What is color? - Colm Kelleher

Watch the whole lesson on http://ed.ted.com/lessons/how-do-we-see-color-colm-kelleher

Have you ever wondered what color is? In this first installment of a series on light, Colm Kelleher describes the physics behind colors-- why the colors we see are related to the period of motion and the frequency of waves.

Lesson by Colm Kelleher, animation by Vitreous.

One of the most striking properties
about life is that it has color.

To understand the phenomenon of color,
it helps to think about light as a wave.

But, before we get to that,

let's talk a little bit
about waves in general.

Imagine you're sitting on a boat on the ocean
watching a cork bob up and down in the water.

The first thing you notice about
the motion is that it repeats itself.

The cork traces the same path over and
over again... up and down, up and down.

This repetitive or periodic motion
is characteristic of waves.

Then you notice something else...

using a stopwatch, you measure
the time it takes for the piece of cork

to go over its highest position down
to its lowest and then back up again.

Suppose this takes two seconds.

To use the physics jargon, you've measured
the period of the waves that cork is bobbing on.

That is, how long it takes a wave to go
through its full range of motion once.

The same information can be expressed
in a different way by calculating the wave's frequency.

Frequency, as the name suggest, tells you
how frequent the waves are.

That is, how many of them
go by in one second.

If you know how many seconds
one full wave takes,

then it's easy to work out
how many waves go by in one second.

In this case, since each wave takes 2 seconds,
the frequency is 0.5 waves per second.

So enough about bobbing corks...
What about light and color?

If light is a wave, then it must
have a frequency. Right?

Well... yes, it does.

And it turns out that we already have a name for the frequency of the light that our eyes detect.

It's called color.

That's right. Color is nothing more than a measure
of how quickly the light waves are waving.

If our eyes were quick enough, we might be able to observe this periodic motion directly,

like we can with the cork and the ocean.

But the frequency of the light
we see is so high,

it waves up and down about
400 million million times a second,

that we can't possibly see it as a wave. But we can tell, by looking at its color, what its frequency is.

The lowest frequency light that we can see
is red and the highest frequency is purple.

In between all the other frequencies form a continuous band of color, called the visible spectrum.

So, what if you had a yellow pencil
sitting on your desk?

Well, the sun emits all colors of light,
so light of all colors is hitting your pencil.

The pencil looks yellow because it reflects yellow light more than it reflects the other colors.

What happens to the blue,
purple and red light?

They get absorbed and the energy
they are carrying is turned into heat.

It is similar with objects
of other colors.

Blue things reflect blue light,
red things reflect red light and so on.

White objects reflect all colors of light,

while black things do exactly the opposite
and absorb at all frequencies.

This - by the way - is why it's uncomfortable
to wear your favorite Metallica t-shirt on a sunny day.