A helium-neon laser of the type often found in physics laboratories has a beam p

Question

A helium-neon laser of the type often found in physics laboratories has a beam power of 8.00 mW at a wavelength of 633 nm. The beam is focused by a lens to a circular spot whose effective diameter may be taken to be equal to 2.00 wavelengths. Calculate the intensity of the focused beam.

Calculate the radiation pressure exerted on a tiny perfectly absorbing sphere whose diameter is that of the focal spot.

Calculate the force exerted on this sphere.

Calculate the magnitude of the acceleration imparted to it. Assume a sphere density of 3.10

Explanation / Answer

(a)

   the intensity is given by

   I = P / (? d2 / 4)

     = ........... W / m2

(b)

   the radiation pressure is

   pr = I / c

        = .........Pa

(c)

   the force exerted will be

   Fr = (P / I) pr

       = ......... N

(d)

   the acceleration of the sphere will be

   a = Fr / m

      = ......... m /s2

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