How Does Double Refraction Occur? Explained with Diagrams
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
FAQs on Understanding Double Refraction: Meaning, Phenomenon & Uses
1. What is the phenomenon of double refraction in Physics?
Double refraction, also known as birefringence, is an optical property of certain materials where a single ray of unpolarised light entering the material splits into two separate rays. This occurs in anisotropic materials, which have different optical properties in different directions. These two resulting rays are polarised at right angles to each other and travel in different directions through the material.
2. What are the Ordinary Ray (O-ray) and Extraordinary Ray (E-ray)?
When a light ray splits during double refraction, it forms two distinct rays:
- Ordinary Ray (O-ray): This ray behaves as expected and obeys Snell's law of refraction. Its refractive index remains constant regardless of its direction through the crystal.
- Extraordinary Ray (E-ray): This ray is unusual because it does not obey Snell's law. Its velocity and refractive index vary depending on its direction of travel relative to the crystal's optic axis.
Both the O-ray and E-ray are plane-polarised, with their planes of polarisation being perpendicular to each other.
3. What is the visual effect of double refraction?
The most common example of double refraction's visual effect is seen with a calcite crystal. If you place a piece of calcite over a single dot or line drawn on paper, you will see two distinct images of that dot or line. Each image is formed by one of the two refracted rays (the ordinary and extraordinary rays), demonstrating the splitting of light.
4. Which types of materials exhibit double refraction?
Double refraction is a characteristic property of all transparent crystals that have a non-cubic crystal structure. These are also known as anisotropic crystals. Common examples include:
- Calcite
- Quartz
- Mica
- Tourmaline
- Ice
Additionally, some materials like plastics or glass can become temporarily birefringent when subjected to mechanical stress, a phenomenon called photoelasticity.
5. Why is one ray called 'Ordinary' and the other 'Extraordinary'?
The names are based on their behaviour with respect to the classical laws of optics. The Ordinary Ray (O-ray) is so named because it behaves 'ordinarily' by consistently following Snell's Law of Refraction. In contrast, the Extraordinary Ray (E-ray) is named for its 'extraordinary' behaviour, as it does not follow Snell's Law and its speed changes with its direction inside the crystal.
6. How is double refraction different from diffraction?
While both are phenomena of light, they are fundamentally different. Double refraction is the splitting of a light ray into two polarised rays due to the anisotropic nature of a material. It is a property of the medium. Diffraction, on the other hand, is the bending or spreading of light waves as they pass an edge or go through an aperture. It is a fundamental property of waves and is not dependent on a specific type of material like birefringence is.
7. Why does double refraction not occur along the optic axis of a crystal?
The optic axis is a unique direction within a birefringent crystal where it behaves as if it were an isotropic material. When light enters the crystal parallel to this axis, it does not split into two rays. This is because, along this specific direction, the refractive indices for both the ordinary and extraordinary rays are equal. Consequently, they travel at the same speed and in the same direction, and no separation is observed.
8. What are some important real-world applications of double refraction?
The principle of double refraction is critical for several optical technologies. Some key applications include:
- Polarising Prisms: Devices like the Nicol prism use birefringence to separate an unpolarised beam into two polarised beams and then eliminate one, producing a single, perfectly plane-polarised beam of light.
- Liquid Crystal Displays (LCDs): The ability to control the birefringence of liquid crystals with an electric field is the fundamental principle behind LCD screens in TVs, monitors, and calculators.
- Retarder Plates: Also known as wave plates, these devices use birefringent materials to alter the polarisation state of light passing through them.
- Medical and Geological Analysis: Birefringence is used in polarising microscopes to identify minerals in geology and to study stresses and structures in biological tissues in medical diagnostics.
