Radio waves coming at ∠ α to vertical are received by radar after reflection from a nearby water surface and directly. The height of the antenna from the water surface so that it records a maximum intensity, should be (wavelength = λ )

Step-by-step solution

Path difference = BN + BA + λ 2 (because reflection occurs from denser medium) In Δ ACB h AB = cos α AB = h cos α ....(1) from right angle Δ ANB BN AB = cos 2 α BN = AB cos 2 α ....(2) Path difference = AB + AB cos 2 α + λ 2 λ = AB 1 + cos 2 α + λ 2 λ 2 = h cos α × 2 cos 2 α λ 2 = 2 h cos α h = λ 4 cos α