Read the following passage and answer the questions.
The figure shows the set-up a student uses to study the photoelectric effect. An evacuated glass tube holds a photosensitive plate C (the emitter) and a metal plate A (the collector). Light from a source S enters through a quartz window and falls on plate C. A battery keeps a potential difference between C and A, and she can change its value. A commutator reverses the polarity, so plate A can be made positive or negative with respect to C. A voltmeter reads this potential difference, and a microammeter reads the photocurrent.
She chooses the frequency of the light by placing coloured filters in its path. She changes the intensity by moving the source nearer to or farther from plate C. For each setting, she records the microammeter reading as the potential of A is changed from a large positive value to negative values.
Consider the following statements about the photoelectrons in this experiment:
(A) The electrons that leave plate C do not all have the same kinetic energy.
(B) When the light is made very dim, emission starts only after a noticeable time lag.
(C) The work function of plate C becomes smaller if light falls on it for a longer time.
(D) The photocurrent is directly proportional to the number of photoelectrons emitted per second.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (D) only
Light above the threshold frequency gives out photoelectrons from plate C at once. The current they carry is then read on the microammeter.
(A) The electrons that leave plate C do not all have the same kinetic energy. — True. The kinetic energies range from zero up to a maximum value. Electrons that are held more tightly in the metal come out with less than the maximum kinetic energy.
(B) When the light is made very dim, emission starts only after a noticeable time lag. — False. Above the threshold frequency, emission starts in about \( 10^{-9}\ \mathrm{s} \) or less, even in very dim light. Dim light only gives fewer electrons.
(C) The work function of plate C becomes smaller if light falls on it for a longer time. — False. The threshold frequency \( \nu_0=\phi_0/h \) does not change with the time for which light falls on the plate. So the work function stays the same.
(D) The photocurrent is directly proportional to the number of photoelectrons emitted per second. — True. Each electron that reaches A adds its charge to the current. More electrons per second give a larger photocurrent in the same proportion.
NCERT (2026-27):
Effect of potential on photoelectric current – “… All the photoelectrons emitted from the metal do not have the same energy. …”
Effect of frequency of incident radiation on stopping potential – “(iv) The photoelectric emission is an instantaneous process without any apparent time lag (~10–9s or less), even when the incident radiation is made exceedingly dim.”
Einstein’s Photoelectric Equation: Energy Quantum of Radiation – “… Thus, there exists a threshold frequency ν0 (= φ0/h) for the metal surface, below which no photoelectric emission is possible, no matter how intense the incident radiation may be or how long it falls on the surface.”
Effect of intensity of light on photocurrent – “… The photocurrent is directly proportional to the number of photoelectrons emitted per second. …”