Question

One student uses a lens of focal length $50cm$ and another of $-50cm$. What is the nature of the lens and its power used by each of them?

• A. Convex, $2D$; Concave, $-2D$
• B. Concave , $2D$; Convex, $-2D$
• C. Concave , $-2D$; Convex, $2D$
• D. None of the above

Answer 1

The correct option is A Convex, $2D$; Concave, $-2D$

$f=50cm=0.5m$

$P=\frac{1}{f}=\frac{1}{0.5}=2D$

For convex lens, $f>0⟹P>0$

For concave lens, $f<0,⟹P<0$

Lens is convex in the first case and concave in the second case. Power is equal to 2 dioptre in the first case and -2 dioptre in the second case.