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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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Answer» Solution :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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