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Photoelectric effect… (Class 12 Physics SQP 2025-26, Q30)

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PhysicsCBSE Class 12 · Sample paper, session 2025-26 · 4 marks · Case-based

CBSE Class 12 Physics sample paper 2025-26: question 30

From the official CBSE sample question paper for 2025-26, 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

Photoelectric effect is phenomenon of the ejection of electrons when the radiation of suitable frequency is made to fall on the surface of a metal. When light of suitable wavelength falls on the emitter C given in the diagram, the photoelectrons are emitted. These photoelectrons are drawn to the collector A. The photoelectric current of the order of a few microamperes can be normally obtained from the device given in figure. The device given converts a change in intensity of illumination into a change in photocurrent. This current can be used to operate control systems and in light measuring devices. The devices are made up of metals with low ionization enthalpies, for example platinum whose work function is 6.35 eV.

  1. (I)

    If infrared radiation of Hz is used as incident radiation, determine the reading of microammeter? Justify mathematically.

    2 marks
  2. (II)

    In the given diagram, if terminal B is shifted towards the left then how will it affect the reading of the microammeter?

    1 mark

  3. OR

    (II) (V.I.)

    If the supplied voltage is decreased, then what will be effect on the reading of the microammeter?

    1 mark
  4. (III)

    Plot a graph showing this variation in reading of micrometre on shifting the terminal B towards the right.

    1 mark

  5. OR

    (III) (V.I.)

    If the intensity of incident radiation is doubled, by what factor will the kinetic energy change?

    1 mark

Figure in the paper: Photocell: evacuated tube with emitter C and collector A, incident light falling on C; external circuit has a battery with sliding terminal B and a microammeter (μA). See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Compare the incident frequency with the threshold frequency .

Hint 2

Moving the sliding contact changes the potential of the collector relative to the emitter.

Hint 3

Maximum KE depends on frequency, while intensity controls the number of electrons.

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) Threshold frequency Hz.

    1 mark

  2. Incident frequency Hz , so no photoelectrons are emitted, whatever the intensity. The microammeter reading is zero.

    1 mark

  3. (II) Shifting B to the left lowers the potential of collector A (accelerating potential decreases and finally becomes retarding). The photocurrent decreases and becomes zero when the retarding potential equals the stopping potential. (Assuming light of frequency above threshold.)

    1 mark

  4. (II) (V.I.) On decreasing the supplied (accelerating) voltage below the saturation value, the photocurrent decreases; with a retarding voltage it falls to zero at the stopping potential.

  5. (III) Shifting B to the right increases the accelerating potential of A. Graph of photocurrent (y-axis) vs collector potential (x-axis): current starts from zero at the stopping potential , rises with potential and then becomes constant (saturation current) — a curve flattening into a horizontal line.

    1 mark

  6. (III) (V.I.) Kinetic energy of photoelectrons depends only on frequency (), not on intensity; so it does not change (factor 1). Only the photocurrent doubles.

Answer: (I) ν₀ ≈ 1.53×10¹⁵ Hz > 3×10¹¹ Hz, reading = 0; (II) current decreases, becomes zero at stopping potential; (III) current rises then saturates; (V.I.) KE unchanged

Where the marks are

  • Calculates threshold frequency ≈ 1.5×10¹⁵ Hz (1 mark)
  • Concludes no emission, reading zero (1 mark)
  • (II) current decreases to zero as collector potential is reduced (1 mark)
  • (III) graph showing current rising and saturating / (V.I.) KE unchanged (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 2025-26: 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.