First slide

Electromagnetic radiation - Dual nature of radiation and matter

Question

How many photons are emitted per second by a 5 mW laser source operating at 632.8 nm?

Easy

A

$1.6 \times 10^{16}$
B

$1.6 \times 10^{13}$
C

$1.6 \times 10^{10}$
D

$1.6 \times 10^{3}$

Solution

$\begin{array}{l} E = \frac{h c}{λ} = \frac{6.6 \times 10^{- 34} \times 3 \times 10^{8}}{632.8 \times 10^{- 9}} J \\ Number of photons per second, n = \frac{P}{E} \\ \Rightarrow n = \frac{5 \times 10^{- 3}}{3.14 \times 10^{- 19}} = 1.6 \times 10^{16} \end{array}$

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