Question

An astronomical telescope of magnifying power `7` consists of two thin lenses `40 cm` apart, in normal adjustment. Calculate the focal lengths of the lenses.

Answer 1

Here, `M = (f_0)/(f_e) = 7` or `f_0 = 7 f_e`
In normal adjustment, distance between the lenses, `f_0 + f_e = 40 cm`
:. `7 f_e + f_e = 40, f_e = (40)/(8) = 5 cm`
`f_0 = 7 f_e = 7 xx 5 = 35 cm`.