Question

Draw a ray diagram to illustrate the formation of an image by a concave mirror when the object is placed at the focus.

Answer 1

Object at the Focus of Concave mirror

1. The above fig shows the object PQ placed at the focus. As usual, ray I is an incident ray parallel to the axis.
2. The reflected ray 1' passes through the focus F. Ray 2 is a second ray incident on the mirror from the point Q.
3. Ray 2 appears to pass through the centre of curvature C so that the reflected ray 2' coincides with the incident ray 2.
4. The two reflected rays I' and 2' constitute a parallel beam and meet inverted, highly enlarged, and at infinity.
5. The fact that the image formed is at infinity is expected on the basis of the definition of the focus itself.