Answer: Photoelectric effect is the phenomenon of emission of electrons from a metal surface when radiation of suitable (high enough) frequency is incident on it.
Experimental set-up: [Diagram: an evacuated glass tube with a quartz window; a photosensitive plate C (emitter/cathode) and a collector plate A (anode) inside; monochromatic light falling on C through the window; A and C connected to a battery through a potential divider (rheostat) and a reversing key; a voltmeter V across the tube and a microammeter µA in series.]
- An evacuated glass or quartz tube contains two electrodes: a photosensitive metal plate C (emitter) and a metal plate A (collector).
- Monochromatic light of known frequency and intensity falls on C through a quartz window (quartz lets ultraviolet light through).
- A variable potential difference is applied between A and C using a battery and potential divider; a commutator (reversing key) allows A to be made positive or negative with respect to C. The voltmeter measures this potential.
- Electrons emitted from C (photoelectrons) are collected by A and the resulting photoelectric current is measured by the microammeter.
- By varying the frequency and intensity of light and the potential of A, one studies the dependence of photocurrent on intensity, the stopping potential (the negative potential of A at which the current becomes zero) and its dependence on frequency.
