Question

When an object is placed at a distance of 15 cm from a concave mirror, its image is formed at 10 cm in front of the mirror. Calculate the focal length of the mirror.

Answer 1

Distance of the object from the mirror 'u' = −15 cm

Distance of the image 'v' = −10 cm
We have to find the focal length of the mirror 'f'.
Using the mirror formula, we get 

`1/f=1/v+1/u` 

`1/f=1/(-10)+1/(-15)=-1/10-1/15` 

`1/f=-3/30-2/30` 

`1/f=-5/30` 

f=-6 cm 

Thus, the focal length of the concave mirror 'f' is 6 cm.