Dual Nature of Radiation and Matter — Class 12 MCQs with Answers

Dual Nature of Radiation and Matter — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Photoelectric effect Nobel went to:

   AEinsteinBBohrCPlanckDCurie
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   Answer: Einstein

   Explanation: Einstein won the 1921 Nobel Prize specifically for explaining the photoelectric effect using light quanta (photons), not for relativity — a common mix-up.

2. Q2 of 6Easy

   de Broglie wavelength:

   Ah·cBhpCp/hDh/p
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   Answer: h/p

   Explanation: de Broglie's hypothesis gives λ = h/p = h/(mv) — momentum in the denominator, so mixing this up as h×p is the usual slip.

3. Q3 of 6Medium

   Below threshold frequency, photoelectrons:

   AIncrease with intensityBAre emitted with low energyCAre not emitted regardless of intensityDAlways emitted
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   Answer: Are not emitted regardless of intensity

   Explanation: If photon energy hν is below the work function (ν < ν₀), a single photon can never eject an electron — piling on more photons (higher intensity) doesn't help.

4. Q4 of 6Medium

   Assertion (A): To understand how we see an object, both the wave picture and the photon picture of light are needed. Reason (R): The focusing of light by the eye-lens is described by the wave picture, while its absorption by the rods and cones of the retina needs the photon picture.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true: seeing is NCERT's example where both descriptions are important. R is true and is exactly the reason, because one stage of seeing (focusing) is a wave process and the other (absorption in the retina) is a photon process.

5. Q5 of 6Hard

   Intensity of incident light affects:

   AWavelength of photonBTheir max KECThreshold frequencyDNumber of photoelectrons emitted
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   Answer: Number of photoelectrons emitted

   Explanation: Light intensity sets how many photons (and hence electrons) arrive per second, while each electron's energy depends only on frequency — intensity changes count, not KE_max.

6. Q6 of 6Hard

   Assertion (A): An electron and a proton having the same kinetic energy have different de Broglie wavelengths, the electron's being the longer. Reason (R): For the same kinetic energy, the lighter particle has the smaller momentum.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: With p = √(2mK), equal K gives the lighter electron a smaller momentum, so it has the longer wavelength. R is true and is exactly the fact A rests on, because λ = h/p.

Dual Nature of Radiation and Matter notes: key concepts (Class 12 Physics)

The evidence that radiation and matter both behave as wave and particle: the photoelectric effect with its threshold frequency and stopping potential, the ways it defies classical wave theory, Einstein's photon explanation, and de Broglie's matter waves confirmed by the Davisson-Germer experiment.

Electron emission

Electrons can be freed from a metal by heating it, by an applied field, by bombardment, or by light — the last of these being the photoelectric effect.

Work function

The minimum energy needed to just free an electron from a metal surface, a property of the metal. It is typically a few electron volts.

Photoelectric effect

The instant emission of electrons when light of sufficiently high frequency falls on a metal surface.

Threshold frequency

The minimum frequency below which no emission occurs at ALL, however intense the light or however long it shines.

Effect of intensity

Raising the intensity increases the NUMBER of photoelectrons, and hence the current, but not their maximum kinetic energy.

Effect of frequency

Raising the frequency increases the maximum kinetic energy of the emitted electrons, but does not change how many are emitted.

Stopping potential

The reverse voltage that just stops the most energetic photoelectron, giving a direct measure of the maximum kinetic energy.

Failure of wave theory

Classical waves predict emission at any frequency given enough time and energy dependence on intensity. Experiment contradicts both, and emission is instantaneous.

Photon

A quantum of electromagnetic radiation carrying energy hν, with zero rest mass, always travelling at c and interacting with one electron at a time.

Einstein's photoelectric equation

One photon gives all its energy to one electron; part pays the work function and the rest appears as kinetic energy. This won Einstein the Nobel Prize.

Momentum of a photon

Despite having zero rest mass, a photon carries momentum h/λ — which is how radiation exerts pressure.

de Broglie hypothesis

All matter has an associated wavelength h/p. It is significant for electrons but utterly negligible for everyday objects, whose momentum is enormous.

de Broglie wavelength of an electron

An electron accelerated through V volts has a wavelength of about 12.27/√V ångström — comparable to atomic spacings, hence observable diffraction.

Davisson-Germer experiment

Electrons scattered from a nickel crystal produced a diffraction pattern, giving direct experimental confirmation of matter waves.

Wave-particle duality

Radiation and matter each display wave behaviour in some experiments and particle behaviour in others, but never both in the same experiment.