Concave Mirror
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Concave mirror is a spherical mirror whose reflecting surface is towards the center of the sphere of which the mirror is a part.
Convex Mirror
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Convex mirror is a spherical mirror whose reflecting surface is away from the center of the sphere of which the mirror is a part.
Concave and Convex Mirror
Let us now define certain physical terms relating to spherical mirrors.
Center of Curvature
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Center of Curvature is the center of the sphere of which the spherical mirror forms a part. It is denoted by the letter C.
Center of Curvature
Radius of Curvature
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Radius of Curvature is the radius of the sphere of which the mirror is a part. It is represented by the letter R.
Radius of Curvature
Linear Aperture
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Linear aperture is the distance between the extreme points (X and Y) on the periphery of the mirror.
XY is the Aperture
Pole
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Pole is the midpoint of the aperture of the spherical mirror. It is represented by the letter P.
Midpoint of xy
Principal Axis
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Principal axis is the straight line passing through the pole and the center of curvature of a spherical mirror.
Principal Axis
Secondary Axis
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Secondary axis is any other radial line passing through the center of curvature other than the principal axis.
Secondary Axis
Normal
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The normal at any point of the spherical mirror is the straight line obtained by joining that point with the center of the mirror. The normal at point A on the mirror is the line AC obtained by joining A to the center of curvature of the mirror. Normal at any point on a spherical mirror is equal to the radius of the sphere of which the mirror is a part.
Normal
Principal Focus or Focus
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The rays of light parallel to the principal axis of a mirror after reflection, either pass through a point (in case of a concave mirror) or appear to diverge from a point (in the case of a convex mirror) on the principal axis and this point is referred to as the principal focus or focal point of the mirror.
The principal focus of a spherical mirror may be defined as a point on its principal axis where a beam of light parallel to the principal axis converges to or appears to diverge from after reflection from the spherical mirror.
Focal Length
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Focal length is the distance between the pole and the focus of a mirror. It is represented by the letter f.
Characteristics of Focus of a Concave and a Convex Mirror
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| Convex Mirror | Concave Mirror |
| The focus lies behind the mirror | The focus is in front of the mirror |
| The focus is virtual as the rays of light after reflection appear to come from the focus | The focus is real as the rays of light after reflection converge at the focus |
Note : A concave mirror is also known as a converging mirror as the parallel rays of light after getting reflected from the concave mirror converge at the focus.
A convex mirror is known as a diverging mirror as the parallel rays of light after reflection appear to come from a point, i.e., the rays diverge after reflection.
