A concave mirror forms a real image three times the size of a real object. The distance between object and image is 20 cm. The radius of curvature of the mirror is:
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A concave mirror forms a real image three times the size of a real object. The distance between object and image is 20 cm. The radius of curvature of the mirror is:

G1 Physics · Accepted Answer

For a real inverted image, |v|=3|u|. Since image and object separation is |v|-|u|=20 cm, 3x-x=20 gives |u|=10 cm and |v|=30 cm. Mirror formula gives 1/f=1/(-30)+1/(-10)=-4/30, so f=-7.5 cm. Radius magnitude R=2|f|=15 cm.