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A source S₁ is producing 10^15 photons per second of wavelength 5000 Å. Another source S₂ is producing 1.02 × 10^15 photons per second of wavelength 5100 Å. Then, (power of S₂)/(power of S₁) is equal to

1. 1.00
2. 1.02
3. 1.04
4. 0.98

Correct answer: 1.04

Solution

The power of a source is proportional to the product of the number of photons emitted per second and the energy of each photon. Energy of a photon is inversely proportional to its wavelength. Comparing the two sources, the ratio of their powers is (1.02 × 5000) / 5100 = 1.04.

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