1.

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 (b) -20 cm (c) -40cm (d) -60 cm​

Answer»

Answer Is Option B) STEPS : Apply the relation – (v / u) = m = hi / ho as= (- 5) / 10= – {(- 30) / u}=u = – 60 cmApply mirror formula as= (1 / v) + (1 / u)= 1 / f= 1 / (- 60) + 1 / (- 30)= 1 / f= 1 / (- 20) = 1 / f or f = – 20 cmHence, b) -20 cm is the answer. The focal length LIES in front of the mirror. Therefore, the sign is negative.



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