Q. 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, $h = 6.63 \times 10^{- 34}$J-s.
A) $0.267 \times 10^{18}$
B) $0.267 \times 10^{19}$
C) $0.267 \times 10^{20}$
D) $0.267 \times 10^{17}$
Explanation
(c) Energy of photon
$E = \frac{hc}{\lambda} = \frac{6.63 \times 10^{- 34} \times 3 \times 10^{8}}{590 \times 10^{- 9}} = \frac{6.63 \times 3}{59} \times 10^{- 18}$
Light energy produced per second $= \frac{90}{100} \times 10 = 9$W
$\therefore\ $Number of photons emitted per sec
$= \frac{9 \times 59}{6.63 \times 3 \times 10} - 18 = 2.67 \times 10^{19}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.