A parabolic mirror is a specially shaped object designed to capture energy and focus it to a single point. It may also work as a way of distributing energy from the focus point back outwards. Parabolic mirrors may also be referred to as parabolic dishes or parabolic reflectors.
Parabolic mirrors are a specific type of paraboloid, known as a paraboloid of revolution. This is a type of elliptical paraboloid that is rotated around its axis and may also be referred to as a circular paraboloid.
One of the earliest uses of the parabolic mirror was in Isaac Newton's reflecting telescope of the 17th century. By using a parabolic mirror, reflecting telescopes correct some of the aberrations which existed in older refracting telescopes. With the use of parabolic mirrors, however, some other problems are introduced. This includes a problem called coma, which exists in all telescopes using parabolic mirrors. Coma causes any objects viewed through the telescope which are not at the center of the field of vision to look slightly wedge-shaped. The further outside the field they are, the more distorted they appear.
Parabolic mirrors are usually made of a low expansion glass, similar to Pyrex glasses. The mirrors are kept as fine as possible to reduce distortion in the image. The processes used to produce extremely high-end parabolic mirrors can take months and cost thousands of dollars.
Aside from amateur telescopes, many people have interacted with a parabolic mirror in the form of a popular optical illusion toy. This small pans has two parabolic mirrors attached to one another and a hole in the top to allow placement of a small object. When an object is placed between the two parabolic mirrors, it appears that the object is in fact resting in the air a few inches above where it actually is.
During the world Olympics, the flame used for the Olympic torch is lit using a large parabolic mirror. This parabolic mirror collects ambient sunlight and focuses it to an intensity sufficient to ignite the torch material.
A most likely apocryphal tradition has it that parabolic mirrors have been used in the past as a way of gathering sunlight to spontaneously ignite enemy ships, or to heat up the armor of the enemy to a point where they were forced to remove it.