Question

The focal length of a convex lens is given 3cm,0.5cm and 0.2cm . What is the power of the lense and nature?

Answer 1

Hint Focal length of a lens refers to the distance of a point from the principal lens axis where the parallel rays of light converge after refraction from the lens. We can calculate power of the lens by reciprocating the focal length value and identify the nature.

Complete Step by Step Solution
The converging lens or convex lens is a type of lens where the light rays converge at a point after undergoing refraction from the lens. The point where the light rays converge after undergoing a series of refraction is called the focal point of the lens. There are 2 types of convex lens, one the plano-convex lens where the one side is bulging outwards and the other remains flat and bi-convex lens, where both the surfaces bulge out.
These types of lenses have thicker centers and have slimmer edges, whereas diverging lenses are thicker at the edges and are very thin at the center. This thickness present in the center has greater power to force down the light ray to converge at a point.
Now, Power of lens is defined as the measure of amount of deviation of light ray that has been produced by the lens. Now, if a lens is said to have greater power, it is said that it has a higher amount of deviating strength. It is generally calculated as the inverse of focal length.
$P = \dfrac{1}{f}$
The given 3 focal lengths in meters are 0.03 m ,0.005 m and 0.002cm. Now power of these lenses are
$\Rightarrow P = \dfrac{1}{{0.03}} = + 33.33D$
$\Rightarrow P = \dfrac{1}{{0.005}} = + 200D$
$\Rightarrow P = \dfrac{1}{{0.002}} = + 500D$
The nature of the image formed is real and upright.

Note Lens with negative power (concave lens) are used as corrective lenses for nearsightedness or myopia , whereas the lens used for long sightedness or hypermetropia is of positive power and is convex in nature.