What is color?

This year's Flame Challenge was to explain what color is. Never heard of the Flame Challenge? It's a really cool contest started three years ago by the Alan Alda Center for Communicating Science. Yes, Alan Alda from M*A*S*H the tv show. The contest asks scientists to explain, at the level of an 11-year-old but without oversimplifying, a basic question. The first year asked "What is flame?" and the second year asked "What is time?". I really recommend you go watch the winning videos--they are awesome.

I would have loved to enter the contest this year, but learned about it too late. The written entry had a word limit of 300 (WAY too short), while the video had a 6 minute limit (into which you can cram more like 1000 words). Next year, I'm going to look for it early so I can actually through together a video.

But I thought there was no reason to do a blog post on the topic!

What is color? Well, that kinda depends on whether you are talking about colored light or a colored object. Because while the answers are similar, they aren't the same. Let's start with what makes colored light, and an analogy!

You've all seen (and probably been forced to learn at some point) a musical instrument. I learned to play clarinet at an earlier point in my life, so I'll use that for this analogy. You know that if you hold certain keys down and blow into the clarinet  (thus vibrating the column of air in said instrument) you get a certain note. If you hold down different keys, you'll get a different note. But unless your clarinet is WILDLY out of tune, you will never ever get a middle C out of the soprano F fingering. What's happening is that by holding some holes open and others closed you are causing the column of air to vibrate at a different frequency, which we perceive as musical notes.

While sound is vibrations in air, light is vibrations in  the electromagnetic field created by electrons jumping around in an atom. It works like this. All atoms have distinct energy levels that their electrons are allowed to inhabit. The electrons are not allowed to be any where but those energy levels.

You may  be on any level you have the energy to reach, but never anywhere in between

Normally, electrons exist in their ground state, or lowest energy level. To get to the next level, they need an extra kick of energy.

Once they are up there though, they can't stay for long. Very few of the non-ground state energy levels are stable, so the electron only gets to hang out a short while before it needs to fall back to the ground state. Only problem is it can't exist in the ground state with this extra energy, which it conveniently gives off as a photon, or a burst of vibrations, in the electromagnetic field. How much energy it needs to give off, which is determined by how far it needs to 'fall', tells you at what frequency it vibrates. Each frequency has its own color attached to it. So if you only have one type of atom, you are only going to be able to get a few colors, or spectral lines.Your eyes don't see them as individual lines, but as a blended single color, just like your ears don't hear individual notes in a chord.

How an non-light-source object  has a color is different. Lets imagine we have a light bulb that gives off white light. That is, it has electrons giving off photons of many different wavelengths across the whole spectrum, which we perceive as white. You could think of this like an orchestra warming up. You just hear a cacophony of noise; to our eyes, a 'cacophony' of light looks white. Then that light strikes an object--lets say a red brick fireplace. All the photons of every color imaginable strike the brick. All the photons that are purple, indigo, blue, green, yellow and orange get absorbed by the atoms in the brick. They cause the atoms to vibrate and if you shone enough light on the brick for long enough, the bricks would get hot (think about a brick patio in the summer time). The red photons don't get absorbed though. They aren't the right frequency to be absorbed, so they get spat out in all directions. The photons that happen to get spat out in the direction of your eye strike your retina, and your brain goes "Hey! That thing is red!".

And that's how we get things with color. If its a light bulb, it's a particular color because that's the color (frequency) that it's electrons give off vibrations at. If it is just a plain old object, it's a particular color because its electrons don't vibrate at that color.

Isn't physics fun? Or should I say, phun?