If we have a reflective sphere and a piece of this is cut out, it is a spherical mirror. If the reflective surface is on the side curved inwards, it is a concave mirror. Concave mirrors are commonly used as shaving mirrors or by dentists and even in telescopes. The reflected image is magnified but the field of view is limited. Concave mirrors are also called converging mirrors.
Similarly, if the reflective surface is curved outwards, it is a convex mirror. Convex mirrors are used extensively as part of car mirrors. Depending on the focal length, the reflected image is reduced in size but the field of view is more. This is very useful for drivers as the blind spots are drastically reduced. Convex mirrors are also called fish eye or diverging mirrors. You can recognize these mirrors easily as the outward bulge is quite evident most of the time.
A spherical mirror is always part of a bigger virtual sphere. Imagine a line passing the centre of this sphere which touches the reflecting surface at its centre. This line is called the principal axis.
The point where the principal axis meets the reflective surface has been marked by point in the figure. Point A is the vertex which is the centre of the mirror.
The centre of the sphere, from which the spherical mirror has been cut, is marked by point C in the figure. This is the centre of curvature. The distance from the vertex to the centre of curvature is the radius of curvature, denoted by the length ‘R’ in the figure.
The point which lies midway between the vertex and the centre of curvature is the focal point, marked as ‘F’ in the figure. The length measured by ‘f’ in the figure is called the focal length of the mirror. It is the distance between the vertex and the focal point of the mirror.
Although the figure above is that of a concave mirror, the properties remain the same for convex mirrors as well. The difference between them is based on the way they reflect light rays. There are also nonspherical mirrors like parabolic reflectors. When and why do we use different kinds of mirrors?
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