A monochromatic source emitting light of 600 nm, has a power output of 66 W. The number of photons emitted by the source per second is (Given, Planck's constant h = 6.6 × 10−34 Js)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2 × 1020
The power of the source is the energy of one photon times the number of photons emitted per second, \( P = N\,hc/\lambda \).
\( \displaystyle E = \frac{hc}{\lambda} = \frac{6.6\times10^{-34} \times 3\times10^{8}}{600\times10^{-9}}\,\mathrm{J} = 3.3\times10^{-19}\,\mathrm{J} \)
\( \displaystyle N = \frac{P}{E} = \frac{66\,\mathrm{J\,s^{-1}}}{3.3\times10^{-19}\,\mathrm{J}} = 2\times10^{20}\,\mathrm{s^{-1}} \)
Note: NTA picked this question from an NCERT (2026-27) example and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Particle Nature of Light: The Photon – …
