PLM Fundamentals:
Interference Colors
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The Visible Light Spectrum |
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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.) |
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Within that spectrum of visible wavelengths different wavelengths of light are perceived as different colors: |
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Interference and the Visible Light Spectrum |
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Remember the
relationships between retardation, interference and the passing/blocking
of light by the ANALYZER...
(see About Birefringence and Interference for full review) |
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If the retardation is a whole number multiple of the wavelength then that wavelength of light will be completely blocked by the ANALYZER |
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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 |
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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 |
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SO HERE IS AN APPLIED EXAMPLE... |
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Imagine light passing through a mineral sample such that the retardation is 12,000 Angstroms... |
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Whole number multiples that equal 12,000 Angstroms will be completely blocked by the analyzer: |
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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) |
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Whole number multiples plus one half that equal 12,000 Angstroms will be completely passed by the analyzer: |
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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) |
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So in the case of a 12,000 Angstrom RETARDATION the visible light will be affected as follows... |
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The resulting light that passes through the ANALYZER will be composed of ... |
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LOTS of BLUE Lesser Amounts of Red, Green and Indigo Even Less Yellow THE RESULTING LIGHT WILL BE BRIGHT BLUE |
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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. |
