Question

State two conditions for sustained interference of light. Draw the variation of intensity with position in the interference pattern of Young's double slit experiment.

Answer 1

We have to understand what interference of light refers to. The phenomenon in which alternate bright and dark fringes are produced due to the superimposition of two waves having the same wavelength and constant phase difference proceeding in the same direction is known as interference of light.

Complete answer:
The conditions for sustained or permanent interference is as follows-
(i) The two sources of light must be coherent which means the two light waves emitted by them must have a constant phase difference or in the same phase.
(ii) The two sources must emit light of the same wavelength but the amplitudes between them should differ as little as possible. The emitted waves should be preferably of the same amplitude to get completely dark fringes.
(iii) The two sources should be very narrow. Otherwise with the increase of slit width, the coherence property will be lost. Hence, no interference pattern will be obtained.
(iv) The two sources must lie very close to each other. Otherwise overlapping of bright and dark points will hinder interference.
(v) At maxima, the path difference between two light waves is always an even multiple of $\dfrac{\lambda }{2}$ and at minima, it is an odd multiple of $\dfrac{\lambda }{2}$.

The variation of intensity with position in the interference pattern of Young’s double slit experiment is-

Note: It must be noted that the intensity of light waves is proportional to the square of the amplitude of the wave. Energy is neither created or destroyed during interference.The energy which is gained during constructive interference is lost during destructive interference and vice-versa.