CBSE Class 10 Science 2017 · Paper 531/2 · Question 21

(a) It is a convex mirror: for every position of the object in front of it, a convex mirror forms a virtual, erect and diminished image, between its pole P and focus F behind the mirror (see the ray diagrams for an object far from and close to the mirror). Convex mirrors are generally used as rear-view (wing) mirrors in vehicles, because they always give an erect, though diminished, image and, being curved outwards, have a wider field of view, so the driver can see a much larger area of the traffic behind. (b) With $u=-20$ cm and $v=+10$ cm, $\frac{1}{f}=\frac{1}{v}+\frac{1}{u}=\frac{1}{10}-\frac{1}{20}=\frac{1}{20}$, so $f=+20$ cm. The focal length is positive, so the mirror is convex, and its focal length is $20$ cm. (The image is virtual, erect and half the size of the object: $m=+0.5$.)