Discussion
Which one of the following does not control the interference colour of a mineral section observed under petrological microscope ?
Answer: D
The intensity of light does not control the interference colour.
Interference colour depends on the retardation between the two rays travelling through the crystal:
\[ \Delta = t\,(n_{1} - n_{2}) \]
where t is the thickness of the section and \(n_{1} - n_{2}\) the birefringence of the section, which depends on its orientation with respect to the crystallographic axes. The orientation of the vibration directions relative to the polariser and analyser also matters, since the mineral goes dark at extinction and shows its colour most brightly at 45°.
Changing the brightness of the lamp makes the colours brighter or dimmer but does not change which colour is seen.
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