Question

An object placed 0.1 m in front of a concave mirror is found to give an erect image 0.3 m
behind the mirror. What its focal length ?​

Answer 1

GIVEN :

An object placed 0.1 m in front of a concave mirror is found to give an erect image 0.3 m

behind the mirror.

TO FIND :

What its focal length ?​

SOLUTION:

◆MIRROR EQUATION ,

1/f = 1/u + 1/v

◆Where, f - FOCAL LENGTH ,

u - object distance = -0.1 m

v - image distance = 0.3 m

◆Substituting,

f = u × v /(u +v)

f = -0.1 ×0.3 /( - 0.1 +0.3)

= - 0.03 / 0.2

f= -0.15m

ANSWER :

Focal length of the mirror , f = -0.15 m.

Answer 2

The focal length is - 0.15 m

Explanation:

The mirror formula is given as:

1/f = 1/u + 1/v

Where,

f = Focal length = ?

u = Object distance = - 0.1 m (negative because placed in front of concave mirror)

v = Image distance = 0.3 m

Now, on substituting the values, we get,

1/f = 1/0.3 - 1/0.1

1/f = 10/3 - 10

1/f = (10 - 30)/3

1/f = - 20/3

∴ f = - 3/20 = - 0.15 m