The largest parabolic mirror in existence is in a telescope located in the Caucasus mountains in Russia. It is nearly 20 feet in diameter and was completed in 1967. Heat waves, as well as light and sound waves, are reflected to the focal point of a parabolic surface. If a parabolic reflector is turned toward the sun, flammable material placed at the focus may ignite. A solar furnace produces heat by focusing sunlight by means of a parabolic mirror arrangement.
Light is sent to it by set of moveable mirrors computerized to follow the sun during the day.
In a real image, the light rays actually come from the image. In a virtual image, they appear to come from the reflected image - but do not. Mirage 's 3-D illusions are similar to the holograms formed by lasers. If a right circular cone is intersected by a plane perpendicular to its axis, part of a hyperbola is formed. Such an intersection can occur in physical situations as simple as sharpening a pencil that has a polygonal cross section or in the patterns formed on a wall by a lamp shade.