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570.


Problem 41.45 (RHK)


A helium-neon laser of the type often found in the physics laboratories has a beam power output of 5.00 mW at a wavelength of 633 nm. The beam is focussed by a lens to a circular spot whose effective diameter may be taken to be 2.10 wavelengths. We have to calculate (a) the intensity of the focussed beam; (b) the radiation pressure exerted on a tiny, perfectly absorbing sphere whose diameter is that of the focal spot; (c) the force exerted on this sphere; and (d) the acceleration imparted to it. We may assume that the density of the sphere is .


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(a)

As the laser beam, wavelength , and power 5.00 mW is focussed to a circular spot of effective diameter

,

the intensity of the focussed beam will be

(b)

The radiation pressure exerted on a tiny, perfectly absorbing sphere whose diameter is that of the focal spot will be

(c)

The force exerted by the laser radiation on the sphere will be

(d)

Mass of the sphere

Therefore, the acceleration imparted to the sphere by the laser beam will be