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TFM
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Homework Statement
A typical “laser pen” pointer has a power output of 3mW at 670nm. If the angular divergence of the beam is 2mrad, estimate the maximum distance at which it could be seen by a day-adapted human eye. You can assume that in bright light, a normal eye will give a signal from 660nm light if [tex] 4 * 10^5 [/tex] photons arrive in 0.1s.
Homework Equations
[tex] E = hf = \frac{hc}{\lambda} [/tex]
[tex] Power = \frac{E}{t} [/tex]
The Attempt at a Solution
Okay, I have used the above equations for the 660nm light
The energy from each photon at that wavelength is [tex] 3*10^{-19} [/tex] Joules
Since [tex] 4 * 10^5 [/tex] Photons are arriving in 0.1s, that means a total of [tex] 1.2*10^{-12} [/tex] Joules.
Now the power of this beam is [tex] 1.2*10^{-13} [/tex] Watts.
I am slightly unsure what to do from here, the power I have calculated for the 660nm Wave seems to be a LOT smaller to the 670nm Wave, which is 3mW, or 0.003 Watts.
Any suggestions/ideas about what to do from here...?
Thanks in advance,
TFM