<P> When light reflects off a mirror, one image appears . Two mirrors placed exactly face to face give the appearance of an infinite number of images along a straight line . The multiple images seen between two mirrors that sit at an angle to each other lie over a circle . The center of that circle is located at the imaginary intersection of the mirrors . A square of four mirrors placed face to face give the appearance of an infinite number of images arranged in a plane . The multiple images seen between four mirrors assembling a pyramid, in which each pair of mirrors sits an angle to each other, lie over a sphere . If the base of the pyramid is rectangle shaped, the images spread over a section of a torus . </P> <P> Note that these are theoretical ideals, requiring perfect alignment of perfectly smooth, perfectly flat perfect reflectors that absorb none of the light . In practice, these situations can only be approached but not achieved because the effects of any surface imperfections in the reflectors propagate and magnify, absorption gradually extinguishes the image, and any observing equipment (biological or technological) will interfere . </P> <P> In this process (which is also known as phase conjugation), light bounces exactly back in the direction from which it came due to a nonlinear optical process . Not only the direction of the light is reversed, but the actual wavefronts are reversed as well . A conjugate reflector can be used to remove aberrations from a beam by reflecting it and then passing the reflection through the aberrating optics a second time . </P> <P> Materials that reflect neutrons, for example beryllium, are used in nuclear reactors and nuclear weapons . In the physical and biological sciences, the reflection of neutrons off of atoms within a material is commonly used to determine the material's internal structure . </P>

Flat surface in nature that has reflection symmetry