Interference Colors (Birefringence)

Interference colors are the colors that you see when a rock thin section is placed between two pieces of polarizing film.  This phenomenon is a side effect of  light slowing down as it passes through different substances. You observe another effect of light slowing down when you see a straw in a glass of water appear to bend where it passes into the water.

The slowing down of light as it passes through a substance is measured in a number called the index of refraction.  Interference colors are an effect produced by the fact that most solids have more than one index of refraction.  The greater the difference between the indices of refraction, the more intense are the interference colors produced.

The difference between indices of refraction is called the birefringence.  Following is a range of effects produced as the birefringence increases.

No Birefringence (Garnet) - appears black

Weak (Quartz) - grays and whites
 

Low (Kyanite) - dull yellows, oranges, and browns
 

High  (Muscovite) - bright pastel-like colors - pinks, blues, greens
 
 

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