JEE Main · 2004 · 24 Apr 2004 · Offline · Modern Physics

A radiation of energy E falls normally on a perfectly reflecting surface. The momentum transferred to the surface is

(A)E/c
(B) $2 E / c$ ✓ Correct
(C)Ec
(D) $E / c^{2}$

Answer: (B)

Solution

A photon of energy

E

carries momentum

\frac{E}{c}

. On striking a perfect reflector it bounces straight back, reversing the sign of its momentum, so its own momentum changes by

\frac{2 E}{c}

. By Newton's third law the surface takes up exactly this amount, receiving momentum

\frac{2 E}{c}

JEE Main · 2004 · 24 Apr 2004 · Offline · Single correct option

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