A monochromatic source of light operating at $200 W$ emits $4 \times 10^{20}$ photons per second. Find the wavelength of the light.

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523 nm

400 nm

320 nm

603 nm

answer is C.

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Detailed Solution

The energy of each photon $E = \frac{200}{4 \times 10^{20}} = 5 \times 10^{- 19} J$

Wavelength $λ = \frac{h c}{E}$ $= \frac{(6.63 \times 10^{- 34}) \times (3 \times 10^{8})}{(5 \times 10^{- 19})}$ $\approx 4 \times 10^{- 7} m = 400 nm$

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