Question

An object is placed 60 cm from a concave lens. The image is formed at a distance of 20 cm from the optical centre. Find the focal length of the lens ? Is the lens converging or diverging?​

Answer 1

Answer:

Explanation:

Given:

Lens being used : concave lens

Position of object, u = -60 cm

Position of image, v = -20 cm

[ Position of image is negative because concave lens always form virtual and erect image ]

To find:

Focal length of concave lens, f =?

Nature of lens whether it is converging or diverging.

Formula required:

Lens formula

     1 / f = 1 / v  - 1 / u

[ where f is focal length of lens, v is position of image and u is position of object ]

Calculation:

Using lens formula

→ 1 / f = 1 / v - 1 / u

→ 1 / f = 1 / (-20) - 1 / (-60)

→ 1 / f = 1 / (-20) + 1 / 60

→ 1 / f = ( - 3 + 1 ) / 60

→ 1 / f = -2 / 60

→ f = -60 / 2

→ f = -30 cm

Therefore,

Focal length of lens is -30 cm. and

Lens is diverging in nature.