर त्रिज्याThe radius of hydrogen's smallest orbit, a0 = 5.29×10⁻¹¹ m.Ch 12 · AtomsC
Cartesian sign conventionकार्तीय चिह्न परिपाटीDistances are measured from the pole or optical centre, positive along the incident light; heights above the axis are positive.Ch 9 · Ray Optics and Optical Instrumentscathode raysकैथोड किरणेंStreams of electrons that leave the cathode of a gas-discharge tube at very low pressure.Ch 11 · Dual Nature of Radiation and Mattercoherent sourcesकला-संबद्ध स्रोतSources of the same frequency whose phase difference stays constant. Incoherent sources have a randomly changing phase difference.Ch 10 · Wave Opticsconduction bandचालन बैंडThe band above the valence band in which electrons can move freely. Its bottom is EC.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitscritical angleक्रांतिक कोणThe angle of incidence in the denser medium for which the refracted ray grazes the boundary: sin ic = n2/n1.Ch 9 · Ray Optics and Optical Instrumentscritical angleक्रांतिक कोणThe angle of incidence in the denser medium for which the angle of refraction is 90 °: sin ic = n2/n1.Ch 10 · Wave OpticsD
de Broglie wavelengthदे ब्रॉग्ली तरंगदैर्ध्यThe wavelength λ = h/p of the wave associated with a moving particle of momentum p.Ch 11 · Dual Nature of Radiation and Matterdepletion regionअवक्षय क्षेत्रThe layer at a p-n junction that has lost its mobile carriers, leaving fixed ions on both sides.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsdiffractionविवर्तनThe spreading of a wave around obstacles and through openings, noticeable when they are not much larger than the wavelength.Ch 10 · Wave OpticsdonorदाताA pentavalent impurity that gives a free electron to the crystal and becomes a fixed positive ion.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsdual natureद्वैत प्रकृतिThe fact that radiation and matter both show wave-like and particle-like behaviour, depending on the experiment.Ch 11 · Dual Nature of Radiation and Matterdynamic resistanceगतिक प्रतिरोधrd = Δ V/Δ I, the slope-based resistance of a device at a point on its characteristic.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple CircuitsE
electron volt (eV)इलेक्ट्रॉन वोल्टThe energy an electron gains when accelerated through 1 V: 1 eV = 1.602×10⁻¹⁹ J.Ch 11 · Dual Nature of Radiation and Matteremission line spectrumउत्सर्जन रेखीय स्पेक्ट्रमBright lines at wavelengths characteristic of an element, from its excited gas at low pressure.Ch 12 · Atomsenergy band gapऊर्जा अंतरालThe forbidden energy range Eg = EC - EV between the valence and conduction bands.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsexcited stateउत्तेजित अवस्थाAny state of an atom above the ground state (n ≥ 2 in hydrogen).Ch 12 · Atomsextrinsic semiconductorबाह्य अर्धचालकA semiconductor whose carrier numbers are set by a deliberately added impurity (doping).Ch 14 · Semiconductor Electronics: Materials, Devices and Simple CircuitsF
field emissionक्षेत्र उत्सर्जनRelease of electrons from a metal by a very strong electric field, of order 10⁸ V/m.Ch 11 · Dual Nature of Radiation and Matterfocal lengthफोकस दूरीThe distance from the pole of a mirror or the optical centre of a lens to its principal focus, f.Ch 9 · Ray Optics and Optical Instrumentsforward biasअग्रदिशिक बायसp-side to the positive terminal: the barrier falls to V0 - V and a large current flows.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsfringe widthफ्रिंज चौड़ाईThe distance between neighbouring bright (or dark) fringes; β = λ D/d for a double slit.Ch 10 · Wave OpticsG
ground stateनिम्नतम अवस्थाAn atom's lowest-energy state; for hydrogen n = 1, E1 = -13.6 eV.Ch 12 · AtomsH
holeहोलA vacancy in a covalent bond that behaves as a mobile carrier of charge +e.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitshydrogenic atomहाइड्रोजन-सम परमाणुA nucleus of charge +Ze with a single electron, such as He+ or Li²+.Ch 12 · AtomsI
impact parameterसंघट्ट प्राचलThe perpendicular distance between a particle's initial line of motion and the centre of the target nucleus.Ch 12 · Atomsinterferenceव्यतिकरणThe redistribution of intensity when coherent waves overlap: bright where they arrive in step, dark where they arrive half a cycle apart.Ch 10 · Wave Opticsintrinsic semiconductorनैज अर्धचालकA pure semiconductor whose carriers come only from thermal excitation, with ne = nh = ni.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsionisation energyआयनन ऊर्जाThe least energy that frees an electron from an atom in its ground state; 13.6 eV for hydrogen.Ch 12 · AtomsisobarसमभारिकNuclides with the same mass number A but different Z.Ch 13 · Nucleiisotoneसमन्यूट्रॉनिकNuclides with the same neutron number N but different Z.Ch 13 · Nucleiisotopeसमस्थानिकNuclides of one element that differ only in their neutron count: equal Z, unequal N and A.Ch 13 · NucleiL
linearly polarisedरैखिकतः ध्रुवितLight whose electric field oscillates along one fixed line; also called plane polarised.Ch 10 · Wave OpticsM
magnificationआवर्धनThe ratio of image height to object height, m = h'/h; negative for an inverted image.Ch 9 · Ray Optics and Optical Instrumentsmagnifying powerआवर्धन क्षमताHow many times larger the angle at the eye becomes with the instrument: the final image's angle divided by the angle of the object viewed directly (at the near point, for a microscope).Ch 9 · Ray Optics and Optical InstrumentsMalus' lawमैलस का नियमI = I0 cos² θ for polarised light of intensity I0 meeting a polaroid whose axis is at θ to it.Ch 10 · Wave Opticsmass defectद्रव्यमान क्षतिHow much lighter a nucleus is than its separate protons and neutrons: Δ M = [Zmp + (A-Z)mn] - M.Ch 13 · Nucleimass numberद्रव्यमान संख्याThe total number of nucleons in a nucleus, A = Z + N.Ch 13 · NucleiN
nuclear fissionनाभिकीय विखंडनThe splitting of a heavy nucleus, such as uranium-235 after absorbing a neutron, into two medium nuclei and a few neutrons, releasing about 200 MeV.Ch 13 · Nucleinuclear forceनाभिकीय बलThe strong, short-range, charge-independent force that binds nucleons; it vanishes beyond a few femtometres and repels below about 0.8 fm.Ch 13 · Nucleinuclear fusionनाभिकीय संलयनThe joining of light nuclei into a heavier, more tightly bound one, releasing energy; it powers the stars.Ch 13 · Nucleinucleonन्यूक्लिऑनA proton or a neutron.Ch 13 · NucleinucleusनाभिकThe tiny, dense, positive centre of an atom, holding all its positive charge and nearly all its mass.Ch 12 · AtomsnucleusनाभिकThe tiny, dense, positively charged centre of an atom, made of protons and neutrons; it holds more than 99.9 % of the atom's mass.Ch 13 · NucleiO
optical fibreप्रकाशिक तंतुA thin glass or plastic thread with a core of higher refractive index than its cladding, guiding light by total internal reflection.Ch 9 · Ray Optics and Optical InstrumentsP
paraxial rayउपाक्षीय किरणA ray close to the principal axis and making a small angle with it.Ch 9 · Ray Optics and Optical Instrumentspath differenceपथांतरThe difference between the distances from two sources to a point; it gives the phase difference ϕ = 2 π Δ/λ.Ch 10 · Wave Opticsphotoelectric effectप्रकाश-विद्युत प्रभावEmission of electrons from a surface when light of high enough frequency falls on it.Ch 11 · Dual Nature of Radiation and Matterphotoelectronप्रकाशिक इलेक्ट्रॉनAn electron set free from a surface by light.Ch 11 · Dual Nature of Radiation and Matterphotonफ़ोटॉनA quantum of electromagnetic radiation, with energy h ν and momentum h/λ, travelling at c.Ch 11 · Dual Nature of Radiation and MatterPlanck's constantप्लांक नियतांकThe constant h = 6.626×10⁻³⁴ J s linking a quantum's energy to its frequency.Ch 11 · Dual Nature of Radiation and Matterpolarisationध्रुवणThe direction in which the vibration of a transverse wave (for light, E) takes place.Ch 10 · Wave OpticspolaroidपोलेरॉइडA plastic sheet of aligned long-chain molecules that passes only the component of E along its pass-axis.Ch 10 · Wave Opticspower of a lensलेंस की क्षमताP = 1/f with f in metres, measured in dioptres (D); positive for converging lenses.Ch 9 · Ray Optics and Optical Instrumentsprincipal focusमुख्य फोकसThe point where rays parallel to the principal axis meet, or seem to diverge from, after reflection or refraction.Ch 9 · Ray Optics and Optical Instrumentsprincipal quantum numberमुख्य क्वांटम संख्याThe whole number n labelling a Bohr orbit; L = nh/2 π and En = -13.6 eV/n².Ch 12 · AtomsQ
Q-valueQ-मानThe energy released in a nuclear process: (∑ m initial - ∑ m final)c², equal to the gain in kinetic energy.Ch 13 · NucleiR
radioactivityरेडियोऐक्टिवताThe spontaneous decay of unstable nuclei with the emission of alpha particles, electrons or positrons, or gamma rays.Ch 13 · NucleirayकिरणA line along which light energy travels; a bundle of rays is a beam.Ch 9 · Ray Optics and Optical Instrumentsreal imageवास्तविक प्रतिबिंबAn image where rays from an object point actually meet; it can be caught on a screen.Ch 9 · Ray Optics and Optical Instrumentsrectifierदिष्टकारीA diode circuit that converts an alternating voltage into one that does not reverse.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsrefractive indexअपवर्तनांकFor a medium, n = c/v; for a pair, n21 = n2/n1 = sin i/sin r.Ch 9 · Ray Optics and Optical Instrumentsreverse biasपश्चदिशिक बायसn-side to the positive terminal: the barrier rises to V0 + V and only a tiny current flows.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple CircuitsS
saturation currentसंतृप्त धाराThe largest photocurrent at a given light setting, reached when every emitted electron reaches the collector.Ch 11 · Dual Nature of Radiation and Mattersecondary waveletद्वितीयक तरंगिकाA small spherical wave spreading from a point on a wavefront, in Huygens' construction.Ch 10 · Wave Opticssemiconductorअर्धचालकA solid whose resistivity lies between that of metals and insulators and falls as it is warmed, such as silicon or germanium.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsspecific chargeविशिष्ट आवेशThe charge of a particle divided by its mass; for the electron e/m = 1.76×10¹¹ C/kg.Ch 11 · Dual Nature of Radiation and Matterstationary stateस्थायी अवस्थाAn allowed state of an atom in which the electron does not radiate and the energy is constant.Ch 12 · Atomsstopping potentialनिरोधी विभवThe retarding potential V0 of the collector at which the photocurrent just becomes zero; eV0 = Kmax.Ch 11 · Dual Nature of Radiation and Mattersuperposition principleअध्यारोपण का सिद्धांतOverlapping waves produce a displacement equal to the vector sum of the displacements each would produce alone.Ch 10 · Wave OpticsT
thermionic emissionतापायनिक उत्सर्जनRelease of electrons from a metal heated strongly enough that some free electrons gain energy above the work function.Ch 11 · Dual Nature of Radiation and Matterthreshold frequencyदेहली आवृत्तिThe lowest frequency ν0 = ϕ0/h that can eject electrons from a given surface, however intense the light.Ch 11 · Dual Nature of Radiation and Mattertotal internal reflectionपूर्ण आंतरिक परावर्तनComplete reflection of light inside a denser medium when it meets a rarer medium beyond the critical angle.Ch 9 · Ray Optics and Optical InstrumentsU
unpolarised lightअध्रुवित प्रकाशLight whose electric field direction changes rapidly and at random, within the plane perpendicular to the ray.Ch 10 · Wave OpticsV
valence bandसंयोजकता बैंडThe band holding the valence electrons; full in a semiconductor at 0 K. Its top is EV.Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuitsvirtual imageआभासी प्रतिबिंबAn image from which rays only appear to diverge; it cannot be caught on a screen.Ch 9 · Ray Optics and Optical InstrumentsW
wavefrontतरंगाग्रA surface on which every point of a wave oscillates in the same phase. Rays are perpendicular to it.Ch 10 · Wave Opticswork functionकार्य फलनThe least energy needed to free an electron from the surface of a metal, ϕ0; it depends on the metal and its surface.Ch 11 · Dual Nature of Radiation and MatterΑ
α-particleऐल्फ़ा कणA helium nucleus (two protons, two neutrons), charge +2e, emitted by some radioactive nuclei.Ch 12 · Atomsncert.ai is an independent learning companion aligned to the NCERT curriculum. It is not affiliated with or endorsed by NCERT.