00:01
Here i'm going to say a few words about the phenomenon known as refraction.
00:07
And the idea is if light goes through a dense, transparent material like water or air or glass, light will actually slow down from its vacuum value.
00:21
So light slows down going through a medium.
00:35
And that's because the photons that make up the light or the electromagnetic waves, as you may want to think about them, interact with material.
00:49
So here we are going to use c as the speed of light in a vacuum, empty space, and v is the speed in the medium.
01:10
And that is simply equal to c divided by n, where n is known as the index of refraction.
01:21
It is bigger than one.
01:29
Another thing i want to say a little bit about is that in general we have the speed of a wave is its frequency times its wavelength.
01:43
So frequency in hertz wavelength in some unit of length measurement.
01:52
The question is, what actually changes about the light as it goes through the material? and it turns out that what changes, the frequency does not change.
02:06
Frequency is a measure of the energy in the light, and it does not change.
02:15
However, the wavelength, because the speed must slow down, the wavelength does change, and it actually must decrease then.
02:28
And if you are looking at wavelengths, let's say that somebody, shines a red laser into water.
02:36
And you're under the water looking at that scattered red laser light.
02:41
The color does not change.
02:43
And it's important to realize that color of light has to do with the frequency, even though we often discuss it in terms of wavelength.
02:54
And that's because we're used to things traveling in air very close to vacuum.
03:00
And so either using the frequency or wavelength.
03:03
As long as the medium does not change is perfectly okay.
03:08
But the wavelength does change going in.
03:12
So here we're going to take as an example, a slab of glass, and we'll try to make some numbers come out nicely.
03:26
We'll make it a few millimeters thick, a fraction of a centimeter thick.
03:39
And we will come in with incoming red light, with wavelength 600 nanometers.
03:52
And we basically have air, which is vacuum, outside.
03:56
And what we would like to do is compare the light beam that goes through air with the light beam that goes through the glass where the index of refraction is.
04:10
We'll just call it 1 .5.
04:14
And it's exactly one for the air.
04:18
And here are the things that we are going to compare.
04:22
Okay, so compare these items, or quantities, i guess.
04:32
They really aren't like shoes.
04:34
They're just quantities.
04:37
For the ray, for the light and glass versus air.
04:56
So the first thing to compare is the speed.
04:59
So here's the glass and the air.
05:04
Make kind of a table here.
05:08
So the first thing to compare is the speed.
05:13
B in meters per second.
05:16
So for the glass it's going to be v divided by n or c divided by n, which is basically two times 10 to the eighth meters per second.
05:31
And for air it is just c three times 10 to the eighth.
05:38
So yeah, it's noticeably slower in the glass.
05:47
And now we're going to look at the time to go through the material...