Question

Draw a labelled ray diagram of a refracting telescope. Define its magnifying power and write the expression for it.
Write two important limitations of a refracting telescope over a reflecting type telescope.

Answer 1

Refracting telescope :

Magnifying power : The magnifying power is the ratio of the angle $\beta$ subtended at the eye by the final image to the angle $\alpha$ which the object subtends at the lens or the eye.

$m=-\frac{\beta }{\alpha }=-\frac{h}{f_e}.\frac{f_0}{h}=-\frac{f_0}{f_e}$

Limitations of refracting telescope over reflecting type telescope :
(i) Refracting telescope suffers from chromatic aberration as it uses large sized lenses.
(ii) The requirement of big lenses tend to be very heavy and therefore, difficult to make.