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This is a classic example of interference effects in light waves. Two light rays pass through two slits, separated by a distance d and strike a screen a distance, L
The path difference (P.d.) = dsinθ.
If the rays were in phase when they passed through the slits, then the condition for constructive interference at the screen is:
dsinθ = mλ,
m = ±1, ±2, … …
whereas the condition for destructive interference at the screen is:
dsinθ = (m + ½)λ,
m = ±1, ±2, … …
The points of constructive interference will appear as bright bands on the screen and the points of destructive interference will appear as dark bands. These dark and bright spots are called interference fringes.
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Waves and Optics