The figure on the left illustrates light traveling from one medium to another.  The angles between the light ray and the normal to the surface in each medium are related to the indices of refraction in those media by Snell's Law:   n1 sin θ1= n2 sin θ2 Here θ1 is the angle of incidence between the light ray and the normal in the first medium. Similarly, the light ray makes an angle of refraction θ2 with respect to the normal in the second medium. The first medium has an index of refraction of n1, and the index of refraction in the second medium is n2.

Aside:  A recent development in materials science has produced a whole new type of refraction in certain manufatured materials.  These so-called left-handed materials have negative indices of refraction.  For more information, you can click here.  For the purposes of these materials, we will stick with conventional materials that all have positive indices of refraction.

The picture we have described here is not completely accurate.  As was indicated in the 5th frame of the animation on the previous slide, not all of the light striking a surface travels across the surface; some is reflected.  You have probably seen this happen when you looked out a window. Often you can see reflections of objects inside the room at the same time as someone outside the window could see the objects. The light you see is reflected; the light the person outside sees has been refracted through the window.  The amount of light reflected depends on several parameters; we will not worry about the specifics, but you should understand that light is generally both reflected and refracted at a given interface between two media.
 
 

How well does this equation match our intuition? Continue to the next page to find out!