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Photo Voltaic Photometer

A photo voltaic cell consists of a copper plate and a layer of cuprous oxide is formed by oxidizing one side of the copper plate.

If the cuprous oxide surface is expressed to light, it emits electrons. The number of photo-electrons emitted depends upon the intensity of the incident radiations. This phenomenon is known as photo-electric effect.

A photo voltaic cell (barrier layer type) can also be used in a photo voltaic photometer. It consists of an iron plate on which there is a layer a selenium. Selenium is coated with a very thin layer of gold or platinum through which light can penetrate to the selenium layer.

1. Comparison of illuminating powers: The source S₁ is placed at a certain distance R₁ from the gold layer of the cell. Photoelectrons are ejected and the current flows in the galvanometer. Let the deflection in the galvanometer be θ.

Replace the source S₁ by the source S₂ and adjust its distance from the cell so that the same deflection is produced in the galvanometer as in the first case. If the distance of the source S₂ from the cell is R₂.

If L₁ is known, L₂ can be calculated.

2. Verification of inverse square law: A Source is placed at different distances from the photo-voltaic cell and the corresponding deflections in the galvanometer are noted. The deflection in the galvanometer is directly proportional to the intensity of the incident radiations.

I ∝ θ

If R is the distance of the source from the cell

∴ The value R² θ will be constant.

If a graph is plotted between θ and 1/R² it will be a straight line.

This verifies inverse square law.