Mirror: an object that reflects light or radio waves.

Ordinarily, mirrors are metallized surfaces smooth enough to reflect electromagnetic waves. They are important tools in the exploration of infinity. For one thing, in mirror and radio telescopes they enable us to collect information from distant regions of the universe. For another, if arranged in certain ways they can yield a visual representation of infinity.

Since mirrors are easier to produce than lenses and, moreover, do not introduce any color distortions, all large telescopes are designed as mirror telescopes. Recent earthbound mirror telescopes make use of adaptive optics to balance any image distortions that may arise when the light is making its way through the atmosphere. This technology is based on computer-controlled adjustment mechanisms placed underneath the mirror surface; these bend one or another region of the surface slightly in response to atmospheric changes, producing an even overall image.

In addition to astronomy, mirrors can be used for playful experiments with infinity. If you place yourself between two exactly parallel mirror surfaces — using a hand mirror and the mirror in your bathroom, for example — you can gaze into an infinite corridor.* A square box with vertically mirrored inner walls will place you on an infinite plain. If you go so far as to furnish your ceiling and floor with mirrors, you will float in an infinite space next to, above, and underneath an infinite number of mirror images of yourself.

* A similar infinity effect can be achieved by means of a video camera that is connected to a TV set and at the same time directed toward it. In this case we also, via recursion, experience certain temporal phenomena such as a rhythmic flickering of the image.

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