Sustained Interference of Light:
The necessary conditions for observing sustained interference of light are-
(1) The sources must be coherent i.e., the initial phase difference between the interfering waves must remain constant with time so that a sustained interference pattern is formed.
(2) The beam must be propagating along nearly the same direction. This is necessary to make them interfere.
(3) If the beams are polarized, their planes of polarization must be same. This means that the electric vector in the interfering beams must be parallel.
If any of these above conditions is not satisfied, the interference cannot be observed at all. Further to observe a good interference pattern, it is desirable to have following as a matter of convenience.
(4) Light from interfering sources must be of nearly equal intensities. This improves contrast, with Imax = 4Io and Imin = 0 for light of equal intensities.
(5) The common original or parent source must be monochromatic i.e., emitting light of a single wavelength.
(6) The sources must be close to each other, otherwise the fringe-width may be too small to observe the fringes distinctly in interference pattern.
(7) The original source must be narrow. A broad source will be equivalent to many narrow sources placed together. Each source will give its own interference pattern and overlapping of patterns will result in uniform illumination.