If an object is placed in front of a. spherical mirror, that may be concave or convex, an image of the object is formed in the mirror. To know the position, size and nature of the image, we have to know the direction of the reflected beam of light emitted by an object. We can draw an image in a spherical mirror considering any two of the three rays described below.
1) Rays incident along the radius of curvature returns back along the same path after reflection in a spherical mirror [Figure: 1].
2) Rays incident parallel to the principal axis are reflected through the principal focus in a concave mirror. Rays incident parallel to the principal axis after reflection in a convex mirror appear to diverge from the principal focus [Figure:2]
In a concave mirror, rays incident through the principal focus are reflected parallel to the principal axis. Rays directed towards the principal focus after reflection become paralle to the principal axis [Figure:3].