Question

The average number of photons emitted per second by a laser of power \(6.6 \times 10^{-3}\) W producing a light of wavelength 600 nm is (Planck’s constant, \(h = 6.6 \times 10^{-34}\) J·s)

• A. \(2 \times 10^{16}\)
• B. \(3 \times 10^{16}\)
• C. \(4 \times 10^{16}\)
• D. \(6 \times 10^{16}\)

Hint:

Use \( E = \frac{hc}{\lambda} \) to find energy of one photon.
Then use \( n = \frac{P}{E} \) to get number of photons per second.

Answer 1

The Correct Option is A

Energy of one photon: \[ E = \frac{hc}{\lambda},\quad h = 6.6 \times 10^{-34},\quad c = 3 \times 10^8,\quad \lambda = 600 \times 10^{-9} \] \[ E = \frac{6.6 \times 10^{-34} \times 3 \times 10^8}{600 \times 10^{-9}} = 3.3 \times 10^{-19} \text{ J} \] Number of photons per second: \[ n = \frac{P}{E} = \frac{6.6 \times 10^{-3}}{3.3 \times 10^{-19}} = 2 \times 10^{16} \]