Interference colour. How is it formed?

 

2 polariser + isotropic mineral + analyser

 

What happens if an isotropic crystal is placed between the nicols?

If it is an isotropic crystal (which allows the light to vibrate in all directions), the radiation from the polariser will pass through it without being modified, and when it reaches the analyser, it will not be able to pass through and the mineral will be seen as black.

All the cuts of an isotropic mineral will be seen as black. They will constantly be black when the microscope stage is turned.

What happens if the crystal is anisotropic and it has in one particular position perpendicular to the optic axis?

 

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