Parallel beam from curved mirrors
In connection with the reversibility of light it is worth noting that a narrow parallel beam of light may be obtained from a point source of light placed at the principal focus of a concave mirror of small aperture. We can regard the image in this case as being at infinity. If a wide parallel beam is required as from a car headlamp then the section of the mirror must be in the form of a parabola (Fig. 22.10).