Question

The power of the lens is $- 5D$.
(1)Find the focal length.
(2)Name the type of lens.

Answer 1

Power of any kind of lens is the inverse of focal length, unit must be in meters of the lens, so if power is given then easily get the focal length and sign of power tells about the type of lens.

Formula Used:
$P = \dfrac{1}{f}$ , $P$ is the power of lens and $f$ is the focal length of the lens.

Complete step by step solution: We want to find the focal length of a lens. So, we use the formula mentioned below.
$P = \dfrac{1}{f}$ as $P$ is given in the question i.e. , $P = - 5D$, where $D$ is the unit of power of the lens called dioptre.
$\therefore f = \dfrac{1}{P} = - \dfrac{1}{5} = - 0.2m$
We have got focal length of the lens -0.2metre. Here negative sign tells about the type of lens used is concave lens power of the lens is a measure of the angle of deviation of light ray produced by a lens. In case of eye lens focal length becomes too short than power lens increased very much therefore, Myopia (vision defect) occurs and for correction of myopia, a concave lens is used having better power of the lens, so the power of the concave lens represents in negative sign which means power of lens is reduced.

Note: If the question would have asked about the focal length and power of the lens is given in positive then we could say that a convex lens is used. Power of a plane glass plate is O having infinite focal length. If the lenses combined and kept close to each other then power of the combination of lenses is equal to the sum of individual power of each lens.