Question

A person is advised to use a lens of power +2 D for correcting his vision. Identify the defect of vision, and find the focal length of the lens.

• A. Myopia, $-0.5m$
• B. Hypermetropia, $0.5m$
• C. Hypermetropia, $-0.5m$
• D. Myopia, $0.5m$

Answer 1

The correct option is B Hypermetropia, $0.5m$

Power, $P=\frac{1}{f\left(inmetre\right)}$
The power and focal length of a convex lens are positive and those of a concave lens are negative. Here, the power of the lens is $+2D$. So, it is a convex lens.
A convex lens is used to rectify hypermetropia. Therefore, the defect with which the person is suffering from is hypermetropia.

The power of the lens is:
$P=\frac{1}{f}$
$\Rightarrow f=\frac{1}{P}$
$\Rightarrow f=\frac{1}{2}$
$\Rightarrow f=0.5m$