A student focused the image of an object on a white screen using a converging lens. He noted down the positions of the object, screen and the lens on a scale as given below: Position of object= 10.0 cm
Position- of lens = 50.0 cm

Position of screen= 90.0 cm

(a) Find the focal length of the converging lens.

(b) Find the position of the image if the object is shifted towards the lens at a position of 30.0 cm.

(c) State the nature of the image formed if the object is further shifted towards the lens.

Question

A student focused the image of an object on a white screen using a converging lens. He noted down the positions of the object, screen and the lens on a scale as given below: Position of object= 10.0 cm
Position- of lens = 50.0 cm

Position of screen= 90.0 cm

(a) Find the focal length of the converging lens.

(b) Find the position of the image if the object is shifted towards the lens at a position of 30.0 cm.

(c) State the nature of the image formed if the object is further shifted towards the lens.

Philoid · Accepted Answer

Given: object distance u = -40 cm

Image distance v =90 -50 = +40 cm

Applying the lens formula:

Therefore, focal length f= +20 cm

(b)Since object is shifted from 10 cm to 30 cm w.r.t 50 cm

So, the object distance u becomes = -20 cm

Focal length = +21 cm

Applying the lens formula, we get

therefore image distance v=1/0 i.e. infinity

(c) When the object gets shifted from 30 cm towards 50 cm, then the object will lie between focus and optical centre of lens and the image formed will be virtual, erect and enlarged in size and also behind the lens.