Question

A telescope consists of two lenses of focal lengths `10cm` and `1cm`. Calculate the length of the telescope, when an object is kept at a distance of `60cm` from the objective, and the final image is formed at least distance of distinct vision.
A. `15.05cm`
B. `12.96cm`
C. `13.63cm`
D. `14.44cm`

Answer 1

(b) Two lenses used are eyepiece and objective.
For eyepiece,
`f_(e)=1cm, D=v_(E)=25cm`
`1/(v_(e))-1/(u_(e))=1/(f_(e))` or `-1/25-1/(u_(e))=1`
`u_(e)=-28/26cm` ltbgt and for objective `u_(0)=-60cm, f_(e)=10cm`
`:. 1/(v_(0))-1/(u_(0))=1/(f_(e))`
`1/(v_(0))+1/60-1/10`
`1/(v_(0))=1/10-1/60`
`1/(v_(0))=5/60`
`v_(0)=12cm`
`:.` Length of telescope `=L=v_(0)-u_(e)=(12+25/26)=12.96cm`