Question

A magnifying lens has a focal length of 100 mm. An object whose size is 16 mm is placed at some distance from the lens so that an image is formed at a distance of 25 cm in front of the lens.

1. What is the distance between the object and the lens?
2. Where should the object be placed if the image is to form at infinity?

Answer 1

a. Given: Focal length (f) = 100 mm

Size of object (h) = 16 mm

Image distance (v) = −25 cm = −250 mm (negative sign because the image is on the same side as the object)

The lens formula is given by:

`1/f = 1/v - 1/u` 

`1/100 = 1/-250 - 1/u` 

`1/u = 1/100 - 1/-250` 

`1/u = 1/100 + 1/250` 

`1/u = 5/500 + 2/500` 

`1/u = 7/500`

u = `500/7`

u = 71.43 mm

u = 7.143 cm

∴ The distance between the object and the lens is approximately 7.143 cm.

b. The object should be placed at the focus for the image to be formed at infinity.