If the laser is pointed toward a pinhole with a diameter of 1.2 mm , how many photons will travel through the pinhole per second? assume that the light intensity is equally distributed throughout the entire cross-sectional area of the beam. (1 w = 1 j/s)

Plug in the information they give you into the equation E = hc/wavelength 

E= (hc)/532x 10^-9 
E = 3.74 x10 ^-19 Joules 

Now take the data for 5 J/s and divide it by the 3.74 x10 ^-19 Joules to get 1.34 x 10 ^ 19 photons/second. 

Then, look for the area of the circle.

Area of a circle is (pi)(r^2) so the ratio will be: 

[(pi (.6^2) ]/ [ (pi) (2.75^2) ] 

Note that since the pi will cancel it'll just be (.6^2)/(2.75^2) and the values .6 and 2.75 are found by dividing the diameters you are given by 2, since this is of course the radius. 

Then multiply (.6^2)/(2.75^2) by 1.34 x 10 ^ 19 photons/second to get 6.39 x 10^17 photons/second.

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