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joex444
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"A 4.5W flashlight with a wavelength of 550nm is shined on a perfect mirror that reflects all the light completely. a) How many photon emanate from the flashlight? b) What force is exerted on the mirror by the beam of light?"
A is easy, 1.25x10^19 photon/sec.
My first attempt tried to exploit momentum, but I kept getting a dt in there, so that my answer was never in N, but rather N/s.
Then, after looking at units, I noticed that a Watt / (m/s) = N. So, I just said F = P / c ... so F = 4.5W / (3x10^8 m/s) = 1.5 x 10^-8 N.
Does this make any sense? Atleast its a small force...
A is easy, 1.25x10^19 photon/sec.
My first attempt tried to exploit momentum, but I kept getting a dt in there, so that my answer was never in N, but rather N/s.
Then, after looking at units, I noticed that a Watt / (m/s) = N. So, I just said F = P / c ... so F = 4.5W / (3x10^8 m/s) = 1.5 x 10^-8 N.
Does this make any sense? Atleast its a small force...
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