Definition

Noun: 1. Optics/Crystallography: The specific angle, measured from a crystal's optical axis, at which it must be oriented to appear maximally dark or extinct when observed under polarized light. This occurs when the vibration direction of the light within the crystal aligns with the polarizer or analyzer, blocking the transmission of light.

Usage Examples

• Noun:
  • Measuring the angle of extinction is a standard technique for identifying minerals in thin section.
  • The crystal showed a sharp angle of extinction of 15 degrees, which helped classify it.
  • In the microscope, the geologist rotated the stage to find the angle of extinction for the mineral grain.

Advanced Usage

• "Symmetrical Extinction": When the angle of extinction is measured relative to a crystal face or cleavage direction. A common type is , where the crystal appears dark when its length is parallel to the vibration direction of the polarizer.
• "Inclined Extinction": When the angle of extinction is measured as a specific number of degrees away from a crystal face or cleavage direction. This angle of extinction is a key diagnostic property.

Variants and Related Words

• Extinction Angle: A synonymous term for angle of extinction.
• Extinction (n.): The phenomenon of a crystal appearing dark under crossed polarizers.
• Pleochroism (n.): A related optical property where a crystal shows different colors when rotated under plane-polarized light (without crossed polarizers).

Synonyms

• Extinction Angle

Related Terms and Concepts

• Crossed Polarizers: The standard microscope setup (polarizer and analyzer perpendicular to each other) used to observe extinction.
• Birefringence (n.): The property that causes light to split into two rays within a crystal, which is directly related to the interference colors seen before extinction is reached.
• Optical Axis (n.): The direction in a crystal along which light propagates without double refraction; a key reference for measuring the angle of extinction.

Noun

1. the angle from its axis that a crystal must be rotated before appearing maximally dark when viewed in polarized light