Two coherent monochromatic light sources produce equal intensities I individually at a point P. The phase difference between the two waves reaching P is 120°. What is the resultant intensity at P?
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Question

Two coherent monochromatic light sources produce equal intensities I individually at a point P. The phase difference between the two waves reaching P is 120°. What is the resultant intensity at P?

G1 Physics · Accepted Answer

For two coherent waves, I(resultant) = I₁+I₂+2√(I₁I₂) cosφ. Here, I₁ = I₂ = I, φ = 120°. Therefore, I(resultant) = I+I+2I cos120° = 2I+2I(−1 ÷ 2) = I. Correct option: C.