Question

The power of a lens is$- 5d$. Find its Focal Length.

Answer 1

Hint If a lens has a shorter focal length it is said to be more powerful. The power of a lens is defined as the reciprocal of the focal length. Lens power is measured in dioptres (D).

Complete Step by step solution

Given :- Power of a lens, $(p) = - 5D$

As we know the formula,

Power of lens, $(p) = 1 \div focal\,length(f)$

$\therefore p = 1 \div f$

Similarly,

$\Rightarrow f = 1 \div p$(metre)

$\Rightarrow f = 1 \div ( - 5)$ (metre)

$\therefore f = - 0.2$ (metre)
Therefore, this negative sign indicates that this lens is a concave lens.

Note When a lens is used to form an image of some object, the distance from the object to the lens u, the distance from the lens to the image v, and the focal length f are related by. The focal length of a thin convex lens can be easily measured by using it to form an image of a distant light source on a screen.