Two coherent sources of equal intensity I illuminate a point P. The resultant intensity at P is 2I. What is the least positive path difference between the waves reaching P?
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Question

Two coherent sources of equal intensity I illuminate a point P. The resultant intensity at P is 2I. What is the least positive path difference between the waves reaching P?

G1 Physics · Accepted Answer

For equal-intensity coherent waves, I(resultant) = 4I cos²(φ ÷ 2). Given, 2I = 4I cos²(φ ÷ 2). Therefore, cos²(φ ÷ 2) = 1 ÷ 2. The least positive phase difference is φ = π ÷ 2. Using φ = 2πΔ ÷ λ, we obtain Δ = λ ÷ 4. Correct option: B.