Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm is ______. (Given : $\mathrm{h}=6.63 \times 10^{-34}$ J.s and $\mathrm{c}=3 \times 10^{8} \mathrm{~m} / \mathrm{s}$ )

JEE Main · 2026 · 28 Jan 2026 · Shift 2 · Modern Physics

Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm is ______.
(Given :

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

J.s and

c = 3 \times 1 0^{8} m / s

)

Answer: (D)

The laser emits energy at the rate

P = n E_{p h o t o n} = n \frac{h c}{λ}

, where

n

is the number of photons per second. Hence

n = \frac{P λ}{h c} = \frac{( 6 \times 1 0 ^{- 3} ) ( 663 \times 1 0 ^{- 9} )}{( 6.63 \times 1 0 ^{- 34} ) ( 3 \times 1 0 ^{8} )} .

The numerator is

6 \times 663 \times 1 0^{- 12}

and the denominator

6.63 \times 3 \times 1 0^{- 26}

, so

n = \frac{6 \times 663}{6.63 \times 3} \times 1 0^{14} = \frac{600}{3} \times 1 0^{14} = 2 \times 1 0^{16}

photons per second.

JEE Main · 2026 · 28 Jan 2026 · Shift 2 · Single correct option