Okay, what is the deal with geometric optics? That pen is out. So the idea with geometric optics is just that. We're going to talk about optical elements and the important components of the optical elements is the geometry of the setup. Okay, and this entire chapter can broadly be categorized under one following statement. Light is a ray. All right we learned about light a little bit in the last chapter when we talked about electromagnetic waves. We said that electric fields and magnetic fields can oscillate together and they can propagate along as an electromagnetic wave, okay? That is what we call light. Specifically, light refers to sort of the visible portion of the spectrum but a lot of times your hero physicists refer to the entire electromagnetic spectrum as light. Okay, so electromagnetic spectrum is pretty big. You've got X-rays at the very short wave length scales, you've got radio waves at the long wavelength scales, and somewhere in the middle, you have the visible spectrum. You got the ROY G. BIV, right. The red, the yellows, the greens, the blues, the indigos, the violets. So light is a ray- what does that mean? It means it travels in a straight line. Okay, now there's a caveat there, which is the following in free space. Pretty soon we're gonna talk about what happens when it meets up with glass or water and then you can see that the ray will actually bend, but in free space, light travels in a straight line. It's a ray. So when you think about a star, here's a distant star. Okay, what is that star doing? Well, it's spitting out electromagnetic radiation in all these different directions. And each little bundle acts like a ray. And so if you're sitting down here on Earth and you're looking up at that star there is a ray that came directly to you. And it's kind of weird to think about, but that little photon, that bundle of energy that left the star, however many millions of years ago traveled in a straight line all the way across free space all the way to your eyeball you detected it, and it was gone. Okay, so that was the source of the ray. This was the sync of the ray. Started there, ended there, traveled in a straight line. The fact that it travels in a straight line lets you determine exactly where that star is. Now it gets a little more complicated once we get to general relativity but for now, just consider those rays moving, really, in a straight line. You look up came from there- that must be where the star is. All right. What about when light rays bounce off of stuff? So let's say that that light ray comes at us but it's not going to hit your eye, it's going to bounce off of a mirror.