Radiation

Activity: The radiation spectrum

https://www.youtube.com/watch?v=hPJPbbT1TsM

The story so far...

One of the chief ways in which heat energy moves is in the form of waves. Sources of heat, such as the sun or a fire or a stove, send out these waves in all directions, as if each wave where the "spoke" or - to use the Latin word - the radius of a wheel. This is why this third kind of heat transfer is called radiation.

And now...

The radiation spectrum

In warm weather, you seem to feel hotter if you're wearing black than if you're wearing white. Is this just your imagination, or are black things really warmer than white things? What could colour have to do with heat? What is colour, anyway? Why are tomatoes red? Or oranges orange? Or buttercups yellow? Or green beans green? Or blueberries blue? Or violets violet? Where do all these colours come from? 

They come from light itself. White light contains all the colours. See for yourself. Go into a dark room. Open a chink in the blinds, and let a narrow beam of light in. Now hold a glass prism in the light beam. Look at the colours: red, orange, yellow, green, blue, violet. That's what white light's made of.

When it shines all these colours on a tomato, the tomato absorbs each of the colours - except red, which it reflects; that's why tomatoes look red. 

Oranges do the same. They absorb all the colours - except orange, which they reflect; so oranges look orange. 

Buttercups only reflect yellow, so they look yellow. The same thing applies to green beans... blueberries... and violets. 

Now if you mix all these colours together again you get white light once more, because white is simply a combination of all the colours. 

Your white suit appears white because it doesn't absorb any of the colours - it reflects them all. On the other hand, your black suit appears black because it absorbs all the colours. It doesn't reflect any of them. Black is simply the absence of any colour.

But what's all this got to do with black being warm, and white being cool?

Well, when the sun radiates waves of heat energy, these waves come in many different forms, which make up a whole band, or spectrum, of energy waves, from radio waves at one end, to x-rays and gamma rays at the other.

Some of this heat energy is visible. These are the light waves made up of all the different colours. But these only take up a little bit of the radiation spectrum. The rest of the spectrum isn't visible at all.

There is, for example, some invisible radiation just beyond, or beneath, the red. Since the Latin word for beneath is "infra", this is called infrared radiation.

Now most of the sun's radiated energy comes to us in the visible and the infrared parts of the spectrum, and that's why black is warm: because it not only absorbs the heat energy of the visible colours, but in doing so it also tends to absorb a lot of the heat energy of the invisible infrared radiation as well.

Conversely, white is cool, because it not only reflects the heat energy of the visible colours, but in doing so it also tends to reflect a lot of heat energy of the invisible infrared radiation.

So now you know that colour really does have something to do with heat - it isn't just your imagination. You also know what lies at the end of the rainbow. It isn't a pot of gold - it's infrared radiation!