So we'll end our discussion of light
with the discussion on the electromagnetic spectrum
as well as a brief discussion on a more modern view of light
as individual packets of energy called photons
which we'll get towards the end of this lecture.
Don't forget where we've been,
we talked about some of the wave phenomena of light
as well as some of the properties of this electromagnetic radiation
And now we're ready to talk about the spectrum,
the electromagnetic spectrum.
We talked a little bit about the wavelength of light,
how a long light wave would be and we can see or ask ourselves,
how big is this wavelength for different kinds of light?
The wavelength for light it turns out can span a huge range.
For example, we could have waves,
electromagnetic waves with wavelengths the size of buildings.
Very, very long waves
and these are towards the more red ends of the spectrum.
On the other end of the electromagnetic spectrum,
we have waves that are very, very tiny.
So for example, we talked a little bit about x-rays
because we said that we wanted to use these rays
to probe some very tiny bits of matter.
We wanted them to diffract off of the individual atoms
or molecules of a substance.
And so we used those waves
because the wavelengths are close to the size of molecules and atoms.
But these wavelengths cross even the size of ordinary, everyday things,
the size of people or even objects that you see around you.
It's also important to know this that the visible part
of this electromagnetic spectrum is in fact very, very tiny.
Electromagnetic waves are not always visible
and of course the infrared waves and the very high frequency waves
which we heard talked about sometimes are of course invisible to humans.
The only visible part of the electromagnetic spectrum is in fact
a very, very small slice. So let's look at that slice in a little more detail.
So in the bottom here what we have is this base,
base green line here
which is representative of the entire electromagnetic spectrum.
From the very high frequencies
all the way down to the very low frequencies of radio waves.
So we've taken just one small slice of this entire spectrum
and blowing it up to see what the visible light wavelengths are.
At the lower frequencies,
we have the red light and then going up towards the higher frequencies
we have more, more blue and then purple and violet light.
Sometimes just for people to remember the arrangement of light
in this visible spectrum going from lower frequencies to higher frequencies
or from higher wavelengths to lower wavelengths.
We have this expression which we've written at the top here, R.O.Y.G.B.I.V.
It is sort of a funny expression.
I'm not sure why it's useful for memorization
but something about the sound of it can stick in your head.
So this is useful to you,
what it's saying is red, orange, yellow, green, blue and then indigo
sometimes is put in there as well and then violet.
So this is just a simple acronym
if you're having trouble remembering the order of the light
as it goes from more red to more blue.
But it's certainly important to be able to order anyone of these colors.
So for example if I told you we have orange light and green light,
you should certainly know that the green light is at a higher frequency
or a lower wavelength.