⚛️ Physics · Ray Optics and Optical Instruments · NEET & JEE

A concave mirror with f=20 cm. At what distance should an object be placed so image is 3 times the object size and real?

Answer: 26.7 cm.

A 26.7 cm
B 30 cm
C 40 cm
D 60 cm

Correct answer: A. 26.7 cm

Explanation: m=-3 (real). -3 = -v/u, so v=3u. 1/v+1/u=1/f: 1/3u + 1/u = 1/(-20). 4/3u = -1/20. u = -80/3 = -26.7 cm. Object at 26.7 cm.

Concave mirror ray diagram with an object drawn as a candle on the principal axis, the focal point F marked, and the parallel ray and focal ray reflecting to form an inverted real image

Concave mirror ray construction: a ray parallel to the axis reflects through F, and a ray through F reflects parallel, locating the inverted real image. Image: Maxmath12, CC0, via Wikimedia Commons.

Concept context

Reflection, refraction, lenses, mirrors, total internal reflection.

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A concave mirror with f=20 cm. At what distance should an object be placed so image is 3 times the object size and real?

Concept context

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