Color I

Interference Normal white light contains all colors in equal proportions. Color may be defined as an imbalance in the visible spectrum reaching the eye. Some processes that produce color: Selective scattering Rayleigh scattering --> blue sky Selective absorption Transmission (colored foil) Refraction and reflection (diamond) Interference ( soapfilm, soap bubbles) Selective Interference Constructive and destructive Interference can select certain frequencies from the visible spectrum. Frauenhofer diffraction occurs when white light passes through a slit and a spectrum appears to both sides of the slit. A polarization interference fringe pattern resulting from a slab of calcite cut perpendicular to its optical axis and viewed through a crossed polarizer and analyzer is shown in this figure. The colors of this interference pattern can be reversed to their complements by orienting the analyzer parallel to the polarizer. Iridescence Iridescence is a process involving thin films or thin slices of material. Selected wave length have destructive interference depending on the thickness. Those wavelengths (frequencies) are removed from the white light. Thus an imbalance is created in the white light and we see color. This effect is responsible for the color of soap bubbles and oil films. Some semiprecious stones like opal or mother of the pearl reflect light from the surface and also from some layers beneath the surface. This creates destructive interference for some frequencies and the stones show color. Selective Emission The selective emission of one wavelength creates one single color. This is the case for the line spectra from atoms. We studied emission of light from atoms in Chapter 6. A close up of a color TV screen shows the additive primary phosphors, which build up the color picture on the screen.