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Write the necessary conditions to obtain sustained interference fringes.

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#### Solution

The necessary conditions to obtain sustained interference fringes are:

(i) The two sources of light must be coherent. In other words, they should emit continuous light waves of same wavelength or frequency, having either the same phase or a constant phase difference.

(ii) The two sources must be very narrow as a broad source is equivalent to a large number of narrow sources lying side by side, which causes general illumination rather than interference pattern.

(iii) The two sources should preferably be monochromatic.

(iv) The coherent sources must be very close to each other.

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