If a concave mirror has focal length 20 cm and object is at 15 cm (inside focus). Image is:

Real at 60 cm, the result if the object were instead at 24 cm

Real at 12 cm, the result if the object were instead at 30 cm

No image formed, even though a virtual image always forms inside f

If a concave mirror has focal length 20…

Answer: Virtual behind mirror at 60 cm.

A Real at 60 cm, the result if the object were instead at 24 cm
B Virtual behind mirror at 60 cm
C Real at 12 cm, the result if the object were instead at 30 cm
D No image formed, even though a virtual image always forms inside f

Correct answer: B. Virtual behind mirror at 60 cm

Explanation: u=-15, f=-20. 1/v = 1/f - 1/u = 1/(-20) - 1/(-15) = -1/20+1/15 = -3/60+4/60 = 1/60. v=+60 cm. Virtual behind mirror.

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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If a concave mirror has focal length 20 cm and object is at 15 cm (inside focus). Image is:

Concept context

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