Practice portal › Ray Optics and Optical Instruments › Reflection by Plane and Spherical Mirrors
Asked in GUJCET 2007 · Mirror formula and magnification
Given: concave mirror, f = 20 cm, object 40 cm in front, so in the Cartesian convention f = -20 cm and u = -40 cm.
Idea: 40 cm = 2f, so the object sits at the centre of curvature R = 2f = 40 cm.
Mirror formula 1/v+1/u = 1/f gives 1/v = -1/(20)+1/(40) = -1/(40), so v = -40 cm.
A negative v here means the image is in front of the mirror, where light really converges, so it is real.
Magnification m = -v/u = -(-40)/(-40) = -1: same size, and the minus sign says inverted.
So the image is real, inverted and of the same size as the object.
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