Everyone, welcome back. So in this video, we're going to start talking about another phenomenon of light, called refraction. Now we're actually not going to talk about the specifics of refraction in this video. We're going to talk about an important variable that you'll need for that video called the index of refraction, and we'll get to it soon. Alright? So let's just get started. Now remember, when we talk about light, we use this variable c, which stands for the speed of light in a vacuum. This is 3×108 meters per second. The problem is in everyday life, light travels in all different kinds of materials, like air, water, and glass, things like that. What you need to know here is that in all other materials, light always travels slower. It's kind of like how we talked about the speed of sound. The speed of sound differs depending on the material, so does light, and light always travels slower in anything that's not a vacuum. Alright? So that actually kind of brings us to this important variable called the index of refraction. It's given by the variable n over here and basically what it is is it's a ratio. It's a ratio of c, so c is the numerator, to the speed of light in that material. So in other words, it's a ratio of the speed of light in a vacuum to the speed of light in a particular material and that's what n is. So you can actually kind of expand this a little bit and rewrite this as just basically just 3×108 meters per second divided by whatever that speed in that material is. So there's really only 2 variables in this equation, n and v because c is a constant. Alright? Now, actually, I'm going to get back to this point in just a second because we can just jump right into our example. I'm going to show you how this works. So we're told here that when light enters water, it slows to a speed of approximately 2.25×108 meters per second. Now we're going to calculate the index of refraction for water. Alright? So you'll see these tables, very commonly in your textbooks. They'll have all different kinds of variables. You'll never have to memorize them. They'll always be given to you, so don't worry about that. Alright? So we're told the speed of light in water. Which variable is that? Is that n? Is that c? Is that v? Well, hopefully, you guys realized that the c is always just going to be a constant and n is the index of refraction. So, really, they're actually just telling you v. They're telling you the speed of light in that material. So how do we calculate n? n is just equal to c over v. So in other words, it's just equal to 3×108/2.25×108. Now here's just a really quick shortcut when you plug this into your calculator. If these have the same base of 10, then you can kind of just ignore them, and you can only just do, you can just do 3 divided by 2.25. Anyway, what you should get here is you should get 1.33. And if you look in your textbook, that is exactly what the index of refraction of light is. Alright. It's 1.33. Now notice that we got a number that was greater than 1, and that actually kind of brings me back to my point over here. Because what we said here is that light always travels slower in any material, so in other words, if v is always less than c, the way we can see from this equation here is that if you always have a number that's lower than c in the denominator, then that means you're always going to end up with a number that is lower or greater than 1, less than or greater. It's actually going to be greater. So it's going to be greater than 1, and notice how all of these numbers over here are going to be greater than 1. That will always be the case. You'll never see something that's less than 1. Alright? So that's just what you need to know about the index of refraction. The very last point that I have to make here actually has to do with air because it's a very common material that you'll see in problems. So if you look at this table here, it says that the index of refraction is very close to 1.0003. So usually what happens is that in most problems, you can kind of just approximate it and use 1 for the index of refraction when you're talking about air. Alright? That's it for this, folks. Let me know if you have any questions, and I'll see you in the next video.
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Index of Refraction
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