CBSE Class 10 Science 2020 · Paper 31/3/1 · Question 30(a)

(a) A security mirror is convex, so $R=+5$ m and $f=+2.5$ m, with $u=-20$ m. From $\frac{1}{v}=\frac{1}{f}-\frac{1}{u}=\frac{1}{2.5}+\frac{1}{20}=\frac{9}{20}$, $v=\frac{20}{9}\approx +2.22$ m. Position: $2.22$ m behind the mirror, between P and F. Nature: virtual and erect. Size: diminished; $m=-\frac{v}{u}=+\frac{1}{9}$, so the image is one-ninth the size of the customer. (b) The mirror is concave. With the tooth held between its pole and its focus, it forms an erect and enlarged (virtual) image of the tooth, so the dentist sees the tooth magnified.