Q. The number of photons emitted per second by a 60 W source of monochromatic light of wavelength 663 nm is ($h = 6.63 \times 10^{- 34}\ Js$)
A) $4 \times 10^{- 20}$
B) $1.54 \times 10^{20}$
C) $3 \times 10^{- 20}$
D) $2 \times 10^{20}$
Explanation
(d)
$Energy,\ E = \frac{nhc}{\lambda}$
$\Longrightarrow 60 \times 1Js = \frac{n \times 6.63 \times 10^{- 34}Js \times 3 \times 10^{8}m}{663 \times 10^{- 9}m}\ \ \left\lbrack \because Power = \frac{energy}{time} \right\rbrack$
$\therefore\ \ \ \ n = \frac{60 \times 1 \times 663 \times 10^{- 9}}{6.63 \times 10^{- 34} \times 3 \times 10^{8}}$
$\ \ \ \ \ \ \ \ \ \ \ = 2 \times 10^{20}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.