Double refraction, also known as birefringence, describes an optical property where specific materials cause a single incident light ray to split into two distinct rays. These two resulting rays travel through the material at different speeds. Furthermore, these rays are polarized in directions perpendicular to each other, meaning their light waves oscillate on different planes.
How Double Refraction Occurs
Double refraction arises from the anisotropic nature of a material’s crystal structure. Anisotropic materials have optical properties that vary depending on the direction light travels through them. This means that light experiences different refractive indices depending on its polarization direction relative to the crystal’s internal arrangement.
When unpolarized light enters such a crystal, it separates into two distinct rays. One is called the ordinary ray (O-ray), which behaves as expected, following Snell’s Law of refraction and traveling at a constant speed in all directions within the crystal. The other is the extraordinary ray (E-ray), which does not consistently obey Snell’s Law, and its speed and angle of refraction change based on its direction of travel relative to the crystal’s optic axis. The optic axis is a specific direction within the crystal where both rays travel at the same speed, and no double refraction occurs if light enters along this axis.
Materials Exhibiting Double Refraction
Many transparent crystalline materials exhibit double refraction, with the exception of those possessing cubic symmetry. This property is quantified as birefringence, which is the difference between the refractive indices of the two rays.
Calcite, also known as Iceland spar, is a widely recognized example of a material that displays strong double refraction. Its distinct rhombohedral crystal structure causes a significant splitting of light into two images. Quartz is another common birefringent mineral, though its birefringence is lower than calcite. Ice also exhibits double refraction due to its crystal lattice. Other materials like mica, sugar crystals, and tourmaline are birefringent.
Observing Double Refraction
One simple way to observe double refraction is by placing a transparent birefringent crystal, such as calcite, over text or an image. When viewed through the crystal, the text or image will appear doubled. This occurs because the single incident light ray is split into two separate rays, each forming its own image.
If you rotate the crystal, one of the images, associated with the ordinary ray, will remain stationary, while the other image, by the extraordinary ray, will appear to rotate around the first. This rotational behavior demonstrates that the two rays are polarized in mutually perpendicular planes. Using a polarizing filter, such as Polaroid, further illustrates this; rotating the filter will cause the two images to alternately vanish as the filter blocks one polarized ray and allows the other to pass.
Applications of Double Refraction
Double refraction has many applications due to its ability to manipulate light polarization. In optical instruments, birefringent materials are used to create polarizers, such as Nicol prisms, which separate unpolarized light into two polarized beams and allow only one to pass. Wave plates, also known as retarders, are another application, which alter the polarization state of light.
Liquid crystal displays (LCDs) fundamentally rely on double refraction. These displays use liquid crystals whose birefringence can be controlled by an electric field, allowing for precise manipulation of polarized light to create images. Beyond consumer electronics, double refraction is also relevant in geology for identifying different minerals under a polarizing microscope. In ophthalmology, it is employed in diagnostic tools.