Even the speed of light varies - it is slightly slower than C in normal air, and MUCH slower in high density materials, such as glass. So, the wavelength changes within optical materials. In fact, it is possible to design optical surfaces entirely by looking at how the change in refractive index alters the speed over the path distance within the optical material. Optical design by such methods was mainly perfected by the late great A. E. Conrady during WWII and by his daughter after his death. It is usually known as the 'Path Difference Method' and [usually used in conjunction with other methods] can yield extremely accurate results in optical design.Ogge wrote:They include changes in wavelength due to temperature variations as well as differences in velocity which drive that wavelength change. The waves are no longer traveling at C (speed of light) as in electrons but at the variable speed of sound in this environment.
The speed of light inside a diamond is LESS THAN HALF its speed in air, though the alteration varies slightly by frequency, of course ['slightly' only because the bandwidth of visible light is incredibly small]. The ratio of speeds of the same light in two media is the exact inverse of the ratio of the media refractive indices. [The refractive index of an optical material as normally given in the tables is relative to normal air - lenses used in the vacuum of space must have their indices adjusted for correct design.]
Another phenomenon of interest is 'internal reflection'. This is most easily observed in a flat-sided glass aquarium, where at certain angles of looking through the front of the tank, the side walls seem to be perfect mirrors, while at other angles they appear transparent. This is the type of reflection where a wave tries to move through an interface from high to low refractive index [in the aquarium example, the observed reflection is off the glass/air interface, not the water/glass interface]. At steep angles, there is no reflection, at shallow angles total reflection. If the angle is VERY shallow, only a slight difference in density is needed for total reflection as observed in mirages off the hot layer of air over hot pavement or desert sand. There must at least be the possibility of this occuring to sound waves at interfaces between cool, dense air and warm, less dense air. A good example might be knowing whether some types of sonar are affected by reflection off thermoclines [interfaces between two temperature layers] in the ocean, or even reflection off the wavy surface. I have no such knowledge, but I suspect that this is true.
Adam, is there such a thing as refraction of radio waves? Can you think of easily explained examples? [Just a curiosity question, not having much application to anything we're talking about.]
L Cottrill
