Question

Monochromatic light of frequency $ 6.0 \times 10^{14} \,Hz $ is produced by a laser. The power emitted is $ 2 \times 10^{-3}\,W $ . The number of photons emitted, on the average, by the source per second is

• A. $ 5 \times 10^{15} $
• B. $ 5 \times 10^{16} $
• C. $ 5 \times 10^{17} $
• D. $ 5 \times 10^{14} $

Answer 1

Answer: (A)

Photons are the packets of energy. Power emitted,
$P=2 \times 10^{-3}\,W$
Energy of photon,
$E=hv$
$= 6.6 \times 10^{−34} × 10^{14}\, J$
$h$ being Planck’s constant.
Number of photons emitted per second
$n=\frac{P}{E}$
$=\frac{2\times10^{-3}}{6.6\times10^{-34}\times6\times10^{14}}$
$=5\times10^{15}$