A mirror which has reflecting surface in the shape of parabola is called parabolic mirror. As the mirror can converge accurately a wide parallel beam of light at the focus,parabolic mirrors are used to increase the intensity of reflected beam. Such beams reflected by parabolic mirrors remains undiminished to a long distance. Parabolic mirrors are free from spherical aberration and hence don’t form caustic curve.
Need for parabolic mirror
If a fine beam of light is incident on a spherical mirror of small aperture, the rays of light meet at a single point after reflection to form the image of a point object. However, if the mirror is large and the beam of light is wide, the light rays do not focus at a single point. They cross each other in the different points in the principal axis. The rays near the axis are called parallax rays and are focused near the pole. The rays towards the margin are called marginal rays and are focused farther from the focus. Hence these rays intersect each other at various points which lie in a curve called caustic curve as shown in figure. The caustic curve has a cusp at the focus of the mirror.
Conversely, if a point source of light is placed at the focus of spherical mirror, all the rays reflected by the mirror wouldn’t be parallel to each other and the intensity of the light decreases.
This kind of defect in spherical mirror is spherical aberration. To make our ‘devices’ free from such defect parabolic mirrors are designed.
Working of parabolic mirror
Parabolic mirrors are so constructed that each and every beam coming from infinity falling in mirror are brought to be focused in the principal focus of the mirror. In this way, parabolic mirrors remove spherical aberration in spherical mirrors.
Similarly, if a point source of light is placed in the focus of the parabolic mirror, the reflected rays are parallel to principal axis. So they can travel a long distance along the principal axis.
Uses of parabolic mirror
- They are used to make search light, in automobile headlight, etc.
- Cinema projectors use parabolic mirror to use them as reflectors.
- Scientists and astronomical companies use parabolic mirrors in optical devices like telescope.