Question

The power of the lens is $-2D$. What is its focal length?
A. 2 m
B. 1.5 m
C. 1.0 m
D. 0.5 m

Answer 1

Hint:To solve this problem we should know about the following term.The power of a lens is defined as the reciprocal of the focal length. Lens power is measured in dioptres (D).So, when we apply a known formula we can easily calculate the focal length of the lens.

Complete step by step answer:
Focal length: The distance between the convex lens or a concave mirror and the focal point of a lens or mirror is called the focal length. It is the point where parallel rays of light meet or converge. And we know the power of the lens will be the reciprocal of the focal length.

As given $$P=-2D$$
As we know,
$P={\displaystyle \frac{1}{f}}$
Here, $P$ is power and $f$ is focal length.
So, keeping value in it. We get,
$P={\displaystyle \frac{1}{f}}$
$\Rightarrow -2D={\displaystyle \frac{1}{f}}$
By cross multiplication
$\Rightarrow f={\displaystyle \frac{1}{-2D}}$
$\Rightarrow f=-0.5m$
As, its focal length is negative. So, it will be a concave lens.Thus, the focal length of the given lens will be 0.5 m.

Hence, the correct option is C.

Note:Application of lens: Light is focused using lenses. Because concentrating light is so vital, lenses can be found in a variety of places. The lenses in people's glasses are probably the most prevalent lenses we see.If the power of the lens is negative it must be a concave lens. If positive power then it must be convex lens