Birefringence is a property exhibited by certain types of crystalline structures in which a ray of light passing through the crystal is broken into two unequal waves which will pass through at different speeds. This property is also known as double refraction. In a classic example of how birefringence works, if a piece of birefringent material is placed onto a piece of paper with a dot on it, two versions of the dot will appear. Tourmaline, calcite, ice, and quartz are all birefringent in nature.
This property is exhibited when a crystal is optically anisotropic. When something is said to be anisotropic, it means that its properties can vary depending on the direction of the measurement, rather than something which is isotropic, in which case the properties will be the same from all angles. In the example above with a block of birefringent crystal placed on top of a piece of paper with a dot, as the paper is rotated, one of the dots will move, because the index of refraction changes with the angle.
In birefringence, two waves of light are generated. The first is the ordinary wave, and it will remain fixed in direction, with the light passing straight through the crystal without interruption. The second wave is known as the extraordinary wave, and it moves as the crystal is rotated. The extraordinary wave circles the ordinary wave, changing as it hits different areas of the crystal and refracts differently. This is because the polarization of the light changes as the object is reoriented.
This property can be harnessed for a number of interesting uses. Birefringent optical filters can be installed on a variety of devices to harness birefringence, for example. Specialized birefringent glass products are used in various scientific experiments and industrial processes, and calcite is also commonly utilized in applications where birefringence is desired. It is also possible to see trirefringent materials, in which there are three indexes of refraction.
People who wear eyeglasses can explore birefringence by experimenting with the edges of their lenses. When glasses are positioned properly, it should be possible to break an image into a doubled pattern, caused by the different indexes of refraction found along the edges of a pair of glasses. Birefringence can also be demonstrated with many types of glass; as the glass is moved, the indexes of refraction should change, making the ordinary and extraordinary rays visible in the form of a double image which moves.