Question

The power of the lenses of a telescope are 0.5 and 20 dioptres.If the final image is formed at the minimum distance of distinct vision (25cm) then what will be the length of the tube ?

Answer 1

Thus the length of the tube is  "L" = 2.05 m

Explanation:

Power of 1 lens P1 = 0.5 D

Power of 2 lens P2 = 20 D

Focal length f1 = 1 / p1 = 1/0.5 =  2 m

Focal length f2 = 1/20 = 0.05 m

Length "L" = fo + fe

Length "L" = 0.05 + 2

Length "L" = 2.05 m

Thus the length of the tube is  "L" = 2.05 m