Question

An object is placed at 12 cm distance from the pole of concave mirror and the image formed at 4 cm from the pole. Find the focal length

Answer 1

Answer:

3cm

Explanation:

mirror formula=

1/f=1/v+1/u

1/f=1/4+1/12

f=3cm

Answer 2

Solution :

As per the given data ,

• Image distance = - 12 cm

[ Distances are always measured from the pole of the mirror ]

• Image distance = + 4 cm

[ image can be formed on the same side as that of the object or on the opposite side ]

If ,

v = + 4 cm

By using mirror formula ,

➜ 1 / f = 1 1/ v + 1 / u

➜ 1 / f =  1/ 4 - 1 / 12

➜ 1 / f = 3 - 1 / 12

➜ 1 / f = 2 / 12

➜ f = 6 cm x

The focal length of a concave mirror cannot be positive .

Hence , the value of v must be - 4 i.e. the image is formed on the same side as that of the object .

As per the mirror formula ,

➜ 1 / f =  1/ v + 1 / u

➜ 1 / f =  - 1/ 4 - 1 / 12

➜ 1 / f = -3 - 1 / 12

➜ 1 / f = - 4 / 12

➜ f = - 3 cm

The focal length of the concave mirror is - 3 cm