A person uses `+ 1.5 D` glasses to have normal vision from `25 cm` onwards. He uses a `20 D` lens as a simple microscope to see an object. Calculate the maximum magnifying power, if he uses the microscope
(a) together with his glasses
(b) without the glasses.

Explanation

Here, `P_1 = 1.5 D : d = 25 cm`,
`P_2 = 20 D`
(a) With glasses on
`M = 1 + (d)/(f_2) = 1 + (25)/(100//20) = 6`
(b) Without glasses, From length of glasses,
`f_1 = (100)/(P_1) = (100)/(1.5) = (200)/(3) cm`.
`u = -25 cm, v = ?`
As `(1)/(v)-(1)/(u)=(1)/(f_1)`
`:. (1)/(v)=(1)/(f_1)+(1)/(u)=(3)/(200)-(1)/(25)=(3 - 8)/(200) = (-5)/(200)`
`v = -40 cm`.
`M = 1 + (v)/(f_2) = 1 + (40)/(100//20) = 9`.