A simple microscope has a magnifying power of 3.0 when the image is formed at the near point (25 cm) of a normal eye. (a) What is its focal length? (b) What will be its magnifying power if the image is formed at infinity?

Explanation

(a) For near point adjustment,
`m=1+ (D)/(f)rArr 3 =1+(25)/(f)` `rArr`   f = 12.5 cm..
 (b) Image formed at infinity le. in normal adjustment `m = D//f`
D = 25 cm, f = 12.5 cm: m = 2