PLM Fundamentals:

Interference Colors

 The Visible Light Spectrum Our eyes are sensitive to a very limited spectrum of light: those rays with wavelengths between 4000 and 7000 Angstroms. (An Angstrom is one ten-billionth of a meter or 1x10-10m.) Within that spectrum of visible wavelengths different wavelengths of light are perceived as different colors: Interference and the Visible Light Spectrum Remember the relationships between retardation, interference and the passing/blocking of light by the ANALYZER... (see About Birefringence and Interference for full review) If the retardation is a whole number multiple of the wavelength then that wavelength of light will be completely blocked by the ANALYZER If the retardation is a whole number multiple of the wavelength plus one half then that wavelength of light will be completely passed by the ANALYZER If the retardation is some other multiple of the wavelength then that wavelength of light will be partly blocked and partly passed by the ANALYZER SO HERE IS AN APPLIED EXAMPLE... Imagine light passing through a mineral sample such that the retardation is 12,000 Angstroms... Whole number multiples that equal 12,000 Angstroms will be completely blocked by the analyzer: 12,000 Angstroms x 1 = 12,000 Angstroms   (Beyond Visible Light)       6,000 Angstroms x 2 = 12,000 Angstroms   (Yellow-Orange Light)       4,000 Angstroms x 3 = 12,000 Angstroms   (Violet Light)       3,000 Angstroms x 4 = 12,000 Angstroms   (Beyond Visible Light) Whole number multiples plus one half that equal 12,000 Angstroms will be completely passed by the analyzer: 8,000 Angstroms x 1.5 = 12,000 Angstroms    (Beyond Visible Light)      4,800 Angstroms x 2.5 =  12,000 Angstroms  (Blue Light)      3,430 Angstroms x 3.5 = 12,000 Angstroms   (Beyond Visible Light) So in the case of a 12,000 Angstrom RETARDATION the visible light will be affected as follows... The resulting light that passes through the ANALYZER will  be composed of ... LOTS of BLUE Lesser Amounts of Red, Green and Indigo Even Less Yellow THE RESULTING LIGHT WILL BE BRIGHT BLUE By selectively passing some wavelengths and blocking others because of the way the different wavelengths interfered, the ANALYZER can produce many different colors which are called INTERFERENCE COLORS.