Practice portal › Ray Optics and Optical Instruments › Reflection by Plane and Spherical Mirrors
Asked in GSEB Board August 2020 · Mirror formula and magnification
Given: concave mirror of radius of curvature 15 cm, object 10 cm in front, so u=-10 cm.
Idea: get the focal length from f=R/2, then v from the mirror formula, then m=-v/u.
f=-(15)/2=-7.5 cm, negative for a concave mirror.
1/v=1/f-1/u=1/(-7.5)+1/(10)=-1/(30), so v=-30 cm.
The minus sign puts the image in front of the mirror, so it is real.
m=-v/u=-(-30)/(-10)=-3
The object sits between the focus at 7.5 cm and the centre of curvature at 15 cm, so a real, inverted, three-times enlarged image is exactly what to expect.
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