# What is Birefringence?

#### Editorial Team - GoPhotonics

Mar 3, 2023

Birefringence, also known as double refraction, is a phenomenon that occurs when light passes through certain materials and is split into two rays, each with a different index of refraction. This effect was first discovered by the Danish scientist Rasmus Bartholin in the 17th century, and it has since been studied extensively in the field of optics.

Light is an electromagnetic wave that contains electric and magnetic fields oscillating in different directions. If the electric field of the light vibrates in more than one direction, it is called an unpolarized light and if the electric field vibration is in a plane that is parallel or perpendicular to the direction of the light beam, it is a polarized light. A polaroid filter is used to convert an unpolarized light into a polarized light.

Birefringence is an optical property of the material with a refractive index that depends on the polarization and propagation direction of the light. It is a phenomenon of double refraction. In birefringence, when an unpolarized light passes through a birefringent material, it gets split into two rays. One ray with polarization in a direction perpendicular to the optical axis of the material and this ray is called an ordinary ray or o-ray and the other ray with polarization in the direction of the optical axis of the crystal and this ray is named an extraordinary ray or e-ray. O-rays produce stationary images that obey the laws of refraction and E-rays produce extraordinary images that do not obey the laws of refraction.

Figure 1: Birefringence inside a crystal

Birefringence occurs in materials that have a crystal structure that is not isotropic, meaning that the properties of the material are not the same in all directions. These types of materials are called anisotropic materials. When light enters such a material, it is split into two rays, one of which follows the normal laws of refraction, while the other is refracted at a different angle due to the different index of refraction. The two rays may then travel at different speeds and with different polarizations, resulting in various optical effects. Figure 1 shows the birefringence occurring inside a crystal.

One of the most common materials that exhibit birefringence is calcite, a form of calcium carbonate that is commonly found in rocks and minerals. When a beam of light enters a calcite crystal, it is split into two rays, one of which travels straight through the crystal, while the other is bent at an angle. This effect can be seen when a calcite crystal is placed over an image, causing the image to appear doubled. Quartz, tourmaline, etc are some other anisotropic materials that exhibit birefringence.

Another example of birefringence can be seen in polarizing filters, which are commonly used in photography and other applications to block out certain wavelengths of light. A polarizing filter consists of a material that has been stretched to create a grid-like structure, causing the material to become anisotropic and exhibit birefringence. When light passes through the filter, it is split into two rays, one of which is blocked by the filter, while the other is allowed to pass through. This effect can be used to selectively filter out certain colors or wavelengths of light, allowing photographers and other users to create unique visual effects.

Types of Birefringent Crystals

Positive and negative birefringent crystals are two categories of anisotropic materials, which have different refractive indices for light waves traveling along different axes.

Figure 3: Wavefronts through Positive and Negative Birefringent Crystals

Negative crystals are those whose refractive index is smaller for the E-ray than for the O-ray in all directions except for the optical axis. This causes the E-ray to travel faster than the O-ray. Examples of negative crystals include calcite, tourmaline, and ruby. In these crystals, the spherical O-ray is located within the E-ray.

On the other hand, positive crystals are those whose refractive index for the O-ray is lesser than the E-ray. In positive crystals, the velocity of the O-ray is greater than or equal to the velocity of the E-ray, and the E-ray is located within the O-ray. Quartz, sellaite, and rutile are examples of positive crystals. Images of wavefronts passing through positive and negative uniaxial crystals are shown in figure 2.

Applications

Birefringence has many practical applications in science, technology, and industry. It is used in the production of polarizing filters, as well as in the manufacture of optical instruments such as polarimeters and spectrometers. Birefringent materials are also used in the study of minerals, as their optical properties can be used to identify different types of crystals.

Polarizing filters are the most common application of birefringence. These filters are used in many optical instruments, such as cameras and microscopes, to reduce glare and improve image clarity. Polarizing filters work by blocking one of the two polarized components of light, and the birefringent material used in the filter is oriented to achieve this effect.

Another application of birefringence is in LCD screens. LCDs use liquid crystals that are birefringent and can change their refractive index when an electric field is applied. By controlling the orientation of the liquid crystals, LCD screens can control the intensity and polarization of light passing through the screen, resulting in the display of images and video.

Birefringence is also used in optical communication systems, such as fiber optic cables. These cables transmit light signals using total internal reflection, and birefringent materials are used to maintain the polarization of the transmitted light. By keeping the polarization constant, the signal can be transmitted with minimal distortion, leading to more reliable communication.

Polarized microscopy is a technique that uses birefringence to study the properties of materials. In this technique, polarized light is passed through a sample, and the birefringent properties of the material cause the light to split into two polarized components. By analyzing the resulting images, researchers can learn about the structure, orientation, and other properties of the material being studied.

Birefringence is also used in biomedical imaging techniques, such as optical coherence tomography (OCT) and polarized light imaging (PLI). OCT uses birefringent materials to generate detailed images of tissues and structures within the body, while PLI can be used to study the orientation and alignment of fibers in biological tissues, such as muscle and cartilage.

TagsOptics