JEE Main 2026 Apr 8 Shift 2, Physics Q13: Electromagnetic Spectrum

JEE Main2026Apr 8, Shift 2Physics

A monochromatic source of light operating at $15$ kW emits $2.5 \times 1 0^{22}$ photons/s. The region of an electromagnetic spectrum to which the emitted electromagnetic radiation belongs to _________. (Take $h = 6.6 \times 1 0^{- 34}$ J.s and $c = 3 \times 1 0^{8}$ m/s).

A
Microwave
B
Infrared
C
Visible
D
Ultraviolet

Solution

Power equals photon rate times energy per photon:

$P = N \cdot \frac{h c}{λ} ⟹ λ = \frac{N h c}{P}$

$λ = \frac{( 2.5 \times 1 0 ^{22} ) ( 6.6 \times 1 0 ^{- 34} ) ( 3 \times 1 0 ^{8} )}{15 \times 1 0 ^{3}}$

$= \frac{2.5 \times 6.6 \times 3}{15} \times 1 0^{22 - 34 + 8 - 3} m = 3.3 \times 1 0^{- 7}$ m $= 330$ nm.

$330$ nm lies in the ultraviolet region (UV: roughly $10$ to $400$ nm).

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JEE Main 2026 Apr 8 Shift 2, Physics Q13: Electromagnetic Spectrum

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