Previous incorrect answers were 1.8*10^-7 and 2.4*10^-6 A beam of light of wavel

Question

Previous incorrect answers were 1.8*10^-7 and 2.4*10^-6

A beam of light of wavelength 560 nm is reflected perpendicularly from a mirror. Determine the force that the light exerts on the mirror when 1020 photons hit the mirror each second [Hint: Refer to the impulse-momentum equation. You may assume that the magnitude of the photons' momenta is unchanged by the collision, but their directions are reversed.] Express your answer to two significant figures and include the appropriate units. alueUnits Submit Previous Answers Request Answe X Incorrect; Try Again; 4 attempts remaining

Explanation / Answer

Wavelength of light is 560*10^-9m. So, its frequency is f = 3*10^8 / 560*10^-9 = 5.36*10^14 Hz

mass of photons of this frequency is m = hf/c^2 = 6.626*10^-34*5.36*10^14 / (3*10^8)^2 = 3.94*10^-36 kg

So, by impulse momentum, F*1 = 10^20 * 3.94*10^-36 kg * (3*10^8 - (-3*10^8)) = 10^20 * 3.94*10^-36 kg * 6*10^8

So, the force that the light exerts on the mirror when 10^20 photons hit the mirror each second is F = 2.36*10^-7 N

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