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
Energy of one photon: $E = \dfrac{P}{N} = \dfrac{15 \times 10^3}{2.5 \times 10^{22}} = 6 \times 10^{-19}$ J.
$\lambda = \dfrac{hc}{E} = \dfrac{6.6 \times 10^{-34} \times 3 \times 10^8}{6 \times 10^{-19}} = 3.3 \times 10^{-7}$ m $= 330$ nm.
This is shorter than violet ($\approx 400$ nm), so the radiation is ultraviolet.
Solution by Sreeraj P, M.Sc Physics