JEE Main · 2026 · 08 Apr 2026 · Shift 2 · Modern Physics

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✓ Correct

Answer: (D)

Solution

The energy carried by each photon is

E = \frac{P}{N} = \frac{15 \times 1 0 ^{3}}{2.5 \times 1 0 ^{22}} = 6 \times 1 0^{- 19} J

. Its wavelength follows from

E = \frac{h c}{λ}

, so

λ = \frac{h c}{E} = \frac{6.6 \times 1 0 ^{- 34} \times 3 \times 1 0 ^{8}}{6 \times 1 0 ^{- 19}} = 3.3 \times 1 0^{- 7} m = 330 nm

. A wavelength of

330 nm

lies below the visible band, so the emitted radiation belongs to the ultraviolet region.

JEE Main · 2026 · 08 Apr 2026 · Shift 2 · Single correct option

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