The Science of Light (Part 2)

For centuries people had been noticing another odd but obvious fact: a straight pole stuck in the water at an angle no longer appears straight to an observer. The underwater part seems to slant off in a different direction. In 1621 a Dutch mathematician named Willebrord Snell finally explained this phenomenon. A ray leaving one transparent medium and entering another, he said, is usually split at the surface. One part is reflected, in keeping with Hero’s rule. The other part continues into the second medium. The reason that the stick appears to bend on entry into the second medium is that the light rays bringing its image to the eyes are suddenly bent at that point.

Well, if light rays are bent when they enter the water, does this not dispose of the Greeks’ old idea that light always travels in straight lines? Not at all, said Snell. All it indicates is that light may be deflected somewhat if it enters a new medium. The light was traveling in a straight line through the air; when it reached the water it changed direction, but continued in a different, deflected straight line under the water. Snell tried to measure this deflection in various transparent substances such as air, glass and water. He found that each one varied in the amount that it could bend light. Whereupon he gave a name to the bending itself–refraction. It took him a long time to work out the principle, because it seemed terribly contrary and slippery until he also discovered something else: the angle of incidence of the light also had something to do with the amount of refraction. For example, a ray of light striking water vertically will not bend at all. But if it enters at a slight slant it will bend a little; at a greater slant it will bend a lot. Later researchers were able to give numerical values–called refractive indexes–for the various bending powers of all transparent substances. What Snell never discovered is why light bends.


Life Science Library - Light and Vision