A 10.00 cm high object is placed 15.00 cm in front of a concave mirror. If the image is 25.00 cm hig

JAMB Physics 2023 308 views
Source: JAMB · 2023

A 10.00 cm high object is placed 15.00 cm in front of a concave mirror. If the image is 25.00 cm high and virtual, what is the focal length of the mirror?

  1. 25.00 cm ✓
  2. 10.71 cm
  3. 18.24 cm
  4. 15.00 cm
Explanation

Magnification = 25 ÷ 10 = 2.5. A virtual image has v = −2.5u, so v = −37.5 cm when u = 15 cm.

Mirror formula: 1/f = 1/u + 1/v = 1/15 − 1/37.5 = (2.5 − 1)/37.5 = 1.5/37.5.

So f = 37.5 ÷ 1.5 = 25.00 cm.

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