What is a photoelectric effect? Write any three laws of the photoelectric effect. Explain these laws by Einstein's equation of the photoelectric effect.

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The photoelectric effect is the phenomenon of the emission of electrons from the metal surface exposed to light energy of suitable frequency.

The three laws of the photoelectric effect are:

For a given photosensitive material, there is a minimum frequency of incident light called threshold frequency(ɳ₀) below which no photoelectric emission take place, whatever may be the intensity of incident light.
The number of photoelectrons emitted per second i.e. photoelectric current is directly proportional to the intensity of incident light, provided that the frequency of incident light is greater than the threshold frequency.
The process of photoelectric emission is instantaneous. The time lag between the emission of photoelectrons and the incidence of radiation is nearly equal to 10^-9 s.

Explanation of the above three laws by Einstein’s equation of photoelectric effect

According to Einstein’s equation of the photoelectric effect:

E_i= Φ_o + KE

Where,

Therefore, Einstein’s equation of the photoelectric effect can also be written as,

hɳ = hɳ_o + KE

Where,

From Einstein’s equation of the photoelectric effect, we conclude that:

If incident frequency ɳ < threshold frequency (ɳ_o), then the kinetic energy of the photoelectron is negative. Hence photoelectric emission is not possible, if the frequency of incident light is less than the threshold frequency.
One photon can emit only one electron from the metal surface, so the number of photoelectrons emitted per second is directly proportional to the intensity of incident light which depends on the number of photons present in the incident light.
Photoelectric emission is due to elastic collision between a photon and an electron in the material. As such there cannot be any significant time lag between the incidence of photon and the emission of photoelectron, the process of photoelectric emission is instantaneous.

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