What Makes Mirrors Reflect?
Perhaps you've been in a situation where you haven't had a mirror on hand and have resorted to using the most reflective surface around you. Depending on the color, shape and texture of the surface, it may have sufficed, but mirrors certainly have a distinct reflective quality most other materials do not. This stems from the unique color, composition and smoothness a mirror has.
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Starting With Metal
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Polished, shiny metals make good mirrors because metal conducts electricity well. Since the electronic field inside the metal must be zero, electrons there will always cancel out a field that is not zero (even if the field originates outside the metal). Since light travels in electromagnetic waves, when it hits a mirror (most often made with sprayed silver and glass), the only way to cancel out the field and set it to zero is to reflect those waves back out, hence a reflection. This process is similar to swinging a long rope attached on one end. If you give a hanging rope with one loose end one, big shake, the rope will wave to the top, then back down. This is what happens when light hits a mirror.
How Color Affects Reflection
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Some molecules hold light and convert some of it to heat. These materials are usually black. White materials have molecules that almost immediately release light after absorbing it. There is an entire range of absorption in different colors. Metal works well for mirrors because it reflects visible light on all parts of the surface at the same time. Silver works well in particular because it's the closest to white and reflects a variety of colors better (copper and gold would not reflect blue well, for example).
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How Smoothness Affects Reflection
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While rough surfaces do reflect light (depending on color and composition), they typically reflect light in all directions. You can see this in concrete, for example. It seems to sparkle because it reflects light, but not in one direction or ordered fashion. Mirrors, however, do reflect in one direction. Because metal (including metal paint) is smooth, it's the best material for mirrors. Mirrors that are warped or not completely smooth give distorted images.
Concave Mirrors
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When a mirror is curved inward, it is called a concave mirror. This type of mirror allows light to hit it at an angle. The light is then reflected back at a different angle (the angle of reflection). In the case of concave mirrors, those angles are stretched, so that an image seems tall and stretched.
Convex Mirrors
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This type of mirror follows the same principles as a concave mirror, but light will hit it at a narrower angle, and so the angle of reflection will be narrower. This tends to condense and squish the image. Where concave lenses are used for viewing wide angles, convex lenses are used for narrow focus.
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References
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- Photo Credit mirror image by Patrizier-Design from Fotolia.com
