Question

A myopic person has been using spectacles of power $-1.0D$ for distant vision. During old age, he also needs to use a separate reading glass of power $+2.0D$. Explain what may have happened.

Answer 1

The power of the spectacles used by the myopic person, $P=-1D$

The focal length of the spectacles,

$f=\frac{1}{P}=-1m=-100cm$

Hence, the far point of the person is 100cm. He might have a normal near point of $25cm$. When he uses the spectacles, the objects placed at infinity produce virtual images at $100cm$. He uses the ability of accommodation of the eye-lens to see the objects placed $100cm$ and $25cm$.

During old age, the person uses reading glasses of power, $P=+2D$

The ability of accommodation is lost in old age. This defect is called presbyopia. As a result, he is unable to see clearly the objects placed at $25cm$.