Question

An object is placed at a distance of 15 cm in front of concave mirror of focal length 5 cm. At what distance in front of mirror is the image formed -

a) 5 cm

b) 7.5 cm

c) 10 cm

d) 12.5 cm

​

Answer 1

QUESTION :

• An object is placed at a distance of 15 cm in front of concave mirror of focal length 5 cm. At what distance in front of mirror is the image formed

GIVEN :

• Object distance = - 15 cm

( object distance is always negative as distance is measured from the pole of the mirror )

• focal length = - 5 cm

( focal length of a concave mirror is negative)

FORMULA :

• 1 / f = 1 / v + 1 / u

here ,

1. f = focal length
2. v = image distance
3. u = object distance

SOLUTION :

As per the mirror formula ,

1 / f = 1 / v + 1 / u

- 1 / 5 = 1 / v - 1 / 15

1 / v = - 1 / 5 + 1 / 15

1 / v = - 3 + 1 / 15

1 / v = - 2 / 15

v = - 7. 5 cm

ANSWER :

• Image distance = - 7 . 5 cm