Question

A person needs a lens of power $-2.5D$ for correcting his vision. What is the defect of vision that the person is suffering from? Also, find the focal length of the lens.

• A. Hypermetropia, $0.4m$
• B. Myopia, $0.4m$
• C. Myopia, $-0.4m$
• D. Hypermetropia, $-0.4m$

Answer 1

The correct option is C Myopia, $-0.4m$

Power, $P=\frac{1}{f(\text{in m)}}$
The power and focal length of the convex lens is positive and that of the concave lens is negative. Here, the power of the lens is $-2.5D$, so it is a concave lens.

Concave lens is used to rectify myopia. Therefore, the defect of vision with which the person is suffering from is myopia.

Power of lens is:
$P=\frac{1}{f}$
$f=\frac{1}{P}$
$f=\frac{1}{-2.5}=-0.4m$