First slide

Electromagnetic radiation - Dual nature of radiation and matter

Question

The number of photons $(λ = 6630 Å)$ that strike per second on a totally reflecting screen (as shown in figure), so that a force of 1N is exerted on the screen, is approximately

Moderate

A

10²³
B

10²⁷
C

10²⁵
D

10²⁶

Solution

$\begin{array}{l} F = \frac{d p}{d t} \\ \Rightarrow F = \frac{N (2 p \sin θ)}{Δ t} \\ \Rightarrow F = 2 n p \sin θ [n = number of photon per sec] \\ \Rightarrow 1 N = 2 (n) \frac{h}{λ} \sin 30^{\circ} \\ \Rightarrow n = \frac{6630 \times 10^{- 10} \times 2}{2 \times 6.63 \times 10^{- 34}} = 10^{27} p h o t o n p e r s e c \end{array}$

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