In
a joint Soviet-French experiment to monitor the Moon’s surface
with a light beam, pulsed radiation from a ruby laser
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Solution: Click For PDF Version The radius of the mirror of the telescope is 1.3 m.
We know that
the spread of the first diffraction peak of light waves of
wavelength
where
Therefore, the angular spread of the laser beam emerging out of the telescope mirror will be determined by the angle
The total
angular spread of the laser beam will be
The Moon’s
distance from the Earth is
The angle subtended by a circular plane mirror of radius 10 cm from the Earth is
The fraction of the energy emitted by the reflecting telescope on Earth by the mirror on the Moon will therefore be
The diffraction angular spread of the laser beam reflected by the mirror on the Moon will be determined by the angle
The angle subtended by the mirror of the telescope with respect to the mirror on the Moon will be
The fraction of the energy reflected by the mirror on the Moon which is received by the mirror of the telescope on Earth will therefore be
The fraction of the original light energy that will be picked up by the detector on reflection from the Moon will therefore be
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was directed to the Moon through a reflecting
telescope with a mirror radius of 1.3 m. A
reflector on the Moon behaved like a circular plane mirror with
radius 10 cm, reflecting
the light directly back toward the telescope on Earth.
The reflected light was then detected after
being brought to a focus by this telescope.
We have to find the fraction of the original
light energy that was picked up by the detector. We may assume that
for each direction of travel all the energy is in the central
diffraction peak.
from a circular object of diameter d
is given by the Rayleigh’s criterion:
is the angle of the first diffraction minimum from the central axis.
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