Question

A student did an experiment with a convex lens. He put an object at different distances 25 cm, 30 cm, 40 cm, 60 cm and 120 cm from the lens. In each case he measured the distance of the image from the lens. His results were 100 cm, 24 cm, 60 cm, 30 cm and 40 cm, respectively. Unfortunately his results are written in wrong order.

 What would be the image distance if the object distance was 90 cm?

Answer 1

 Lens formula is given by: 

`1/f=1/v-1/u` 

`1/20=1/v-1/-90` 

⇒`1/20=1/v+1/90` 

⇒`1/v=1/20-1/90` 

⇒`1/v=(9-2)/180` 

⇒`1/v=7/180` 

⇒`v=180/7` 

⇒`v=25.7`cm 

The image distance will be 25.7 cm if the object distance is 90 cm.