Distance between an object and three times magnified real image is 40 cm. The focal length of the mirror used i $s _____$ cm.
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Distance between an object and three times magnified real image is 40 cm. The focal length of the mirror used i s _____ cm.

G1 Physics · Accepted Answer

A three-times magnified real image formed by a mirror is inverted. Therefore the magnitude relation is |v| = 3|u|. For a concave mirror, both the real object and real image lie in front of the mirror. The distance between object and image is |v|-|u| = 40 cm. Hence, 3|u|-|u| = 40. Therefore, 2|u| = 40, so |u| = 20 cm. Thus, |v| = 60 cm. Using the Cartesian sign convention, u = -20 cm, v = -60 cm. Mirror formula: 1/f = 1/v + 1/u. Therefore, 1/f = -1/60 - 1/20 = -4/60 = -1/15. Hence, f = -15 cm. C…