Draw a labelled ray diagram of a refractive telescope. Deduce an expression of magnifiying power of it. Write two main limitations of a refracting type telescope over a reflecting type telescope.

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Solution

As per the given diagram, light from a distant object enters the objective and a real image is formed in the tube at its second focal point. The eyepiece magnifies this image producing a final inverted image. The magnifying power $M$ is the ratio of the angle $β$ subtended at the eye by the final image to the angle $α$ which the object subtends at the lens or the eye. Hence
$M \approx \frac{β}{α} \approx \frac{h}{f_{e}} \times \frac{f_{0}}{f_{e}} = \frac{f_{0}}{f_{e}}$
Magnifying power of telescope $M = \frac{f_{0}}{f_{e}}$
In this case, the length of the telescope tube is $f_{0} + f_{e}$

Reflecting telescope is superior in comparison to refracting because:
(1) Reflecting telescope do not suffer as much from chromatic aberrations.
(2) Reflecting telescope are cheaper to make.
(3) Light reflects off the primary mirror rather than passing through it so only one side of the reflector needs to be perfect unlike a refracting lens.

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