Question

Calculate the power of the convex lens of focal length 20

Answer 1

$\large{\bf{\gray{\underline{\underline{\orange{Given:}}}}}}$

Focal length of lens = 20cm = 0.2m

Type of lens : $\tt\red{convex}$

$\large{\bf{\gray{\underline{\underline{\green{To\:Find:}}}}}}$

We have to find Power of the lens.

$\large{\bf{\gray{\underline{\underline{\pink{Solution:}}}}}}$

⇒ Focal length of concave lens is taken negative abd that of convex lens is taken positive.

⇒ Relative b/w focal length and power of lens is given by

• P = 1 / f

⇒ Unit of power = D ($\sf{m^{-1}}$)

Let's calculate :D

$:\implies\tt\:P=\dfrac{1}{f}$

$:\implies\tt\:P=\dfrac{1}{(+0.2)}$

$:\implies\boxed{\bf{\purple{P=+5D}}}$

Answer 2

Here ,

• Focal length (f) = 20 cm or 0.2 m

As we know that ,

The reciprocal of focal length is called power of lens

Mathematically ,

$\boxed{ \sf{Power \: of \: lens \: (P) = \frac{1}{f \: \: (in \: m)} }}$

Thus ,

P = 1/0.2

P = 5 D

∴The power of convex lens is 5 D

Remmember :

• In case of convex lens , focal length is positive