WHAT COLOR IS THAT?

(http://www.webexhibits.org/causesofcolor/3.html)

Let’s go back to our friend, the Sun. How would you answer this question – what color is the Sun?

Think about it for a moment… ask a friend.

What do you think?

Well, it doesn’t really have ONE color. Let’s explore this idea. You have probably heard of prisms and you’ve probably seen rainbows. What do these things have in common? (That’s a tricky little question, by the way.)

The answer is a little strange. First, some background. Light, as you may know, travels at a constant speed, 3 x 10^8 m/s (186,000 miles/second).

c = 3 x 10^8 m/s

Note that the letter c is used to denote “constant”, though some prefer to think of it as c for celerity (aka speed).

Now, that’s not just fast – it’s FAST! As in….

F A S T ! ! !

That’s nearly 7 times around the Earth’s equator in one second, or up to the Moon and back in around 2.5 seconds. FAST. In fact, that’s as fast as anything could ever be – to make things weirder, the only things that can go that fast are things with NO mass – in other words, light. Plus, it’s not just one type of light that travels that fast – it’s ALL types of electromagnetic radiation, visible and otherwise. So, that means radio waves, microwaves, infrared light, ROYGBV, ultraviolet, x-rays… All travel at the same speed – IN A VACUUM. That’s the important part.

I’ll use visible light as an example, but this applies to all electromagnetic waves. Light travels at a constant speed in a vacuum, yes, but vacuums are hard to come by. If light enters something that is NOT a vacuum its speed changes (lowers) – sometimes a lot, sometimes a little, but always. Mind you, even dropping by 50% (as light’s speed does in many types of glass) keeps the speed still extremely fast (1.5 x 10^8 m/s). Basically, half of really fast is still really fast!

So, what does this have to do with the color of the Sun? Light from the Sun comes in many different wavelengths – each particular wavelength corresponding to a color (in the case of visible light) or if you prefer, an energy (in the case of all of the types of electromagnetic radiation). The Sun emits virtually every possible wavelength of electromagnetic radiation in some amount.

How do we know this?

For this part of the experiment, you will need a prism and the Sun – make sure to call ahead to see if the Sun is available.

What do you expect to see when your prism is brought into the open sunlight? Discuss with your friend before trying it.

So, what do you see? Is it what was expected? What can this mean?

If you have a regular piece of thick glass (not a prism), try it again. Does it have the same effect? Why?

As noted above, light travels at a constant speed in a vacuum. But when NOT in a vacuum, the story changes a little. Light travels at a speed that depends entirely on the medium – its so-called optical density (usually embodied in a term called “index of refraction”) – relative to light’s wavelength. In other words, different wavelengths of light travel at different speeds in a prism (or whatever it’s going through) and, therefore, emerge at (slightly) different times. Another way to think about this: different wavelengths are slowed down in matter by amounts that depend on the relative sizes of the wavelengths and atomic size of the medium.

A natural consequence of this slowing is the bending of the light as it exits the prism. Since violet is slowed more than red, red emerges first. But why the bending? Here is a common analogy. Think of light as you would a car on the road – if you start to go off the road to the right, maybe going from pavement to dirt, your right tires will experience different friction than the left tires. As a result, the tires on the left will have a different speed than those on the right – naturally, the car will turn somewhat. The story is similar for light. Different resistance means different speed and different amounts of bending.

So, back to our friend, the Sun. What color is it? If you said yellow or orange, you’re somewhat right. The truth is, it emits light of many colors – virtually every color you can imagine – AND nearly all other types of electromagnetic radiation. That’s right – from the Sun we get radio waves, microwaves, infrared light, ultraviolet light, x-rays and gamma rays. WOW! But why the yellow/orange color?

Why do you think it appears to be this color?

Well, it’s a combination of two things: Yellow/orange is the color given off with the greatest intensity, but also (and very importantly): this color is close to the most sensitive range of human sight. If we could see the Sun with microwave or x-ray sensitive eyes, we would see quite a different object in the sky.

Have a look for yourself. Find images at the Solar Data Analysis Center.

http://umbra.nascom.nasa.gov/images/

Now, the Sun doesn’t emit all types of electromagnetic radiation in equal amounts. In fact, it is close to being what physics types call a blackbody emitter.

(See image above.)

This curve (which is a pretty good representation of the Sun’s output) looks a little strange, doesn’t it? In this case, the peak is in the orange/yellow range. But look! There are plenty of other types of electromagnetic radiation emitted from the Sun, just in smaller amounts. But you know this already, don’t you. For example, why do you wear sunscreen?

That’s all for now. See you again, chromatic friends!

ALL TEXT AND IMAGES COPYRIGHT SEAN LALLY 2009 (except where noted)