Explanation

(a) When ultraviolet rays fall on zinc plate, electrons are liberated from the zinc plate. This phenomenon is called photoelectric emission. In general the emission of electrons from matter as a result of electromagnetic wave falling on the matter is referred to as photoelectric effect.Emission of electrons depends only on the threshold frequency. But the rate of emission depends on the intensity of light. The threshold frequency is not the same for all metals. The energy of emitted electrons varies from zero to a maximum value.

(b) photo cell

When light photons fall on the caesium metal, it emits electrons which move to the collector to generate current flow. As the intensity of Iight varies, the rate of emission of electrons varies and thus, the current flow varies. 
(c) Applications of photoelectric emission.

(1) Production of television cameras (2) Burglary/ alarms (3) Automatic switches for putting on lights at dusk. (4) Industrial controls and counting operations.

(d) The energy of the light is absorbed by the photocell and are used to overcome the force that brids the electrons but may not be sufficient to remove the electron until light with sufficient threshold frequency. One factor that may affect the number of emitted electrons is the intensity of the light. 

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