Question

The focal length of the lens of a myopic person is 40 cm. What is the power of the lens?

Answer 1

As the person is myopic (short sighted), the lens of his glasses is concave.

Focal length of the concave lens (f) = −40 cm [f for concave lens is −ive]

\[ \text{Applying, } \mathrm{P} = \frac{100}{f \text{(in cm)}} \]

\[ \Rightarrow \mathrm{P} = \frac{100}{-40} \]

\[ \Rightarrow \mathrm{P} = - \ 2.5 \ \text{D} \]

Minus sign indicates that the lens is concave in nature.