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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Solution: Click For PDF Version (a)
As the laser
beam, wavelength
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
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.
,
and power 5.00 mW is focussed to a circular spot of effective
diameter
,
