First slide

Quantum Theory of Light

Question

Monochromatic light of frequency 6.0 x $10^{14}$ Hz is produced by a laser. The power emitted is 2 x $10^{- 3}$ W. The number of photons emitted. on the average, by the source per second is

Easy

A

$5 \times 10^{15}$
B

$5 \times 10^{16}$
C

$5 \times 10^{17}$
D

$5 \times 10^{14}$

Solution

Photons are the packets of energy. power emitted,
$\begin{array}{l} P = 2 \times 10^{- 3} W \\ Energy of photon, E = hv = 6 .6 \times 10^{- 34} \times 6 \times 10^{14} J \\ h being Plank' s constant . \\ Number of photons emitted per second \\ n = \frac{P}{E} = \frac{2 \times 10^{- 3}}{6 .6 \times 10^{- 34} \times 6 \times 10^{14}} = 5 \times 10^{15} \end{array}$

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