An object is placed in front of a concave mirror of focal length 18 cm, at a distance of 12 cm. The magnification produced by the mirror is ________.
- A.-0.6
- B.+3
- C.-3
- D.+0.6
Correct answer
Explanation
The correct answer is B, +3. The object stands between the pole and the focus of the concave mirror, so the image formed is virtual, erect and enlarged, and the magnification is positive. Using the sign convention, u is -12 cm and f is -18 cm. From the mirror formula, 1/v + 1/u = 1/f, we get 1/v = 1/f - 1/u = -1/18 + 1/12 = 1/36, so v = +36 cm, an image formed 36 cm behind the mirror. Magnification m = -v/u = -36 divided by -12, which is +3, so the image is three times the size of the object. This is exactly why a concave mirror is used as a shaving mirror and by dentists. A is wrong because a negative magnification means a real and inverted image, which a concave mirror gives only when the object lies beyond the focus. C is wrong for the same reason. D is wrong because an image smaller than the object is never formed in this position. Exam tip: an object inside the focus of a concave mirror always gives a virtual, erect and magnified image.
Practice Questions
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- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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- C.Urea is produced from digestion of starch.
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- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
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- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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