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A sodium street lamp rated at 200 W emits yellow light of wavelength 0.6 μm. Only 25% of the electrical power is converted into light. If the number of yellow photons emitted per second is approximately 1.5 * 10^x, find x.
- 20
- 21
- 22
- 23
Correct answer: 20
Solution
With 25% efficiency the lamp radiates 50 W of light. Each yellow photon carries energy hc/lambda = 3.315*10⁻¹⁹ J, so the photon rate is 50 / 3.315*10⁻¹⁹ ~ 1.51*10²⁰, giving x = 20.
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