Question

A student uses a lens of focal length 50 cm and another of – 20cm. Write the nature of and power of each lens.​

Answer 1

Explanation:

focal length= 50 cm

p= 1/f

p= 1/50

p= 0. 02 D

thus,nature of lens is convex .

focal length= -20 cm

p= 1/f

= 1/-20

= -0.05 D

thus, nature of lens is concave

hope it helps you

Answer 2

Given:-

• focal length of $\bf lens_{1}$ = 50cm. = 0.5m
• focal length of $\bf lens_{2}$ = -20cm. = -0.2m

To find:-

• power and nature of $\bf lens_{1}$
• power and nature of $\bf lens_{2}$

Concept to be applied:-

• A lens having +ve focal length is a Converging lens or Convex lens.
• Hence, $\bf lens_{1}$ is Convex lens.

• A lens having -ve focal length is a Diverging lens or Concave lens
• hence, $\bf lens_{2}$ is a Concave lens

Formula for finding power:-

$\underline{\boxed{\bf power = \dfrac{1}{f(in\: m )}}}$

Here,

• f = focal length
• it should be in meters to find the power of the lens

Solution:-

Power of lens 1 :-

$\longrightarrow$ power = $\bf \dfrac{1}{0.5}$

$\longrightarrow$ power = 2D

Power of lens 2 :-

$\longrightarrow$ power = $\bf \dfrac{1}{-0.2}$

$\longrightarrow$ power = -5D

_____________________________

Additional information:-

• focal length of a concave mirror is -ve
• focal length of a convex mirror is +ve