Question

When an object of height 1 cm is kept at a dist of 4 cm from a concave mirror,its erect image of height 1·5 cm is formed at a dist of 6 cm behind the mirror.find the focal length.

Answer 1

Given - height of object = 1 cm
Height of image = 1.5 cm
Object distance ( u ) = -4 cm
Image distance ( v ) = 6 cm
To find - focal length ( f )
Solution - According to mirror formula
1/v + 1/u = 1/f
1/6 + (1/-4) = 1/f
1/6 - 1/4 = 1/f
2-3/12 = 1/f
-1/12 = 1/f
-12 = f
Focal length of the concave mirror is -12 cm

Answer 2

hii !! answer will be -12cm.

Explanation:

• As ,height of object=1,distance = -4,height of image is=1.5and of image is 6cm.So 1/f=1/v+1/u.
• 1/f=1/6+(-1/4)=-1/12.
• f=-12cm.

hope helpful