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How many photons per second emanate from...

How many photons per second emanate from a 50 mW laser of 640 nm ?

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Number of photons emanate per second `n_(p) = (P)/(E) = (P lambda)/(hc)`
`= (50 xx 10^(3) xx 640 xx 10^(-9))/(6.6 xx 10^(-34) xx 3 xx 10^(8)) = (32000 xx 10^(-6))/(19.8 xx 10^(-26)) = 1616.16 xx 10^(20)`
`n_(p) = 1.61 xx 10^(17) s^(-1)`
P = 50 mW
`lambda = 640` nm
`h = 6.6 xx 10^(-34)` Js
`c = 3 xx 10^(8) ms^(-1)`
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