Question

The objective and eyepiece of an astronomical telescope are double convex lenses with refractive index 1.5. When the telescope is adjusted to infinity, the separation between the two lenses is 16 cm. If the space between the lenses is now filled with water and again telescope is adjusted for infinity, then the present separation between the lenses is

Answer 1

fsumnew=16*(1.5-1)/(1.5-1.33)
= 16(.5/.17)= 47 cm

and because the diatance between the lenses is the sum of the two focal distances, that is the new distance.