Question

When an object of height 1 cm is placed in front of a lens, the image formed is real, inverted and of height 2cm. The distance between the image and object is 90 cm. Find, the focal length of the lens used.​

Answer 1

Answer:

Let, image distance=v

object distance=u

v+u=90 cm

or, v=90-u

Here,

Height of the image/Height of the object=Image distance/Object distance

or, 2/1 = v/u

or, 2/1 = (90-u)/u

or, 2 u = 90-u

or, 3 u = 90

∴ u= 30 cm

∴ v = 90 - u = 90 - 30 = 60 cm

Now,

Using lens formula,

1/f = 1/u + 1/v

or, 1/f = 1/30 + 1/60

or, 1/f = 3/60

or, 1/f = 1/20

∴ f = 20 cm