In a YDSE, screen distance is D and slit separation is d. Find the nearest distance from the central maximum where the resultant intensity equals the intensity due to one slit alone.
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Question

In a YDSE, screen distance is D and slit separation is d. Find the nearest distance from the central maximum where the resultant intensity equals the intensity due to one slit alone.

G1 Physics · Accepted Answer

Let the intensity from either slit alone be Iₛ. Then I=4Iₛ cos²(φ/2). Setting I=Iₛ gives cos²(φ/2)=1/4. The nearest solution from the central maximum is φ=2π/3, so δ=λ/3. Using δ≈dy/D gives y=Dλ/(3d). Option C.