Question

Power of a lens is −4D. Answer the following questions:

1. Is the lens convex or concave?
2. What is the focal length of lens?

Answer 1

(i) As the power is −4 D (negative), therefore the lens is concave.

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

\[ \Rightarrow - \ 4 = \frac{100}{f \text{(in cm)}} \]

\[ \Rightarrow f = -\frac{100}{4} \]

\[ \therefore \ f = - \ 25 \ \text{cm} \]

Thus, the focal length of the lens is −25 cm (or −0.25 m).