Question

An eyepiece of telescope consists of two plano-convex lenses L1 and L2 each of focal length f separated by a distance of 2f/3. where L1 should be placed relative to the focus of the objective lens of the telescope so that the final image through L2 is seen at infinity?

Answer 1

                       focal length of combination of lenses L1 and a
1/f' =1/f+1/f-2f/3/f²=4/3f
f' = 3/4f
so for image  at infinite image of objective should be in focal plane of eyepieceso distance of L1
so distance of L1 from focal plane of objective =3f/4−2f/6=5f/12