The Spectrum of Light


Does this look familiar?

People think of rainbows as a symbol of happiness and fortune. There are even myths that leprechauns hide gold at the end of a rainbow. That’s more of a tease than good fortune, if you ask me, because it’s impossible to reach the end of a rainbow.

That’s right. Impossible.

Some people wonder if rainbows look the same from the back. The answer’s no. They don’t. You wouldn’t see a rainbow if you were standing behind it.

Whoa…why would that be?

It’s because a rainbow is not a physical thing. It’s a trick of the light, the astronomer’s most valuable tool.

When you look at a rainbow, you’re seeing waves.


No, not ocean waves…

Light waves. But as a matter of fact, light waves are very much like ocean waves. They both look, well, wavelike.

Here’s a drawing of ocean waves…


See how they’re…well…wavy?

I guess you already knew that. The word “wavy” comes from waves, and we use it to describe things all the time. You know what a wave looks like, no matter if it’s made of water or not.

Light waves really aren’t that much different from water waves. In fact, they both carry energy—not just light waves. The only thing that makes them different is that with light, you’ve never actually seen its wavy shape.

light waves.jpg

Well, now you have.

This is literally what light waves look like. They’re…waves. Nothing else to it.

Almost nothing.

Thing is…light likes to make life difficult on us. And it can’t decide whether to act like a wave…or like a particle.


light prism refracted.gif

Uh…yeah. Here’s an image of light getting bent as it travels through a prism. This is something called refraction, and we’ll talk more about it in later posts. For now, just notice one key thing…those are definitely not waves.

They’re photons. A photon is basically a bundle of waves.

But I digress…

What if I told you that light is radiation?

Gosh, please don’t go dive under the covers to hide from the light. Light isn’t going to hurt you. People think of radiation as something dangerous, but it’s really just anything that spreads outward from a source.

There are dangerous forms of radiation, like ultraviolet rays and x-rays. Ultraviolet rays are the kind that give you sunburns after a long day at the beach.

Light itself—or, to be specific, visible light—is a very small portion of what we call the electromagnetic spectrum.

electromagnetic spectrum.jpg

And here it is, in all its glory. Everything we can ever know about the universe beyond our solar system depends on this little spectrum of radiation.

When you look through a telescope, you see visible light. That’s all the colors of the rainbow. But since visible light is such a tiny portion of the electromagnetic spectrum, we’ve made instruments that can detect the other forms of radiation, too.

Are you surprised to see radio waves up there? Those are a form of sound, right?

Wrong. If radio waves were sound, then communication on the moon wouldn’t work.


Imagine standing within mere feet of one another…and not being able to hear a sound.

The moon has no atmosphere. Its surface is a vacuum. Sound is a type of wave, like light, but it can’t travel through a vacuum. Astronauts can use the radio to talk to one another because radio waves are part of the electromagnetic spectrum.

But your home radio emits noise, you say? You don’t see your radio glowing from emitting radiation, do you?

Well…doesn’t it heat up?

Try it. Touch it in different places while it’s turned on. I’ll bet you anything it heats up, just as computers and other machines do when they’re working. Radiation emits energy, which we feel as heat. And radio waves aren’t even close to visible light.

Oh, also, your radio emits noise because it translates the radio waves into something you can hear. So yeah, you’re “hearing” electromagnetic radiation. Is that cool or what?

So, now that we’ve established the different types of radiation…for goodness sake, how do we tell the difference?

Visible light is obvious. But all the other waves…it’s heat or it’s heat or it’s heat. I don’t get it.

The wavelength changes.

The diagram of the electromagnetic spectrum above shows this, but I’ll replicate it for you here:


Here’s a couple visible light wavelengths and a couple invisible radiation wavelengths. Notice here that the shape of the wave matters. Those numbers—700, 400, etc—tell you the distance between the crests of a wave.

What are the crests of a wave? The same you would expect from water waves. The tops. The highest points.

Likewise, the lowest points are called troughs, which makes sense, given what a trough is in the first place. And the distance from trough to trough will be the same as the distance from crest to crest.

Technically, it doesn’t matter if you measure the wavelength from crest to crest, trough to trough, or any other point on the wave to an equal point on the wave…but we say that we measure it from crest to crest. It’s just easier to remember that way.

Notice that blue light has a shorter wavelength than red light. Ultraviolet and x-rays have even shorter wavelengths, and this explains why they can burn. Because, get this…

The shorter the wavelength, the greater the energy.

And what’s energy again? You can think of it as heat. Technically, it also manifests as visible light…but not always. There’s always heat involved. This is the danger of ultraviolet rays. They’re invisible, undetectable to human eyes. And yet they burn.

It’s why you have to put sunscreen on if you spend a lot of time in the sun. Sunblock blocks ultraviolet rays.

Ever gotten an x-ray taken?


They’re super useful. They let your doctor see through things that would otherwise be completely opaque—like your skin.

Imagine walking around all day if your skin was see-through?

The thought makes me shudder.

And yet, that’s a problem for doctors. They need to see what’s going on inside you. X-rays solve that problem.

But that introduces another problem to the equation…x-rays have a lot of energy. They have the most energy of almost any other wavelength. And that means they’re even more dangerous than ultraviolet rays.

If ultraviolet rays—aka sunburns—can eventually cause cancer, then yes, so can x-rays.

That’s why you have to wear a cool smock, which works like sunblock—but for x-rays. And your doctor always steps out of the room.

Coming up, we’ll talk about how telescopes gather light.

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