The two lenses of a compound microscope are
of focal lengths 2 cm and 5 cm. If an object is
placed at a distance of 2.1 cm from the objective
of focal length 2 cm the final image forms at
the final image forms at the least distance of
distinct vision of a normal eye. Find the
magnifying power of the microscope.
l) 20
2) 60
3) 120

4)6

Focal length of the objective lens (f1) = 2.0 cm
Focal length of the eyepiece (f2) = 6.25 cm
Distance between the objective lens and the eyepiece (d)= 15 cm
(a) Least distance of distinct vision, (d1) = 25 cm
Therefore, Image distance for the eyepiece (v2) = – 25 cm
Object distance for the eyepiece =  u2
According to the lens formula,
1/ v2 - 1/u2 = 1/ f2
1/u2 = 1/ v2 -1/ f2  = 1/-25 - 1/6.25 = -1-4/ 25 = -5/25
 u2 = -5 cm
Image distance for the objective lens, (v1) =- d + u2 = 15-5 = 10 cm
Object distance for the objective lens = u2
According to the lens formula,
1/ v1 - 1/u1 = 1/f1
1/u1 = 1/ v1 - 1/f1
        = 1/10 - 1/2 = 1-5/10 = -4/10
u1 = -2.5 cm
Magnitude of the object distance (u1) = 2.5 cm
The magnifying power of a compound microscope
m = v1/ u1 (1 + d1/f2)
    = 10/2.5 (1 + 25/6.25)
    = 4 (1 +4) = 20
Hence, the magnifying power of the microscope is 20.