The photoelectric experiment

The apparatus is an evacuated glass or quartz tube holding a thin photosensitive plate C, the emitter, and a metal plate A, the collector. Monochromatic light from a source S enters through a quartz window W, which lets ultraviolet through, and falls on C.

A battery keeps a potential difference between C and A that can be varied, and a commutator reverses it, so A can be made positive or negative with respect to C. A voltmeter reads the potential difference and a microammeter the photocurrent.

When A is positive, it attracts the emitted electrons, and a current flows round the circuit. Four things can be varied: the intensity of the light, its frequency, the potential difference between A and C, and the material of C.

Coloured filters or coloured glass in the light's path change its frequency. Moving the source nearer to or farther from the emitter changes its intensity.

Effect of intensity: with the frequency and the accelerating potential held fixed, the photocurrent rises in direct proportion to the intensity of the light (a straight line through the origin, Fig. 11.2).

Each electron that crosses the tube adds to the photocurrent, so the current counts photoelectrons per second. Doubling the intensity doubles that count: emission rate is directly proportional to intensity.