Question

A nearsighted person cannot see beyond 25 cm. Assuming that the separation of the glass from the eye is 1 cm, find the power of lens needed to see distant objects.

Answer 1

Given:
As the person wears spectacles at a distance of 1 cm from the eyes.
Distance of the image from the glass, v = Distance of the image from the eye − Separation between the glass and the eye
v = 25 − 1 = 24 cm = 0.24 m
For the near-sighted person, u = ∞.
The lens formula is given by
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
On substituting the values in the above formula, we get:
$\frac{1}{f}=\frac{1}{\left(-0.24\right)}-\frac{1}{\infty }=-42$
Power of the lens, P = $\frac{1}{f}$ = − 4.2 D
Thus, the power of the lens required to see distant objects is − 4.2 D.