What are the necessary conditions of interference of light waves?

Answer 1

They must arrive 180 degrees out of phase. (Waves must be of the same frequency.) In a situation where two identical waves (amplitude and frequency) arrive 180 degrees out of phase, the crests meet the troughs and the troughs meet the crests. The waves effectively "cancel each other out" here, or, as a mechanic might say, the vector sums of the waves total zero. If the waves are not equal in amplitude (but are in frequency), they will at least sum to a minimum energy. Put two speakers 10 to 20 feet apart and point them toward each other. Hook everything up "normally" and walk between the speakers. Then reverse the speakers wires to one speaker. (No, it won't hurt the speakers or the audio system.) Walk between them again. Biiiiiiig difference. Note that this experiment is a 3-dimentional test. Study a bit on the 2-dimentional problem before moving to 3-D. (It's just that this experiment is fun! It makes the phenomenon so real to the observer.) This a problem in what is sometimes called two-point source interference. Use the link to visit a site where drawings are posted. A picture is probably worth a thousand words in this case.

Answer 2

1. The two sources of light should emit continuous waves of same wavelength and same time period i.e. the source should have phase coherence.
2. The two sources of light should be very close to each other.
3. The waves emitted by two sources should either have zero phase difference or no phase difference.

Answer 3

the condition is "the width of obstacle or aperture must be less than or comparable with wavelength of light used."