Polished metal is a very good reflector, but there is another, surprising answer: bubbles. Imagine a bubble in water. The index of refraction of water is 1.33, and air is 1.00001 or so. The surface of the bubble is curved, of course; and the light rays, traveling from water to air, are refracted away from each other. As you move a light ray from the centre of the bubble toward the edge, you eventually pass the critical angle, where the refracted light ray is bent 90° from the surface normal. Beyond that, there is total internal (or external, depending on your viewpoint) reflection and the bubble appears to be a perfect curved mirror. You can explore this using free software called POV-Ray, the Persistence of Vision Raytracer, available from http://www.povray.org/ . It's an optical simulator that uses math to replicate the behaviour of light.