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## Homework Statement

The laser cavity is formed by two mirrors separated by 15 cm. One of the mirrors has an ultra-high reflectivity and the output mirror has the much lower reflectivity of 99.5 %. How many photons are there in the cavity?[/B]

The power of the laser is 1mW and the wavelength is 600nm

## Homework Equations

E= hc/λ and N=P/E Hence N = Pλ/hc

Power out = Power in x (1 - reflectivity) ........... Not sure about this formula

## The Attempt at a Solution

I have different views to what is happening as I think the question is vague. So I only have worked out certain elements.

At full power N = Pλ/hc ...... N=3.01x10

^{15}

We could work out the time inside the cavity t = 30x10

^{-2}/3x10

^{8}which gives 1 nano second....... Not sure if this is relevant to the question.

Using the formula Power out = Power in x (1 - reflectivity) gives a power of 1x10

^{-3}x(1 -0.05)

=9.5x10

^{-4}

Using this new power N = Pλ/hc......... N= 2.86x10

^{15}photons/s

Would I then do 3.01x10

^{15}- 2.86x10

^{15}= 1.6x10

^{14}??

Not sure if this is correct. Is the time element I tried supposed to be used?