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The awesome physics behind how mirrors work

Don't believe your eyes.

DAVID NIELD
5 FEB 2016
 

With mirrors around us every day, we tend to take them for granted, but what exactly is happening on a scientific level when we peer into one? And on that note, do you know the physics involved in why we see a mountain range reflected in the clear, calm lake below? 

Essentially, a mirror is made up of a shiny piece of extremely smooth metal, kept in place with a glass front and a thin layer of backing (usually aluminum). Key to the way a mirror functions is how the physics of light behave in our Universe: the same laws that make a banana appear yellow and a piece of paper appear white.

 

The colour of something is defined by which colours of the visible spectrum it absorbs or reflects. Our aforementioned banana, for example, absorbs every colour except yellow - so the yellow light returns to our eyes (unless there's no light, in which case the banana is as black as everything else). White objects, meanwhile, reflect all the colours of the visible spectrum, and thus appear colourless.

The metals inside mirrors perform the same trick, reflecting all the colours of the visible spectrum, but the difference is they're ultra-smooth on a microscopic level. A piece of paper might seem smooth to you, but it's not even in the same smoothness league as a mirror, and that's how a mirror image is formed: all of the light is bouncing straight back in the direction it's just come from.

Anna Green over at Mental Floss uses the analogy of a bunch of tennis balls thrown at a wall, which will usually bounce back in the same direction they came from. Try the same exercise on a craggy rock face, and the balls will spin off in all kinds of directions. A metal and glass mirror is the wall and the balls are the beams of light hitting our eyes.

The same effect is happening when ripples hit a pond - the surface is no longer flat, the light is no longer bouncing straight back, and you can no longer see your face when you peer into it. Ever since we've been able to perfect the manufacture of mirrors, they've become useful in science, transportation and many other fields.

And if you're wondering why mirrors flip the image they see, well... actually, they don't. It's we who are flipping the image when we put on a t-shirt, turn a sign around, or hold up our hand. All the mirror is doing is reflecting exactly what's placed in front of it, without any flipping taking place at all. For more on that, watch this:

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