Answer 1

Distance of the object 'u' = -20 cm

Image is formed in front of the mirror at a distance (v) of -15 cm. 

Therefore, the focal length of the mirror 'f' is 

⇒`1/f=1/v+1/u` 

⇒`1/f=1/-15+1/-20` 

⇒`1/f=-4/60-3/60=(-7)/60` 

⇒`f=(-60)/7 cm `  

Thus, the mirror is concave and has a focal length of 60/7.

 

(ii) Distance of the object 'u' = -20 cm

Image is formed behind the mirror at a distance (v) of 15 cm.

Then, the focal length of the mirror 'f' will be 

`1/f=1/v+1/u` 

⇒`1/f=1/15+1/-20` 

⇒`1/f=4/60-3/60=1/60` 

⇒`f=60 cm ` 

Thus, the mirror is convex and has a focal length of 60 cm.