A monochromatic light source operating at a power of 200 W emits 4 * 10²⁰ photons per second. What is the wavelength of the light?

200 nm
400 nm
100 nm
1800 nm

Correct answer: 400 nm

Solution

Each photon has energy E = hc/lambda. Power P = N * E = N*h*c/lambda. So lambda = N*h*c/P = (4e20 * 6.626e-34 * 3e8) / 200 = (4e20 * 1.9878e-25) / 200 = (7.95e-5) / 200 = 3.975e-7 m ~ 400 nm.

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