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 _(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`