The experimental set-up used for the photoelectric effect is shown. It consists of an evacuated glass/quartz tube having a thin photosensitive plate C and another metal plate A. Light from the source S of sufficiently short wavelength passes through the quartz window and falls on the photosensitive plate C (emitter). The electrons emitted by the plate C are collected by the plate A (collector) by the electric field created by the battery. The battery maintains the potential difference between the plates C and A, which can be varied. Thus, the plate A can be maintained at a desired positive or negative potential with respect to the emitter C. When the collector plate A is positive with respect to the emitter plate C, the electrons are attracted to it. The emission of electrons causes a flow of electric current in the circuit, called the photoelectric current.
The light from the source S that falls on the plate C in this set-up should be
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → monochromatic (of a single frequency)
Each observation in this set-up is taken for a particular frequency of the incident radiation. This holds for the photocurrent, the saturation current and the stopping potential. The frequency is then changed to study its effect. So the light falling on C must have one known frequency at a time.
Option (1) of any colour — Incorrect. Light of a colour whose frequency is below the threshold frequency of the plate ejects no electrons at all. So the light must be of sufficiently short wavelength.
Option (2) monochromatic (of a single frequency) — Correct. With monochromatic light every photon has the same energy \( h\nu \). So a definite stopping potential \( V_0 \) can be measured for that frequency. The frequency can then be varied in a controlled way.
Option (3) white light — Incorrect. White light is a mixture of many frequencies. Photoelectrons ejected by the different frequencies have different maximum kinetic energies, so no single stopping potential belongs to a given frequency.
Option (4) infrared — Incorrect. Infrared radiation has a long wavelength and low photon energy, below the work function of the photosensitive plate. The set-up needs light of sufficiently short wavelength. The quartz window is there to let ultraviolet light through.
NCERT (2026-27):
Experimental Study of Photoelectric Effect – “… Monochromatic light from the source S of sufficiently short wavelength passes through the window W and falls on the photosensitive plate C (emitter). A transparent quartz window is sealed on to the glass tube, which permits ultraviolet radiation to pass through it and irradiate the photosensitive plate C.”