- #1

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