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NEET 2026
Exam
NEET
Year
2026
Subject
Physics
Chapter
Chapter

NEET 2026 · Physics · Chapter

A monochromatic point source of light operates at a power of 60 W...

A monochromatic point source of light operates at a power of 60 W60\text{ W} and emits light of wavelength 663 nm663\text{ nm}. Assuming 100%100\% efficiency in converting electrical power into light, what is the total number of photons emitted per second by the source?

(Take Planck's constant h=6.63×10−34 J sh = 6.63 \times 10^{-34}\text{ J s} and speed of light c=3×108 m/sc = 3 \times 10^8\text{ m/s})

Options

  1. A2×10192 \times 10^{19}
  2. B3×10193 \times 10^{19}
  3. C3×10203 \times 10^{20}
  4. D2×10202 \times 10^{20}

Correct answer

D. 2×10202 \times 10^{20}

Step-by-step solution

Energy of a single photon is given by E=hcλE = \dfrac{hc}{\lambda}

Substituting the given values:

E=6.63×10−34×3×108663×10−9E = \dfrac{6.63 \times 10^{-34} \times 3 \times 10^8}{663 \times 10^{-9}}

E=6.63×3×10−266.63×10−7=3×10−19 JE = \dfrac{6.63 \times 3 \times 10^{-26}}{6.63 \times 10^{-7}} = 3 \times 10^{-19}\text{ J}

The total power emitted by the source is P=60 W=60 J/sP = 60\text{ W} = 60\text{ J/s}

Let nn be the number of photons emitted per second. Then P=nEP = nE

n=PE=603×10−19n = \dfrac{P}{E} = \dfrac{60}{3 \times 10^{-19}}

n=20×1019=2×1020n = 20 \times 10^{19} = 2 \times 10^{20}

Answer: 2×10202 \times 10^{20}
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