Monochromatic light of frequency $6 \times 10^{14} \mathrm{~Hz}$ is produced by a laser. The power emitted is $2 \times 10^{-3} \mathrm{~W}$. How many photons per second on an average, are emitted by the source ? (Given $\mathrm{h}=6.63 \times 10^{-34} \mathrm{Js}$ )

JEE Main · 2024 · 01 Feb 2024 · Shift 2 · Modern Physics

Monochromatic light of frequency

6 \times 1 0^{14} Hz

is produced by a laser. The power emitted is

2 \times 1 0^{- 3} W

. How many photons per second on an average, are emitted by the source ?
(Given

h = 6.63 \times 1 0^{- 34} Js

)

(A) $9 \times 1 0^{18}$
(B) $6 \times 1 0^{15}$
(C) $5 \times 1 0^{15}$ ✓ Correct
(D) $7 \times 1 0^{16}$

Answer: (C)

Solution

Power radiated equals the number of photons emitted per second times the energy of each photon:

P = nh ν

. Rearranging,

n = \frac{P}{h ν} = \frac{2 \times 1 0 ^{- 3}}{6.63 \times 1 0 ^{- 34} \times 6 \times 1 0 ^{14}}

. The denominator works out to

3.978 \times 1 0^{- 19}

J, giving

n = 5.03 \times 1 0^{15} \approx 5 \times 1 0^{15}

photons per second.

JEE Main · 2024 · 01 Feb 2024 · Shift 2 · Single correct option

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