I'm inside the impressive All Saints' Church,
in Atlanta, but something's missing.
It's the part of the electromagnetic spectrum
that we can see.
The part that makes this... look like this.
(snaps fingers) Color.
It's magical, but there's a lot of science behind it.
Let's find out what color is
and some of the amazing ways it works,
in this segment of "Physics in Motion".
♪♪
(Adrian Monte) When you look at this window, you see an array of colors.
What exactly is color anyway?
It's the part of the electromagnetic spectrum
we can see, called, logically enough,
the visible spectrum.
So, what makes us see particular colors?
Objects absorb some wavelengths of light
and not others.
The ones we don't see are the ones it absorbs,
and the colors that we do see
are those that are reflected back.
The spectrum of color ranges from violet to red,
and each color has a very specific wavelength.
Red has the longest and violet the shortest.
Just beyond our sight, at the red end,
is the infrared region of the electromagnetic spectrum,
and at the other end, the ultraviolet range.
Human sight can perceive wavelengths
from about 390 to 790 nanometers.
A nanometer is pretty short, one billionth of a meter.
How many colors are there in the universe?
It's been estimated that we distinguish between
18 decillion colors.
That's 18 followed by 33 zeroes.
A lot of choice, huh?
You may have been taught that the three primary colors
are red, blue, and yellow,
but those are the primary colors of pigments,
which are substances that add color to materials.
The primary colors of light are red, green, and blue.
But what happens when you mix all three of those?
We get this. White light.
It was that genius of physics
we've heard a lot about in this series, Isaac Newton,
who figured out that combinations of wavelengths
spread out along the visible spectrum make up white light.
So, white actually isn't a color at all.
It is the presence of all of the colors of visible light.
He also understood that what we see as
is the absence of light.
We see color when white light is refracted
or bent at an angle.
The wavelengths are distributed along a spectrum,
what we often think of as a rainbow.
We'll talk more about color a bit later,
but there are other cool characteristics of light,
and we're in a good spot to talk about those.
See how the light comes through the window?
But the glass is not clear, is it?
If the material that light is moving through is rough,
even microscopically rough,
it will diffuse or spread out the light.
We call that kind of material translucent,
and that's what this glass is.
It's a medium that allows some but not all light
to pass through it,
so the objects cannot be seen clearly.
If no light passes through a material at all,
we say that material is opaque.
If all the light passes through a material,
like through this window, the material is transparent.
Look at this painting.
And now look at it.
Looks a lot brighter, doesn't it?
That's because the light we just turned on
is reflecting off the surface of the painting.
We call that measure of brightness luminance,
which is the amount of light reflected off a surface
measured in candelas per square meter.
The unit you might be more familiar with, lumens,
is the unit for luminous intensity,
which measures the brightness of light in a given direction.
You can see that word on any light bulb package.
It's different than wattage,
which measures the electrical power a device uses.
Luminance is inversely related
to the square of the distance from the light source.
The farther away you are, the less the luminance.
The brightness decreases at an exponential rate.
Now that we've talked about some characteristics
of the way light behaves,
we've come outside to show you something awesome
that goes on every day all over the planet.
All this green.
But why does this leaf look green?
Because it's reflecting the wavelength of green light
and absorbing all the other colors.
The chemical that makes this happen is chlorophyll,
which you might know is involved in photosynthesis.
The process that plants use to make nutrients out of light.
Chlorophyll is a light trapping pigment.
It absorbs red light,
which is the most effective wavelength
when it comes to making glucose,
a form of sugar, from light.
What we call the red edge,
near the infrared end of the spectrum,
is where photosynthesis for plants is most active.
So, we have our own little factory going on here,
powered by absorbed light
near the infrared part of the spectrum.
Let's explore more about color in our everyday world.
We brought in this color projector to do this that.
Remember, our primary colors of light
are red, green, and blue.
They are specific wavelengths of light.
When we overlap all three of them in equal intensities,
we get white light.
Where the three primary colors come together,
you can see a small triangle of white light.
Why?
What do you remember about the nature of color?
When we overlap two of the primary colors,
we get what we call the secondary colors of light,
which are yellow from combining red and green,
magenta from combining red and blue,
and cyan from combining green and blue.
That may sound familiar from your printer's ink cartridges,
and we'll talk about that more in a moment.
But what about the amazing array of colors
we can see from light?
They arise from what we call additive color mixing.
When wavelengths from different parts of the visible spectrum
overlap to create subtle, beautiful, and endless colors.
By blending different wavelengths of light
from the visible spectrum,
I can produce any color of light I want.
Within additive color mixing,
we can also produce white light using complementary colors.
These are pairs of colors which, when combined,
produce white light.
For example, some complementary combination of colors
are green and magenta, blue and yellow,
red and cyan.
When any two of them are combined,
they do a very specific thing.
Let's look at green and magenta...
now yellow and blue...
and red and cyan.
See?
All produce white light.
Each of these pairs is made of one primary color
and a secondary color.
The secondary colors themselves
are made of two other primary colors,
allowing for all of the wavelengths of light
to be present to create white light.
There's another kind of color mixing by light.
It's called subtractive.
This is when materials reflect and absorb light
rather than only reflecting it,
as in additive color mixing.
Subtractive mixing occurs when the source of color
is pigment or dye.
The refractive properties of the molecules
within the pigments and dyes
absorb some wavelengths of light and reflect others.
So, when these dyes subtract or take away the wavelengths
by absorbing them,
we are left with the colors that are reflected.
If you mix too many pigments
so that all the wavelengths are being absorbed,
what would you get?
Almost black because nothing is being reflected.
Mixing paints is the exact opposite of mixing light,
and here's an example of how painters use their properties
to create an amazing array of colors.
We can add color to objects like materials or canvas
in a few ways.
Pigment is one,
which is usually made from inorganic sources
that change the color of reflected or transmitted light
as a result of wavelength selective absorption.
Pigments are insoluble, suspended in a medium,
and don't dissolve.
For instance, cadmium sulfide is called cadmium yellow,
chromium (III) oxide is chrome green,
and iron (III) oxide is red ochre.
A dye, on the other hand, is biological in nature.
People have extracted substances from plants and insects
for thousands of years to create dyes.
Dyes are liquids or other substances
that dissolve in a medium and transmit color
when a material is soaked in it.
Our eyes perceive light with what we call rods and cones.
They are nerve cells that are photo receptors.
They absorb reflected light
and turn it into an electrical impulse,
which goes from our eyes to our brain.
We have about 120 million rods
and only about 6 or 7 million cones.
The rods are more sensitive to light than cones,
but not sensitive to color.
Cones are what perceive color,
transmitting the information about wavelengths to our brain.
These rods and cones contain molecules that absorb light
that is reflected from our surroundings.
They allow us to perceive so many awesome things.
In nature, color is used as a signal to attract,
to repel, and to disguise.
Without it, things would get pretty dull.
But, luckily, we have color all around us.
That's it for this segment of "Physics in Motion."
We'll see you next time.
For more practice problems, lab activities,
and note-taking guides,
check out the "Physics in Motion" toolkit.