Question

State the nature of the lens and the focal length if its power is +4D.

Answer 1

(i) As the power is +4D, therefore the lens is convex.

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

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

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

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

Thus, the focal length of the lens is 25 cm.