What will be the magnifying power of a simple microscope of focal length 10cm when image is formed at infinity?

For the simple microscope, we have:
Least distance of clear vision, D = 25 cm
Magnifying power, m = 3Now,

Let the focal length be f.

(a) When the image is formed at D = 25 cm, the magnifying power of the simple microscope is given by

m = 1 + `(D/f)`

`=> 3 =1+(25/f)`

`=> 25/f = 2`

`=> f = 25/2 = 12.5  "cm"`

(b) When the image is formed at infinity in normal adjustment, the magnifying power is given by

`m = D/f = 25/12.5 =2.0`

So, the magnifying power is 2 if the image is formed at infinity.