00:02
So this question involves total internal reflection.
00:08
To understand the question, let's talk about what that means.
00:12
Let's say we have an interface between two materials.
00:18
In this case, we're going from glass to air, and glass will have a higher index refraction than air does.
00:36
So when a light ray is incident on this interface, the direction of travel of the light ray will bend away from the normal line.
00:56
And the angles we use are those defined by, as i said, the normal line, which is this line here, the line that is normal to the interface between the two substances.
01:11
So the angle out here in the air is bigger than the angle in here in the glass.
01:20
So this is the incoming light ray hits the interface, and this is the refracting light ray here.
01:29
Now, because it's bending away from the normal, what can happen if you go in at a big enough angle is the following.
01:38
Let us say that we go into an even bigger angle to the interface.
01:46
The incident light is at an even bigger angle to the interface.
01:55
So it's like this, and that angle is so big that the angle of refraction is 90 degrees.
02:07
We'll just draw that here, if i can.
02:19
There we go.
02:23
Well, that means the light's not being refracted then.
02:26
If the angle of refraction is 90 degrees, then the light isn't really refracting.
02:31
It's just staying inside of the glass.
02:34
In fact, what happens when light is incident on a interface here is that usually it's a combination of both reflection and refraction.
02:46
But here, none of the light can be refracted because it can't get out of the medium.
02:53
The angle that it's making in the air is 90 degrees.
02:58
So what actually happens is all the light then gets reflected.
03:01
And of course, we know when light is reflected, the rule we follow is different.
03:05
It's just the angle of incidence equals the angle of reflection.
03:08
So the light's going to be reflected back in...