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563.


Problem 41.23 (RHK)


A HeNe laser, radiating at 632.8 nm, has a power output of 3.10 mW and a full-angle beam divergence of . We have to find (a) the intensity of the beam 38.2 m from the laser; (b) and calculate the power output of an isotropic source that provides the same intensity at the same distance.

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(a)

The area subtended by a cone of angular size at a distance r from its vertex is .

Therefore, the area subtended by the laser beam of full-angle divergence of at a distance of 38.2 m will be

The power output of the HeNe laser is 3.10 mW. Therefore, the intensity of the laser beam at a distance of 38.2 m from the laser will be

(b)

The power output of an isotropic source that will provide the intensity of at a distance of 38.2 m will therefore be