What is photoelectric effect?Describe with neat circuit diagram an experimental setup of photoelectric effect.

Answer:

Photoelectric effect: The emission of electrons from a metal surface when electromagnetic radiation of suitable (sufficiently high) frequency falls on it is called the photoelectric effect. The emitted electrons are called photoelectrons.

Experimental set-up:

[Figure: an evacuated glass tube with a quartz window. Inside are a photosensitive plate C (emitter/cathode) and a metal plate A (collector/anode). Monochromatic light from a source passes through the quartz window onto C. A and C are connected to a battery through a potential divider (rheostat) and a commutator (reversing key), so that the potential of A relative to C can be varied and reversed. A voltmeter V is connected across A and C, and a microammeter (μA) is in series in the circuit.]

• The tube is evacuated so that the photoelectrons do not collide with gas molecules.
• The quartz window transmits ultraviolet as well as visible light.
• When suitable light falls on C, photoelectrons are emitted. If A is positive with respect to C, they are attracted to A and a current (photocurrent) flows, read on the microammeter.
• The potential divider changes the accelerating potential; the commutator reverses it so that A can be made negative (retarding potential).

Observations made with this set-up: the photocurrent increases with intensity of light; as the accelerating potential is increased the current reaches a saturation value; when A is made negative the current decreases and becomes zero at the stopping potential \(V_0\), which depends on the frequency but not on the intensity; no electrons are emitted below a threshold frequency \(\nu_0\) however intense the light; and emission is instantaneous.