Waco Gem and Mineral Club, Waco, Tx

17.3 Birefringence or Double Refraction


17.3: Birefringence, or Double Refraction

Birefringence is one long, fancy word! A simpler term is double refraction. Take a piece of paper and draw a single line on it. Place a certain kind of clear crystal over the line, gaze through the crystal, and you‘ll see two lines! How does that happen?

When we direct our eyes at a line on a piece of paper, light is bouncing off the paper and into our eyes, allowing us to perceive the line on the paper. When light travels through certain crystals, the structure of the crystal causes the light to split into two rays traveling in slightly different velocities. When they bounce into our eyes, we perceive a double image. What is actually a single line, when viewed through the stone, appears as two lines.

Refraction, by the way, can occur in air when its density varies. You may have noticed this when gazing down a roadway on a hot summer day or across the basin of a desert, where hotter, less dense air radiates off the surface beneath somewhat cooler, denser air above. Light is refracted. The result? A mirage! Or, something your eyes see, even though your mind knows it‘s not really there.

The mineral most commonly associated with double refraction is calcite, particularly clear rhombohedral calcite crystals known as Iceland spar. Nice specimens from Mexico are on sale at nearly every gem and mineral show and are also often sold in museum gift shops.