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Answer the following giving… (Class 12 Physics 2025, Q26)

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PhysicsCBSE Class 12 · Board exam 2025, set 55/1/1 · 3 marks · Short answer

CBSE Class 12 Physics 2025: question 26

Set in the CBSE Class 12 board exam of 2025, paper 55/1/1, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerDual Nature of Radiation and Matter

Answer the following giving reason :

  1. (a)

    All the photo electrons do not eject with the same kinetic energy when monochromatic light is incident on a metal surface.

    0 marks
  2. (b)

    The saturation current in case (a) is different for different intensity.

    0 marks
  3. (c)

    If one goes on increasing the wavelength of light incident on a metal surface, keeping its intensity constant, emission of photo electrons stop at a certain wavelength for this metal.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 55/1/1.

Hints

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

Hint 1

Think about where in the metal the electrons come from and how much energy each loses before escaping.

Hint 2

Relate saturation current to the number of photons per second. For (c), relate λ to photon energy and compare it with the work function.

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. (a) Each photon has the same energy , but electrons are bound in the metal with different energies. Electrons at the surface need only the work function to escape, while deeper or more tightly bound electrons lose more energy. So the emitted electrons have kinetic energies from zero up to a maximum .

    1 mark

  2. (b) Saturation current means all emitted electrons are collected, so it measures the number of photoelectrons emitted per second. This number is directly proportional to the number of photons falling per second, i.e. to the intensity. Different intensities therefore give different saturation currents.

    1 mark

  3. (c) As λ increases, the frequency and the photon energy decrease. When λ exceeds the threshold wavelength (i.e. ), is less than the work function. No electron can then escape, whatever the intensity, so emission stops.

    1 mark

Answer: (a) electrons have different binding energies, so KE ranges from 0 to K_max; (b) saturation current ∝ photoelectrons per second ∝ intensity; (c) beyond the threshold wavelength λ₀, hν < work function, so no emission.

Where the marks are

  • Reason based on different binding energies of electrons (1 mark)
  • Saturation current ∝ number of photoelectrons per second ∝ intensity (1 mark)
  • For λ > λ₀, photon energy is below the work function, so emission stops (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 paper2025 board paper, set 55/1/1: 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.