Question

What will be the number of photons emitted per second by a $10\, W$ sodium vapour lamp assuming that $90 \%$ of the consumed energy is converted into light
[Wavelength of sodium light is $590\, nm$, and $\left.h=6.63 \times 10^{-34}\, J\, - s \right]$

• A. $0.267 \times {10}^{18}$
• B. $0.267 \times {10}^{19}$
• C. $0.267 \times {10}^{20}$
• D. $0.267 \times {10}^{17}$

Answer 1

Answer: (C)

Energy of photon, $E=\frac{h c}{\lambda}$
$=\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{590 \times 10^{8}}$
$=\frac{6.63 \times 3}{59} \times 10^{-18}$
Light energy produced per second $=\frac{90}{100} \times 10$
$=9\, W$
Number of photons emitted per sec
$=\frac{9 \times 59}{6.63 \times 3 \times 10^{-18}}=2.67 \times 10^{19}$