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sharkybait
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Homework Statement
My homework question is: A flashlight beam produces 2.5W of electromagnetic radiation in a narrow beam. Although the light it produces is white ( all visible wavelengths), make the simplifying assumption that the wavelength is 550 nm, the middle of the spectrum. What force would the beam exert on a perfect mirror ( reflects light completely)?
Homework Equations
E=hc/wavelength
Force=momentum/Time
Power=Energy/Time
E=pc
The Attempt at a Solution
We need to find the force so I decided the best way to go about this is to use the relationship F=Momentum/Time.
1. E=hc/wavlength used to find the momentum P=E/C.
2. Once I found the momentum I used the relationship Power=Energy/Time and then solved for Time yielding Time = Energy/Power.
3.Now that I had both the Momentum and Time I simply solved the equation F=momentum/time.
The answer I am supposed to get is 1.67*10^-8 N while the answer I got was 8.36*10^-9 N. The correct answer turns out to be exactly two times the answer I calculated. Could anyone give me some insight into why this is the case? I think there is some conceptual aspect to this problem I may be missing. Thanks for any help.