Question

Fill in the Blank : The focal length of a thin biconvex lens is 20 cm. When an object is moved from a distance of 25 cm in front of it to 50 cm, the magnification of its image changes from $m_{25}$ to $m_{50}$. The ratio $\frac{m_{25}}{m_{50}}$ is ......

Answer 1

Answer:

The ratio of m₂₅ / m₅₀  =6

The image of an object formed at a particular distance, d=25 is

(1 / v) - (1 / u) = (1 / f)

(1 / v) -  (1 / -25) = (1 / 20)

v = 100cm.

Magnification m₂₅ = v / u = 100 / -25 = -4

The image of an object formed at a particular distance, d=50 is

(1 / v) - (1 / u) = (1 / f)

(1 / v) -  (1 / -50) = (1 / 20)

v = 100 / 3 cm.

Magnification m₅₀ = v / u = (100/3 ) / -50 = -2/3

Hence,

The ratio of m₂₅ / m₅₀ = - 4  / (-2/3)  = 6