Question

# Define: (i) Focus (ii) Focal length, for a spherical mirror. If the focal length of a convex mirror is 20 cm, find its radius of curvature.

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Solution

## Focus: Focus of a concave mirror is a point on the principal axis at which the light rays incident parallel to the principal axis converge after reflection from the mirror. Focus of a convex mirror is a point on the principal axis at which the light rays incident parallel to the principal axis, appear to diverge from after reflection from the mirror. Focus is represented by letter 'F'. Focal length: The distance of the focus from the pole of the mirror is called the focal length of the mirror. F=R2 Given, for the convex mirror: F=20 cm ⇒R=2×F ⇒R=2×20=40 cm

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