Q 12-11-034JEE MainJEE Main 2026 (8 Apr, Shift 2)Easy
A monochromatic source of light operating at $15$ kW emits $2.5 \times 10^{22}$ photons/s. The region of an electromagnetic spectrum to which the emitted electromagnetic radiation belongs to ______. (Take $h = 6.6 \times 10^{-34}\ \text{J.s}$ and $c = 3 \times 10^8\ \text{m/s}$).
Answer: (D) Ultraviolet
Concept. Energy per photon $=\dfrac{P}{n}$; then $\lambda=\dfrac{hc}{E}$ tells the band.
Step 1. Energy. $\dfrac{15\times10^3}{2.5\times10^{22}}=6\times10^{-19}$ J.
Step 2. Wavelength. $\dfrac{6.6\times10^{-34}\times3\times10^8}{6\times10^{-19}}=3.3\times10^{-7}$ m $=330$ nm, below the visible.
Answer. Ultraviolet (Option D).
Solution by Sreeraj P, M.Sc Physics
