Question

State the principles of image formation in the case of a spherical mirror. Use this principle to draw a ray diagram to illustrate the formation of an image by a concave mirror of an object placed halfway between the centre of curvature and the focus

Answer 1

Rules for Reflection of Light from a Concave Mirror:

1. A ray incident parallel to the principal axis passes through the principal focus after reflection.
2. An incident ray passing through the centre of curvature is reflected back along the same path.
3. A ray of light incident at the pole of a concave mirror and makes an angle (O) with its principal axis is reflected back making the same angle (6) with the principal axis.
4. An incident ray passing through the principal focus of the mirror becomes parallel to its principal axis after reflection.

Ray diagram to illustrate the formation of an image by a concave mirror of an object placed halfway between the centre of curvature and the focus

1. The above Fig. shows object PQ lying in between the focus and the centre of curvature C. Again ray 1 is an incident ray parallel to the principal axis and reflected ray 1' passes through F.
2. The second incident ray 2 from point Q of the object is one that appears to pass through the centre of curvature C of the mirror.
3. The reflected ray 2' is coincident with the incident ray but travels in a direction opposite to the incident ray. In other words, ray 2 retraces its path as ray 2'.
4. The two reflected rays actually meet at point Q', which is the image of point Q on the object. Clearly P'Q' is the image of the object PQ.
5. The image formed is real, inverted, enlarged and lies beyond the centre of curvature C, as shown the fig.