Question

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

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