What is interference of light? Write two essential condition for sustained interference pattern to be produced on the screen.
Draw a graph showing the variation of intensity versus the position on the screen in Young's experiment when (a) both the slits are opened and (b) one of the slit is closed.
What is the effect on the interference pattern in Young's double-slit experiment when:
(i) screen is moved closer to the plane of slits?
(ii) separation between two slits is increased. Explain your answer in each case.

Solution

Interference of light:
The phenomenon of redistribution of light energy in a medium on account of superposition of light waves from two coherent sources is called interference of light.
Conditions for sustained interference:
The two essential conditions of sustained interference are as follows:
(i) The two sources of light should emit light continuously.
(ii) The light waves should be of same wavelength. (Monochromatic).

When both the slits are open, we get interference pattern on the screen. Then the following intensity distribution curve is obtained.

When one of the slits is closed, diffraction pattern is obtained on the screen. The following intensity curve is obtained.

Fringe width, $β = \frac{Dλ}{d}$
where,
D is the distance between the screen and the slit and,
d is the distance between the slits.

(i) When screen is moved closer to the plane of slits, D decreases and hence, fringe with of the interference pattern also decreases.

(ii) Fringe width β decreases if, separation d between two slits is increased.

Some More Questions From Wave Optics Chapter

Let us list some of the factors which could possibly influence the speed of wave propagation:
(i) nature of source.
(ii) direction of propagation.
(iii) motion of source and/or observer.
(iv) wave length.
(v) intensity of the wave.
On which of these factors, if any, does
(a) the speed of light in vacuum,
(b) the speed of light in a medium (say, the glass or water) depend?

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