JEE Main · 2023 · 31 Jan 2023 · Shift 1 · Modern Physics

If a source of electromagnetic radiation having power 15 kW produces

1 0^{16}

photons per second, the radiation belongs to a part of spectrum is.
(Take Planck constant

h = 6 \times 1 0^{- 34}

Js)

(A)Micro waves
(B)Ultraviolet rays
(C)Gamma rays✓ Correct
(D)Radio waves

Answer: (C)

Solution

Dividing the radiated power by the number of photons emitted each second gives the energy of a single photon,

E = \frac{15 \times 1 0 ^{3}}{1 0 ^{16}} = 1.5 \times 1 0^{- 12}

J. Its frequency follows from

E = h ν

, so

ν = \frac{E}{h} = \frac{1.5 \times 1 0 ^{- 12}}{6 \times 1 0 ^{- 34}} = 2.5 \times 1 0^{21}

Hz. A frequency this high lies far beyond X-rays, placing the radiation in the gamma-ray region of the spectrum.

JEE Main · 2023 · 31 Jan 2023 · Shift 1 · Single correct option

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