Sustained interference: The interference in which the position of maxima and minima of light remains fixed all along the screen is called sustained 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 the 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)].
