Question

12.A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is(a) -30 cm​

Answer 1

Explanation:

Apply the relation – (v / u) = m = hi / ho as

= (- 5) / 10

= – {(- 30) / u}

=u = – 60 cm

Apply mirror formula as

= (1 / v) + (1 / u)= 1 / f

= 1 / (- 60) + 1 / (- 30)= 1 / f

= 1 / (- 20) = 1 / f or f = – 20 cm

so, -20 cm is the answer. The focal length lies in front of the mirror. Therefore, the sign is negative.