Photo-emissive cell

Photo-conductive cell

Photo-voltaic cell

It consists of an evacuated glass or quartz bulb containing anode A and cathode C. The cathode is semi-cylindrical solid on which a thin layer of photo-sensitive material is coated.

It is based on the principle that conductivity of a semiconductor increases with increase in the intensity of incident light.

It consists of a Cu plate coated with a thin layer of cuprous oxide (Cu₂O). On this plate is laid a semi transparent thin film of silver.

When light incident on the cathode, it emits photo-electrons which are attracted by the anode. The photo-electrons constitute a small current which flows through the external circuit.

In this, a thin layer of some semiconductor (as selenium) is placed below a transparent foil of some metal. This combination is fixed over an iron plate. When wave is incident on the transparent foil, the electrical resistance of the semiconductor layer is replaced. Hence a light starts flowing in the battery circuit connected.

When light fall, the electrons emitted from the layer of Cu₂O and move towards the silver layer. Then the silver layer converts into negatively charged and copper plate becomes positively charged. A potential difference is set up between these two and current is set up in the external resistance.

Compton effect: The scattering of a photon by an electron is called Compton effect. The momentum and energy is conserved. Scattered photon will have less power as compare to incident photon (less wavelength). The power lost by the photon is taken by electron as kinetic energy.

The change in wavelength due to Compton effect is called Compton shift. Compton shift