Question

A doctor has prescribed a corrective lens of power +2.5 D to a person. Which of the following is/are correct about the lens?

• A. Focal length of the lens is 0.4 cm.
• B. Focal length of the lens is 40 cm.
• C. The lens is a converging lens.
• D. The lens is a diverging lens

Answer 1

Answer: (B), (C)

The correct options are
B Focal length of the lens is 40 cm.
C The lens is a converging lens.

Let $P$ be the power of lens, $f$ be the focal length of lens
Given, $P=2.5D$
Power of lens is defined as
P = $\frac{1}{f\left(inm\right)}$
where the unit of power is Dioptre.
So, f = $\frac{1}{Power}$ = $\frac{1}{2.5}=0.4m=40cm$

Since focal length comes out to be positive, it is a convex or converging lens.