Hi Michael,

That's a very good question and you're right; there is a confusion in the terminology. As you know, light can be thought of as a wave. This is how light seems to behave. However, unless the light waves are going through apertures that are on the order of their wavelength, light can also be approximated by rays. These rays are directed line segments that are perpendicular to the light wave (please see the figure below). These rays aren't real rays; they're just mathematical idealizations or models that work as long as the apertures are bigger than the wavelength of the light. This is because if the apertures are on the order of the wavelength of the light, the wave aspect of light, including phenomena like diffraction, comes into play. The study of light that describes the way light behaves when it's thought of as a wave is called physical optics. But, in the approximation where diffraction effects don't occur, considering light as a ray allows great simplification of the optics (this is called geometrical optics). I'm including a picture below to show a light wave and it's associated ray. I tried a search on the net for a better picture to no avail but you can find good pictures of waves and their associated rays in Physics for Scientists and Engineers by Halliday & Resnick in the chapter on Optics (my picture was "derived" from theirs :). If you have any further questions or would like further clarifications, please feel free to drop me an email and I'll be more than happy to address this further.

Regards,

Rick.