The nature of light has been the subject of controversy for thousands of years. The debate over the theories of light has included such prominent scientists as Euclid, Galileo, Isaac Newton, and Albert Einstein. Even today, while scientists know how light behaves, they do not always know why light behaves in certain ways.

The Greeks were the first to theorize about the nature of light. Led by the scientists Euclid and Hero (first century a.d.), they came to recognize that light traveled in a straight line. However, they believed that vision worked by intromission--that is, that light rays originated at the eye and would travel to the object being seen.

Even with this erroneous hypothesis, the Greeks were able to successfully study the phenomena of reflection and refraction and derive the laws governing them. In reflection, it was determined that the angles of...