Snell's law and the concept of rays are the foundation of geometrical optics and what we're going to be using for the whole semester. We're going to be tracing these rays bending and surfaces with Snell's law in relatively complex systems with lots of surfaces, and we don't want to do that by hand. We want to start learning that way, but not to design it that way. That motivates the use of a computer-aided design tool that can automate these calculations for us. In this course, we're going to be using Zemax's OpticStudio and this is a preview of what that can do for us. So what I show here is much like the calculations we showed in the notes, is I have a block of glass, I have a set of parallel rays, the blue lines, that represent a plane wave. Remember, phase fronts normal to those rays. That intersects that glass surface, and the program knows and uses Snell's law to calculate a new refraction angle for each ray, when I enter and exit the block of glass. Now, that in itself isn't terribly exciting, but I can now curve these surfaces as such. Now, locally, the refraction angle, theta in Snell's law, depends on where I hit the surface because the surface is curved. That means that the refraction angle, the direction the ray goes inside the glass, depends on where the ray hit the glass. So when I refract through a curved surface and back out into air, I've converted this plane wave into something which is like a spherical wave and converging. I can only really know if it is a spherical wave if I went over here and looked at where the rays finally all come together, and see if they actually come to an absolute point. If they don't all intersect at a point, then this is something that's a converging wave that isn't actually a sphere. Again, that would be a hard thing to describe mathematically, but these rays are straight lines and thus very easy to calculate. So, we're going to learn how to use this program, how to put optical systems together with lenses, prisms, things like that, and then the program will run the ray equations that we've just learned, Snell's law of refraction and reflection, in order to calculate properties of the optical system which really are derived from Maxwell's equations, as we've just learned.