Question

A myopic person has been using spectacles of power –1.0 dioptre for distant vision. During old age he also needs to use separatereading glass of power + 2.0 dioptres. Explain what may have happened.

Answer 1

Given: A myopic person has been using spectacles of power −1.0 D and during old age he uses separate reading glasses of power +2 D.

The focal length of spectacles is given as,

f= 1 P

Where, P is the power of spectacles.

By substituting the given values in the above expression, we get

f= 1 −1× 10 −2 =−100 cm

So, the far point of the person is 100 cm. He may have a normal near point of 25 cm. When he uses his spectacles, any object placed at infinity produces virtual image at 100 cm. Hence he uses the ability of accommodation of the eye-lens to see the objects placed between 100 cmand 25 cm

During old age, he requires reading glasses of power +2 D. This is because the ability of accommodation of eye is lost in old age. This defect is called presbyopia. As a result, he is unable to see clearly the objects placed at 25 cm.