Magnifying Power
M = v o / u o { 1 +( D / f o)
Where v o = distance of image, formed by
objective lens and u o = distance of object from the objective
( ii) When final image is formed at infinity, then M = v o / u o. D / f e
Astronomical Telescope
It is also a combination of two lenses, called objective lens and eye piece, separated by a distance. It is used for observing distinct images of heavenly bodies like stars, planets etc.
Magnifying Power
( i) When final image is formed at least distance of distinct vision( D), then M
= f o / f e { 1 +( D / f e)} where f o and f e are focal lengths of objective and eyepiece respectively.
Length of the telescope( L) =( f o + u e) where, u e = distance of object from the eyepiece.( ii) When final image is formed at infinity, then M = f o / f e Length of the telescope( L) = f o + f e
For large magnifying power of a telescope f o should be large and f e should be small.