Color I

So far, we considered two different phenomena involving light: Geometrical Optics Here light encounters material obstacles of a size much larger than the wavelength of light. We treated light as light rays and considerd mirrors and lenses. Wave Optics Here light encounters material obstacles of a size comparable to the wavelength of light. We considerd diffraction and discussed that different frequencies of visible light correspond to different colors. When light interacts with small, isolated object we have to consider a different phenomenon, Scattering The scattering of light is responsible for many of the colors we see in nature. We had learned that light is an electromagnetic wave and that electromagnetic waves are the result of `wiggling charges'. Therefore, another item needed to specify a light wave is the direction in which its electric field oscillates. This is called the Polarization So far we have pretended that a substance like glass has one index of refraction for all kinds of light, no matter what the frequency is. But the index of refraction, n=c/v, is specified by the speed of light in the glass, which depends on the way the charges in the glass respond and radiate when they are wiggled. We have seen that the amplitude of the charges' motion depends on the frequency with which they are wiggled: the closer one gets to a resonance frequency, the the charges will oscillate for the same applied force. Thus the index of refraction will depend on frequency. This effect is called Dispersion For mechanical waves we have learned that when the crests and troughs of two waves keep in step or have a constant phase difference, the waves are coherent and will interfere in regions where both waves are present. The Interference is constructive (increase in intensity) where crest meets crest and trough meets trough, that is, when the waves are in phase. It is destructive (decrease in intensity), when crest meets trough, that is, when the waves are out of phase.