A modern 200-watt sodium street lamp emits yellow light of wavelength 0.6 µm. Assuming it to be 25% efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is:
Correct answer: C. 1.5 x 10^20
- A. 6.2 x 10^20
- B. 3 x 10^19
- C. 1.5 x 10^20
- D. 6 x 10^18
Explanation
EEnergy of photon = (6.6 x 10 ^-34)(3 x 10^8)/0.6 x 10 ^-6E = 33 x 10 ^-20 JNumber of photons emitted per second is given by:N = 25/100 P/EN = 25/100 x 200 W/33 x 10 ^-20 JN = 1.5 x 10^20 OR This question can be solved by another method. View the attached image for further explanation.
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Classical physics cannot explain blackbody radiation and related observations, leading to Planck’s quantum theory, in which energy is emitted or absorbed in discrete packets. Photons provide the particle model of light, with energy and momentum linked to frequency and wavelength, including the photoelectric effect and its threshold frequency.
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