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In 1905, Albert… (Class 12 Physics SQP 2026-27, Q30)

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PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 4 marks · Case-based

CBSE Class 12 Physics sample paper 2026-27: question 30

From the official CBSE sample question paper for 2026-27, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedDual Nature of Radiation and Matter

In 1905, Albert Einstein explained the photoelectric effect by proposing that light consists of discrete packets of energy called photons. Each photon carries an energy . When a photon strikes a metal surface, it can transfer its energy to an electron. If the photon’s energy exceeds the work function of the metal, the electron can be emitted with maximum kinetic energy . This discovery challenged the classical wave theory, which predicted that electron emission should depend only on the intensity of light. Instead, experiments showed that emission of electrons depends on frequency too, proving the particle nature of light.

  1. I.

    Define the term work function.

    1 mark
  2. II.

    Why the increase in the intensity of light does not increase the maximum kinetic energy of photoelectrons?

    1 mark
  3. III.

    A metal surface has a work function of 2 eV. The light of frequency Hz falls on it. Which statement is correct?

    1. (A)No electron is emitted because the intensity is low.
    2. (B)Electrons are emitted with maximum kinetic energy less than 2 eV.
    3. (C)Electrons are emitted with maximum kinetic energy greater than 0 eV.
    4. (D)Electron emission depends only on the intensity of light and not on the frequency.
    1 mark
  4. IV.

    Which observation/s directly contradicts the classical wave theory of light?

    1. (A)Photoelectric current increases with intensity
    2. (B)The maximum kinetic energy depends only on the frequency and not on the intensity.
    3. (C)The electrons are emitted instantaneously.
    4. (D)Both B and C.
    1 mark

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

Open one hint at a time and try again before the next.

Hint 1

Use E = hν with h = 4.14 × 10⁻¹⁵ eV s.

Hint 2

Ask yourself: does brighter light mean more photons or more energetic photons?

Hint 3

Classical waves would spread energy over time — would emission then be instantaneous?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (I) Work function is the minimum energy that must be given to an electron to just remove it from the surface of a metal.

    1 mark

  2. (II) Intensity only increases the number of photons per second, not the energy of each photon. Since one photon is absorbed by one electron, depends only on frequency; greater intensity only raises the photoelectric current.

    1 mark

  3. (III) eV; eV . Option (C).

    1 mark

  4. (IV) Wave theory predicts should rise with intensity and that emission should need a time lag; observations B (K_max depends only on frequency) and C (instantaneous emission) contradict it. Option (D).

    1 mark

Answer: I. Minimum energy to free an electron from the metal surface; II. photon energy depends only on ν; III. (C), K_max ≈ 3 eV; IV. (D)

Where the marks are

  • Correct definition of work function (1 mark)
  • Intensity changes number of photons, not energy per photon (1 mark)
  • III: hν ≈ 5 eV, Kmax ≈ 3 eV, option C (1 mark)
  • IV: option D (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Dual Nature of Radiation and Matter, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.