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When an object is placed in front of a concave mirror, light rays from the object fall on the mirror and get reflected. The reflected rays produce an image at a point where they intersect or appear to intersect. Formation of an image by mirrors is usually shown by constructing ray diagrams. To construct a ray diagram, we need at least two rays whose paths after reflection from the mirror are known. These rays must be chosen according to our convenience. Any two of the following rays can be considered to obtain the image.
- A ray of light parallel to the principal axis after reflection from a concave mirror passes through its focus.
- A ray of light passing through the focus of a concave mirror after reflection emerges parallel to the principal axis.
- A ray of light passing through the centre of curvature of a concave mirror retraces its path after reflection as the ray passing through the centre of curvature acts as a normal to the spherical mirror.
- A ray of light which strikes the mirror at its pole gets reflected according to the law of reflection.
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