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How can I find the fast and slow axes of a retarder?

There is also a simple way to find the axes of a retardation plate. This requires two linear polarizers. Orient one linear polarizer so the axis is horizontal. Put the other linear polarizer in front of the first, oriented so that the axis is vertical. Place the retardation plate between the two crossed polarizers. Rotate only the retardation plate until maximum transmission is reached. The fast and slow axes will be at ±45° from horizontal.

To determine which axis is fast and which is slow, hold the retarder along one of the axes. For example, hold the plate by the left side and the right side. Rotate the retardation plate about this axis, so that the light is passing though a slightly thicker cross section of the retardation plate. Then repeat, using the other axis. If the color of the light changes from a bluish color to gray and then to black, then you are rotating about the fast axis. If the color changes from white to yellow and then to interference colors, then it is the slow axis.

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