Radio waves coming at angle $α$ to vertical are received by a ladder after reflection from a nearby water surface directly. What can be height of antenna from water surface so that it records a maximum intensity (a maxima) (wavelength = $λ$ ).

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$\frac{λ}{4 \sin α}$

$\frac{λ}{4 \cos α}$

$\frac{λ}{2 \cos α}$

$\frac{λ}{2 \sin α}$

answer is D.

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Detailed Solution

Total path difference = AB + BC + $λ / 2 = λ$ for maxima
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$h \sec α \cos 2 α + h \sec α = λ / 2 h \sec α (2 \cos^{2} α) = λ / 2 h = \frac{λ}{4 \cos α}$

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