What is Polarization of Light? Explained

Polarization of light is a fascinating phenomenon in optics that reveals the wave nature of light. Like other phenomena such as reflection, refraction, diffraction, dispersion, interference, and scattering, polarization is also an important topic for discussion in the world of optics. 

In this blog post, we will delve into the concept of polarization, its various types, the role of polarizers and analyzers, the significance of Polaroids, and the methods used to achieve polarization. So let's explore the world of Polarization of light.

What is Polarization of Light?

Definition: Polarization of light is a phenomenon of restricting the vibration of the electric field vector of a light wave along a specific axis on a plane perpendicular to the direction of light propagation. 

In simpler terms, it involves aligning the oscillations of light waves in a particular direction.

Unpolarized Light: Unpolarized light, such as sunlight, candlelight, or electric lamp light, consists of random vibrations of electrons, ions, or charged particles. These vibrations occur in all directions within a specific plane, making it ordinary light. 

Unpolarized light can be considered as the sum of two equally spaced perpendicular components of transverse vibration. Ordinary light is electromagnetic waves. The electric field È and magnetic field B of such waves always oscillate perpendicular to the direction of wave motion. The wave travels in a direction perpendicular to this plane.

Polarized Light: Polarized light is created when ordinary light waves are passed through a polarizer. For example, when an ordinary light wave passes through a crystal of tourmaline or similar medium, its random transverse vibration is converted into a unidirectional transverse vibration. This phenomenon is called polarization of light and that light is called polarized light.

Terminology of Polarization

To understand polarization, we need to familiarize ourselves with a few key components:

Polarizer: A polarizer is a mechanical system used to polarize unpolarized light. 

For example, tourmaline crystals are used in most light-based experiments, and other common polarizers that we use in everyday life are our sunglasses, photographic filters, and liquid crystal displays.

Analyzer: An analyzer is another mechanical system that determines whether light is polarized or not.

An optical tool like a Nicol prism is capable of producing plane-polarized light and is used to see how objects affect this polarized light after it passes through.

Plane of Vibration: The plane where the vibrations of incident light are confined is known as the vibrational plane.

Plane of Polarization: The plane where the light beam is perpendicular to the vibrational plane is referred to as the plane of polarization.

Linear Polarized Light: Light can vibrate in any direction from a point on a surface perpendicular to the normal. When the vibrations occur along a straight line within a specific plane perpendicular to the direction of light propagation, it is termed linear polarized light. This is often called plane-polarized light.

Angle of polarization: The particular angle of incidence for which the degree of polarization by reflection is maximum is called the angle of polarization (ip).

The plane-polarized light that can be produced by reflection was first discovered by the scientist Malus in 1808. He showed that when ordinary light i.e. unpolarized light is reflected by a transparent medium (e.g. water or glass), the reflected ray is partially polarized. 

The degree of polarization of the ray depends on the angle of incidence. The value of this angle depends on the reflector surface and the wavelength of the incident light.

In the case of glass, the value of the angle of polarization is about 56° and in the case of pure water, the angle of polarization is 53°.

Methods of Polarization

Polarisation of light can happen by several methods for example - Polarization by Reflection, Polarization by Refraction, Polarization by Transmission, and Polarization by Scattering. But here we will discuss only the Polarization by reflection and refraction.

Polarization by Reflection:

As we already discussed when ordinary light is reflected from a transparent medium, the reflected rays become partially polarized. The degree of polarization depends on the angle of incidence and the medium's properties. This angle is known as the angle of polarization, sometimes called Brewster's angle.

Brewster's Law:

Brewster's law describes the relationship between the angle of polarization and the refractive index of the medium. It states that the tangent of the angle of polarization is numerically equal to the absolute refractive index of the medium.

Here's the simple idea behind Brewster's Law:

When light hits a surface, it can bounce off in different directions.

Brewster's Law tells us that when light hits at a specific angle, called the "Brewster angle" (we'll call it ip), something interesting happens.

At this angle, the reflected light becomes special. The light that bounces back is "polarized," meaning its waves vibrate in a specific direction.

Now, let's put this into math:

Imagine you have a beam of light hitting a surface. We'll use "μ " to represent the refractive indexes, and "ip" for the Brewster angle.

Brewster's Law in this simple equation:

Where:

ip is the Brewster angle.

μ₁ is the refractive index of the material where the light is coming from (like air).

μ₂ is the refractive index of the material where the light is going (like glass or water).

So, to find the Brewster angle, you just need to know the refractive indexes of the two materials. But remember, since the refractive index of the medium depends on the wavelength of light, it can be said that the polarization angle also depends on the wavelength.

When light hits at the Brewster angle, the reflected light will be polarized. This means it will be less likely to create glare, which can be useful in things like sunglasses or reducing reflections on windows.

Polarization by Refraction:

Imagine light as lots of tiny waves moving in different directions. When these waves meet a clear glass surface at a special angle, called the "angle of polarization," something cool happens.

Around 85% of the light decides to go inside the glass, which we call refraction. The remaining 15% says, "No thanks," and bounces back, which we call reflection.

The light that goes inside the glass isn't all neatly organized. It's like a mix of waves going every which way. Some of the waves inside the glass start lining up their vibrations in one direction. This makes them somewhat polarized but not fully.

To get stronger polarization, we use many thin glass plates, all lined up next to each other. The light goes through these plates multiple times, each time getting more and more organized.

Eventually, the light splits into two groups. One group becomes reflected light with vibrations up and down, and the other group becomes refracted light with vibrations side to side. These two groups of light go in different directions, and their vibrations are at right angles to each other, like a cross.

So, by using multiple glass plates, we can make light become strongly polarized, which means its waves move in just one direction. This is useful for many things like reducing glare or improving image quality on certain devices.

What is Polaroid?

Polaroid is like a special sheet that turns regular light into polarized light. It does this using tiny cells called herapathite cells on a film.

Here's how it works:

These cells are arranged in a way so that all their optical axes (imagine this as their preferred direction) are lined up in the same direction.

These cells are picky about which light they like. They absorb one type of light that comes from birefringence (a fancy word for how light splits into two rays in some materials). They don't like this one.

But, they let the other type of light pass through. This light is special because it's now "plane polarized," which means all its waves move in the same direction.

So, when light goes through Polaroid, it only lets through the light that's moving in one direction, and that's why it's used in things like sunglasses to reduce glare.

Uses of Polaroid:

1. Polaroids can generate and analyze polarized light very easily and at a very low cost.

2. Advanced sunglasses use Polaroid instead of glass which is more comfortable for the eyes.

3. Polaroids are affixed to car headlights and window screens. As a result, the strong light coming from the front cannot enter the car.

4. Polaroid film is used to make three-dimensional movies (3D pictures).