The laser in my research lab produces 1 watt of power at a wavelength of 800 nan

Question

The laser in my research lab produces 1 watt of power at a wavelength of 800 nanometers. Suppose I focus this laser beam down to a circular spot of diameter 10 microns. If I place a piece of black paper in front of this laser beam, what is the force exerted on the paper by the photons? Assume that all of the photons are absorbed by the paper, so you don't have to worry about recoil from reflected photons. Suppose the laser was pointing upward (so that it was exerting its force against the force of gravity). What is the mass of paper that we could support by balancing the light pressure against the pull of gravity?

Explanation / Answer

given :
power = 1 watt
wavelength () = 800 nm
diameter = 10 microns
we know that :
power = force * velocity
therefore, force = power / velocity
force = 1 / speed of photons
now speed of photons = speed of light
speed of light = 3.00*108 m/s

force = 1/3.00*108 N

now,
mass of paper = force / acceleration due to gravity

(taking acceleration due to gravity = 10 m/s2)

mass of the paper = 1/3.00*109 Kg

note : the above answer is in SI unit....

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