Interference of Light: The phenomenon of redistribution of energy in a medium due to superimposition of waves from two coherent sources of light is called Interference of Light.
Constructive Interference: At points, where the crest of one wave falls on the crest of the other or a trough of one falls on the trough of the other, the amplitude of the resulting wave becomes maximum. Hence the energy or the intensity of light at such points becomes maximum. This is called Constructive Interference.
Destructive Interference: At some other points where the trough of one falls on the crest of the other or crest of one falls on the trough of the other, the amplitude of the resulting waves becomes minimum. Hence the energy or intensity becomes minimum. This is called Destructive Interference.
Law of conservation of energy is obeyed: It should be clearly understood that in interference of light no light energy is destroyed. The loss of energy at the points of destructive interference appears as the increase of energy at the points of constructive interference.
Conditions for sustained interference: To produce sustained (or stationary) interference following conditions should be fulfilled:
1. Two sources must be coherent, so the sources emit continuous waves of th same wavelength (or frequency) which are either in the same phase or a constant phase difference.
2. The waves should be preferably of the same amplitude to get complete darkness in case of destructive interference.
3. Two sources must be very close to each other, if it is not so, the path difference at particular point of observation will be large and the maximas will be very close to each other and may overlap.
(∵ ß ∝ \(\frac{1}{\text{distance between the sources}}\))
4. Two sources must be very narrow, a broad source of light is equivalent to a large number of narrow sources and each set of two sources will give its own interference pattern and their overlap will result in general illumination.
5. The distance between two sources and the screen should be large, so that the dark and bright fringes are of large width [∵ ß ∝ D (distance between two sources and screen)].
Graphs
(a) When both the slits are opened See Fig.
(b) When one of the slits is closed
The graph is shown in Fig. 11 for S1 or S2.
Since fringe width, ß = \(\frac{\lambda D}{d}\)
So (i) when screen is moved closer to the plane of slits, D decreases and hence ß will also decrease.
(ii) When separation between the slits is increased, d increases and hence ß will decrease.
