ï»¿ So, we have a mirror right here. Here's a mirror, can you guys see that? Probably if I look just like that, you can maybe see my reflection. Can you see my reflection? It's a really dirty mirror. Okay? That's a mirror. What is a mirror? A mirror is very simple. It is a coating of aluminum or silver on glass. Okay, there is a glass substrate and on that glass substrate, you provide a little bit of aluminum or silver. If you have a lot of money, you can buy gold mirrors, right you can put gold foil down onto a glass substrate. And so it looks like this. Here is our glass. And then right on top, there is a very thin coat of metal. Okay, and it doesn't have to be very thick at all, the metal is what does the reflection. So when a light ray comes in- here comes our light ray. It hits the mirror, and it bounces off. Why does that happen? Why does light bounce off of this mirror? And the combined two things here the glass and the metal is what constitute the mirror. Why does that happen? It happens because we know that light is made up of electric fields and magnetic fields. And when those electromagnetic fields hit the metal the little charges in the metal start to vibrate. And they vibrate at exactly the same frequency as that light ray that was coming in and it reradiates that stuff out into another ray at the same frequency, okay. This is why you have to have metal there. If you have an insulator, like a piece of wood it doesn't reflect the light ray. And that's because those little charges in the wood can't move quite as easily as the charges in the metal. So the metal is in fact what does the reflection. Metal causes that reflection to happen. What is the purpose of the glass? Let me ask you guys. Chime in on the chat room and tell me what the purpose of the glass substrate is . The purpose of the glass substrate is purely to keep the metal flat. So if I think about a piece of metal, like this it can be kind of flimsy. Right, it's easy to bend. And if I bend this you can see the reflection change shape, okay. Think about when you open a brand-new can of aluminum foil. Right, it comes out and it looks like this. And it doesn't make a very good reflector but if you pull it out very slowly and lay it on the table then it's flat. And so that's it. The whole purpose of the glass is to keep it flat. There's an additional purpose which is to keep it from oxidizing so typically when you look in the bathroom mirror there is metal coated on glass but the very front side is glass. Then there's metal behind it, and then there's some other sort of protective backing on it.