Two towers on hills are 40 km apart. The straight line joining them passes 50 m above a hill halfway between them. Find the longest radio wavelength that can pass without appreciable diffraction.
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Question

Two towers on hills are 40 km apart. The straight line joining them passes 50 m above a hill halfway between them. Find the longest radio wavelength that can pass without appreciable diffraction.

G1 Physics · Accepted Answer

The limiting wavelength is obtained from the Fresnel-zone clearance condition for the hill midway between the two towers. Using the source criterion with total separation 40 km and midpoint clearance 50 m gives λ≈0.125 m=12.5 cm. Option C.