 QUESTION

# What is the photoelectric effect? State the laws of photoelectric emission.

Hint: Photo electrons are responsible for photoelectric effect because we know that, above a certain frequency of the incident light, photo-electrons emit. There is also a minimum frequency or cut off frequency below which no photo-electrons are produced.

Photoelectric effect:

When the radiation such as γ-rays, X-rays, UV and even visible light fall on metals, electrons are emitted. This phenomenon of emission of electrons is known as photoelectric effect.

Laws of Photoelectric Emission: The experimental observations on photoelectric effect may be summarized as follows, which are known as the fundamental laws of photoelectric emission.

(i) For a given photo sensitive material, there is a minimum frequency called the threshold frequency, below which emission of photoelectrons stops completely, however great the intensity may be.

(ii) For a given photosensitive material, the photoelectric current is directly proportional to the intensity of the incident radiation, provided the frequency is greater than the threshold frequency.

(iii) The photoelectric emission is an instantaneous process i.e., there is no time lag, between the incidence of radiation and the emission of photoelectrons.

(iv) The maximum kinetic energy of the photoelectrons is directly proportional to the frequency of incident radiation, but is independent of its intensity.

Note: Whenever we come up with this type of problem then we should also know that the photons energy or energy consisting in a photon of a particular light is proportional to the frequency of that light and hence that is inversely proportional to the wavelength of the light wave.
E = h.f = $\dfrac{{{\text{hc}}}}{\lambda } \Rightarrow {\text{ E}} \propto {\text{ }}\dfrac{1}{\lambda }$