Question

The power of a lens is -5D, what is the nature of lens and also find its focal lenght​

Answer 1

Given

The power of a lens is -5D.

To find

what is the nature of lens and also find its focal lenght.

Solution

Power → It is defined as reciprocal of focal length

• Power of lens = - 5D

→ P = 1/f

→ -5 = 1/f

→ f = -1/5

→ f = -0.2m

Minus shows that it is a concave lens

Hence, focal length of given lens is

- 0.2 m

Answer 2

→GIVEN←

Power of a given lens = -5 D

$\underline{\rule{150}2}$

→TO FIND←

• The nature of the lens.
• The focal length of the lens.

$\underline{\rule{150}2}$

→SOLUTION←

We know,

$\rm{\bigstar\ Power=\dfrac{1}{Focal\ length} }$

$\rm{\rightarrowtail -5=\dfrac{1}{f} } \\\\\rm{\rightarrowtail f=\dfrac{-1}{5} } \\\\\boxed{\rm{\rightarrowtail f=-0.2\ m}}$

◙ The focal length of the lens is -0.2 m.

◙ Since the focal length is negative, therefore the lens is a concave lens.

$\rule{190}5$

Notes :-

• Focal length must me in metres(m).
• If the focal length is positive, it would be a convex lens.