A laser emits a light pulse of duration `tau = 0.13 ms` and energy `E = 10J`. Find the mean pressure extered by such a light pulse when it is focussed into a spot of diameter `d = 10mu m` on a surface perpendicualr to the beam and possessing a reflection coefficient `rho = 0.50`.

A laser emits a light pulse of duration `tau = 0.13 ms` and energy `E

1.	A laser emits a light pulse of duration `tau = 0.13 ms` and energy `E = 10J`. Find the mean pressure extered by such a light pulse when it is focussed into a spot of diameter `d = 10mu m` on a surface perpendicualr to the beam and possessing a reflection coefficient `rho = 0.50`.
Answer» The mean pressure `lt p gt` is related to the force fexerted by the beam by `lt p gt xx (pid^(2))/(4) = F` The force `F` equals momentum transferred per second. This is (assuming that photons, not reflected, are absorbed) `2rho(E)/(ctau) +(1-rho)(E)/(ctau) = (1+rho) (E)/(c tau)`. The first term is the momentum transfered on reflection (see problem (261)), the second on absorption. `lt p gt = (4(1_rho)E)/(pid^(2)c tau)` Substituting the values we get `lt p gt = 48.3` atomsphere.

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A laser emits a light pulse of duration `tau = 0.13 ms` and energy `E = 10J`. Find the mean pressure extered by such a light pulse when it is focussed into a spot of diameter `d = 10mu m` on a surface perpendicualr to the beam and possessing a reflection coefficient `rho = 0.50`.

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