Question

An eye specialist prescribes a number +4.5 D to a person for his glasses. What is the nature of the lens? What is the focal length of the lens?

Answer 1

(i) As the power is +4.5 D, therefore the lens is convex.

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

\[\Rightarrow \ 4.5 = \frac{100}{f \text{(in cm)}}\]

\[f \text{(in cm)} = \frac{100}{4.5} \ \text{cm}\]

\[\therefore f \text{(in cm)} = + \ 22.22 \ \text{cm} \]