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.
Solution
\textbf{Magnifying power} of a telescope is the ratio of the angle β subtended at the eye by the final image to the angle α subtended by the object:
m=αβ=−fefo(normal adjustment)
\textbf{Limitations of a refracting telescope:} (i) It suffers from \emph{chromatic and spherical aberration} because it uses large lenses. (ii) A very large objective lens is heavy, sags under its own weight and is difficult to make and support.
