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Intensity Explanation in YDSE

Why don't we ...

Question

Why don't we have interference when two candles are placed close to each other and the intensity is seen at a distant screen? What happens if the candles are replaced by laser sources?

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Solution

In case of candles we can't say that the sources are coherrent.
i.e. in same phase, their angular frequencies $w$ can be different which will make a pattern dependent on time and our eyes can't see the pattern formed clearly . The avg intensity becomes $2 I$ because $cos θ = 0$ .
And if candles are replaced by light sources, interference happen due to coherrent sources.

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are placed in front of a screen. [Fig.(a)] The pattern of intensity distribution seen in the central portion is given by Fig(b).
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