Question

The focal length of a concave lens is 10 cm. Calculate its power.

Answer 1

As the lens is concave, its focal length is negative.

Focal length of the concave lens (f) = −10 cm [f for concave lens is −ive]

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

\[ \Rightarrow \mathrm{P} = \frac{100}{-10} \]

\[ \Rightarrow \mathrm{P} = - \ 10 \ \text{D} \]

Minus sign indicates that the lens is concave in nature.