Double Refraction

The double refraction of light is the phenomenon of birefringence. It is an optical property in which a single ray of unpolarized light enters an anisotropic medium and splits into two rays, each travelling in a different direction. We can think of double refraction as the end which divides into two roads. Here, the end is the anisotropic medium, the person travelling is the unpolarized light, while the two roads are the two rays, each travelling their paths. 

Birefringence is characterized by crystallographic materials with different recurrence indicators concerning different crystallographic directions. Birefringence occurs when light passes through transparent objects, ordered by molecules, indicating a differential difference in reception at refractive indices.

In this article, we will understand the definition of double refraction, explain the phenomenon of double refraction in depth.

Types of Birefringence and its measurement

• Intrinsic Birefringence: The anisotropy in crystals causes this sort of birefringence. Birefringence is caused by the atomic arrangement of the crystal. Calcite, tourmaline, and other minerals are examples.

• Stress-Induced Pressure-induced birefringence: This sort of birefringence is caused by applying pressure to the property. Glass and polymers, for example, exhibit a combination of strain birefringence.

Changes in the spacing of light waves can be used to determine birefringence. Polarimetry is the name for this measurement procedure. The birefringence of lipid bilayers is measured using a technique known as dual-polarization interferometry. 

Explain Double Refraction

We observe that in double refraction light is unpolarized and it divides into two rays when passed through the doubly refracting crystal that deviates the rays into different directions.

We can also observe the Double refraction of light by comparing two materials, viz: glass and calcite.  If a mark is drawn upon a sheet of paper with a pencil and then covered with a piece of glass, only one image is visible; but if the same paper is covered with a piece of calcite, and the crystal is adjusted in a specific direction, then two marks are visible.

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The figure above shows the phenomenon of double refraction via calcite crystal. An incident ray is seen to split into two rays, the first is the ordinary ray CO and another is the extraordinary ray, i.e., CE on entering the crystal face at point C. 

Explain the Phenomenon of Double Refraction

The optic axis of a Calcite crystal (doubly refracting crystal) is defined by the symmetry of the crystal lattice. In calcite compounds or CaCO₃, the CO₃ (Carbon trioxide) forms a triangular cluster and the optic axis lies perpendicular to this.  When light enters along with the optic axis of the crystal, nothing happens and the light comes out unpolarized. However, when the light enters at a certain angle to the optic axis, the asymmetry of the lattice splits the ray into two with mutually orthogonal polarizations, as shown in the below diagram of Birefringence in a calcite crystal. 

From the below figure, we see that one ray is the Ordinary ray, for which Snell's law holds, while the other is the Extraordinary ray that does not obey Snell's law.

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Observation

On observing an object through the above crystal, we see a double image of the object. While analysis through the Polaroid sheet shows that these images have axes of polarization perpendicular to each other; therefore, rotating the Polaroid makes the images alternately disappear. 

Things get interesting when you place a second crystal just on the top of the first. Now, you have four images instead of two, but when you rotate it, the second crystal functions as an analyzer for the first one, and you get two images again.

Double Refraction of Light

In double refraction of light, the ordinary ray and the extraordinary ray are polarized in planes oscillating at right angles to each other. Furthermore, the refractive index, i.e a number that determines the angle of bending specific for each material medium of the ordinary ray is observed to be constant in all directions.

The refractive index of the extraordinary ray changes according to the direction taken because it has both parallel and perpendicular components to the crystal’s optic axis.  It’s because the speed of light waves in a medium that is equal to their speed in a vaccum is divided by the index of refraction for that wavelength, an extraordinary ray can move both faster and slower than an ordinary ray.

Do You Know?

All transparent crystals like calcite crystals except those of the cubic system that is normally optically isotropic possess the phenomenon of double refraction: in addition to calcite, some well-known examples are sugar crystals, ice, mica, quartz, and tourmaline. 

• Other materials may become birefringent under special circumstances. Now, let’s consider some examples for the same:

• Solutions of long-chain molecules exhibit double refraction when they flow, and this principle is called streaming birefringence.

• Plastic materials formed of long-chain polymer molecules can also become doubly refractive when compressed or stretched. This phenomenon is called photoelasticity. 

• There are some isotropic materials like glass that also exhibit birefringence when placed in a magnetic field or electric field or when exposed to external stress.

Enhancing Knowledge!

The effect of birefringence was first described by the Danish scientist named Rasmus Bartholin in 1669.

Birefringence Applications

Birefringence finds use in the following applications:

• Polarizing prism and retarder plates

• Liquid crystal displays

• Medical Diagnosis