Question

A person suffering from myopia (near-sightedness) was advised towear corrective lens of power - 2.5 D. A spherical lens of same focal length was taken in the laboratory. At what distance should a student place an object from this lens so that it forms an image at a distance of 10 cm from the lens ?​

Answer 1

We have

P = - 2.5 D

W. K. T

P= 100/f in centimeters

f= 100/ P

f =- 100/ 2.5

= - 40cm

Now we have

f = - 40 cm

v = - 10cm ( since concave lens)

u =?

By lens formula

1/f = 1/v - 1/u

1/ u = 1 / v - 1/ f

= 1/-10 - ( 1/ - 40)

= - 1 /10 + 1 / 40

= - 4/40 + 1 /40

1/ u = - 3 / 40

u = - 40 / 3

u = 13.33cm

At a distance of 13.33cm the student should place an object from this lens so that it forms an image at a distance of 10 cm from the lens.

Hope it helps you ☺️