Any reflective material can act as a mirror, because it will throw back enough light to form an image of its surroundings. The surface may be a plane, concave or convex. Planar or flat mirrors present a virtual image that reverses the object being reflected. Curved surfaces act more like lenses, without the aberrations to which glass lenses are prone. Nowadays such surfaces can be made from glass, metal, or plastic, often with a thin metal coating. Prisms can also act as mirrors.
Mirrors create a virtual image, which you can easily see in a hallway mirror, but they also produce a real image, which may not be detectable with diffuse lighting. If you place a single, bright filament bulb in front of a mirror, a reflection can be cast on a screen held between the two, a little above the bulb. The image of the bulb may seem to disappear once you remove the screen. If you keep looking at the space it once occupied and move away from the direct path of the light, however, you will be able to see the real image floating in that space.
Convex spherical mirrors, or fish-eyes, are used as rearview mirrors on cars or trucks. These form only a virtual image within the body of the mirror. A concave version is more common on magnifying makeup mirrors. Conic mirrors are parabolic or elliptic in shape, and will reflect perfect images from a certain distance only. Mirrors are also used as solar collectors and to focus military searchlights. In a Newtonian telescope, a conic mirror reflects an image backwards onto a retractable plane mirror, so the observer does not block any of the incoming light rays. Cassegrain telescopes are made from a pair of mirrors, one hyperbolic and the other paraboloid. The Cassegrain system provides a long focal length without taking up much space. Both these telescopes aid the detection of very distant stars.
In the age of the Hubble Telescope, the mirrors for such giant structures often use as much as 20,000 lb (9,000 kg) of a Pyrex-like glass called borosilicate. Such a vast stretch of material, 7 yd (6.5 m) across, cannot be ground normally. Surface irregularities could too easily develop. The material is melted and whirled in a huge circular furnace until it settles into shape. Afterwards the exterior is polished by a computer-programmed machine, to avoid human error as much as possible.
Mirrors have not changed structurally in thousands of years, since the ancient Egyptians used the first known metal hand mirrors. The so-called "true mirror," which reflects a person's image without inverting it the way an ordinary looking-glass does, is actually made of two plane mirrors joined at a 90° angle to reflect a fused image. You can still make your own mirror for a small telescope by grinding two flat discs of glass together until the top piece becomes concave and the lower one convex. Both discs can then be finished separately with minute polishing.
See also Lens.