Q. Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm is ________. (Given : h = 6.63 × 10⁻³⁴ J.s and c = 3 × 10⁸ m/s)

Correct Answer: 2 × 10¹⁶

Solution

Power of the laser source is:

P = 6 mW = 6 × 10⁻³ J/s

Energy of one photon is given by:

E = hν = hc / λ

Substituting the given values:

E = (6.63 × 10⁻³⁴ × 3 × 10⁸) / (663 × 10⁻⁹)

E ≈ 3 × 10⁻¹⁹ J

Number of photons emitted per second is:

N = P / E

N = (6 × 10⁻³) / (3 × 10⁻¹⁹) = 2 × 10¹⁶

Hence, the correct answer is 2 × 10¹⁶.

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