Monochromatic light of frequency 6 × 10¹⁴ Hz is produced by a laser. The power emitted is 2 × 10⁻³ W.
How many photons per second on an average, are emitted by the source ?
(Given h=6.63 × 10⁻³⁴ Js )

Solution

Physics sir · 3 steps

Power is energy per second, so divide it by the energy of one photon to count the photons.

1. Energy of one photon. Each photon carries E = hν = 6.63 × 10⁻³⁴ × 6 × 10¹⁴ = 3.978 × 10⁻¹⁹ J.
2. Link power and photons. The laser gives out P joules every second. If n photons leave per second, then P = n hν.

   n = P/hν = (2 × 10⁻³)/(3.978 × 10⁻¹⁹) ≈ 5.03 × 10¹⁵ s⁻¹

3. So. About 5 × 10¹⁵ photons are emitted per second, which is option (a).

The idea to keep. Photons per second is always power divided by hν. Find the energy of one photon first, then see how many fit into one second's worth of energy.

Answer from the NTA final answer key. Solution by PrepWiser.