First slide

Quantum Theory of Light

Question

Monochromatic light of wavelength 667 nm is produced by a helium neon laser. The power emitted is 9 rnW. The number of photons arriving per second on the average at a target irradiated by this beam is

Moderate

A

$9 \times 10^{17}$
B

$3 \times 10^{16}$
C

$9 \times 10^{15}$
D

$3 \times 10^{19}$

Solution

$\begin{array}{l} Here λ = 667 \times 10^{- 9} m, P = 9 \times 10^{- 3} W \\ Power = \frac{energy}{time} = \frac{nhc}{λt} = \frac{Nhc}{λ} \\ Where N is number of photons emitted per \sec . \\ \Rightarrow N = \frac{P \times λ}{hc} = \frac{9 \times 10^{- 3} \times 667 \times 10^{- 9}}{6 .6 \times 10^{- 34} \times 3 \times 10^{8}} = 3 \times 10^{16} s \end{array}$

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