Interference colors are observed under cross-polarized light as which description?

• A. Interference colors are colors derived from a single light ray
• B. Colors produced by fluorescence only
• C. Spectral colors caused by the interaction of two light rays seen under cross-polarized light
• D. Inclusions that alter color tone

Answer: (C)

Interference colors come from birefringence in a crystal. When light enters a transparent mineral with different refractive indices along different directions, it splits into two rays—the ordinary and the extraordinary—that travel at different speeds. Under cross-polarized light, most light is blocked unless these two rays acquire a phase difference as they pass through the stone. That phase delay depends on thickness, the material’s birefringence, and the light’s wavelength, so the recombined light shows a spectrum of colors produced by interference. So interference colors are the spectral colors caused by the interaction of two light rays seen under cross-polarized light. The other descriptions don’t capture this two-ray interference phenomenon.