Question

A converging lens of focal length f is placed at a distance 0.3 m from an object to produce an image on a screen 0.9 m from the lens. With the object and the screen in the same positions, an image of the object could also be produced on the screen by placing a converging lens of focal length.

Options

• f at a distance 0.1 m from the screen

• f at a distance 0.3 m from the screen

• 3 f at a distance 0.3 m from the screen

• 3 f at a distance 0.1 m from the screen

Answer 1

f at a distance 0.3 m from the screen

Explanation:

Given,

u = –30 cm

v = 90 cm

f = ?

`1/v - 1/u = 1/f`

⇒ f = 22.5 cm

When the distance between the object and the image is greater than 4f, there are two lens positions in which the image of the object on the screen is clear and distinct.

v = ?

Let u = 90 cm

f = 22.5 cm

`1/v = 1/u + 1/f`

= `1/(-90) + 1/22.5`

`v = (90 xx 22.5)/(90 - 22.5)` = 30 cm

Put lens of focal length f at distance 0.3 m from screen.