Light Filtering Polaroid Films - Polarization

Introduction

Light is represented by an Electromagnetic Wave (EMW), where it travels by oscillating electric and magnetic field intensities simultaneously. Here, the electric field will be perpendicular to the magnetic field and vice versa. Not only that, these two components together oscillate perpendicular to the direction of propagation of light waves.

Mainly, oscillations due to electric fields in a plane are considered for the light as electromagnetic waves when compared to magnetic fields. Hence, light electromagnetic waves are also called transverse waves. Since the atoms that produce EMW can vibrate or oscillate in all directions, it will be difficult to focus the light at a spot we need. For some applications the intensity of light should be reduced like in vehicles to prevent accidents, there shouldn’t be a back reflection of bright light.

DECHAMMAKL, Electromagnetic waves, CC BY-SA 4.0

To overcome this type of issue, the brightness of light wave intensities should be reduced by starting the atoms to oscillate in all directions and thus allowing light waves to propagate in one direction only. This process is called polarization and it is done by using polaroids.

What is Polarization?

We know the light is a transverse nature of electromagnetic waves where the electric field is considered and it travels perpendicular to the light propagation in a plane. The particles in the light oscillate in different directions, giving rise to the vibration of the electric field components in vibrating all the directions in a plane. So, in normal light, the vibration of the electric field will be in different directions leading to an unpolarised light. Suppose we constrained all the vibrations of the electric field in every direction except in one direction, it is said to be polarised light and this process of obtaining polarised light is termed polarization.

Dave3457, Circular.Polarization.Circularly.Polarized.Light Circular.Polarizer Creating.Left.Handed.Helix.View,

Types of polarisation of light

• Linear polarization

• Circular polarization− Same amplitude with 90 degrees phase difference.

• Elliptical polarization− Different amplitudes with 90 degrees phase difference.

What are Polaroids?

Polarization of light can be obtained by passing unpolarized light through a thin sheet of crystal material. This thin crystal material is known as Polaroids or polarizers. The molecules in the polaroids are positioned in a particular direction, which is said to be the axis of the polaroids, which helps in allowing only those oscillated electric fields which are in line with this axis. Hence, producing a polarized light.

What are Polaroid Films?

Polaroid films are made up of organic materials having long chains of molecules or atoms that are aligned in a particular direction. In Polaroid films, the crystal material molecules are placed between two glass-like sheet substances. In making polaroid films, the crystal material we consider is quinine sulfate or herapathite which are placed between two transparent nitrocellulose polymer films. These crystals are then aligned by applying electric or magnetic fields in a particular direction giving the axis of the polaroids.

How light is polarized using polaroid film?

When an unpolarized light having a transverse electric field oscillating in all directions passes through the polaroid film, a polarized light wave oscillating in one direction will get transmitted out of the polaroid. The direction of transmitted polarised light depends on the direction of alignment of the crystal of atoms or molecules inside the polaroid film. This helps in reducing the intensity or brightness of the light which is used for many light applications. If $\mathrm{I_{0}}$ is the full intensity of an unpolarized light passing through the polaroid films, then the transmitted light intensity from the polaroid will be I. Suppose the angle between the direction of polarized light to the axis of the polaroid is $\mathrm{\theta}$, then the amplitude of transmitted wave will be $\mathrm{Ecos\:\theta}$. Here E is the amplitude of the electric field component. Then the relation between the intensity of unpolarized light to the intensity of transmitted polarized light is given according to Malu’s law as,

$$\mathrm{I=I_{0}cos^{2}\theta }$$

The direction of Polarization of Light

The direction of polarization of light is nothing but the direction of oscillation of the electric field after polarization. An unpolarized light where the electric field oscillates in all directions can be converted into polarized light, in which the electric field oscillates in one direction only. This process of allowing only one direction of EMW is achieved by using polaroids. Hence, polaroids can control the directions of light propagation. It means the direction of propagation of polarised light will be the direction of the polaroid axis on the film.

Uses of Polaroids

• In sunglasses to reduce harmful sun rays falling on our eyes.

• In headlights in vehicles control of the brightness of light helps in reducing the accidents to occur.

• In cameras for getting better-colored pictures.

• In trains and airplanes glass windows, where the polaroid films help in reducing the bright intensity of light passing through.

FAQs

Q1. What are different methods for polarizing a light other than polaroids?

Ans. The polarization of light can be obtained by another process like light scattering, light reflection and by Nicol prism.

Q2. What is meant by a dichroic substance in polaroid?

Ans. In polaroids, dichroic crystals are used, which means these types of substances allow only a particular direction of oscillating waves and absorb the remaining.

This process where the substance selects one direction waves to transmit through It is called dichroism and such substances are called dichroic substances.

Q3. Are there any organic materials that can polarize the light on their own?

Ans. Yes, Liquid crystals are one of the organic materials that can polarize the light and transmit, since all the molecules inside the liquid crystals will be specifically aligned in one direction.

Q4. What is an analyzer in polaroids?

Ans. We can use two polarizers in the process of polarization of light. Analyzer acts as a second polarizer, that helps in finding out whether the light transmitted from the first polarizer is plane polarized or not.

Q5. Explain polarized light when polarizer and analyzer are perpendicular to each other?

Ans. Suppose the polarizer axis is parallel aligned, then from the unpolarized light only those oscillating electric fields allowed that are parallel in direction to the polarizer axis. After that, if we put an analyzer in which its axis is perpendicularly aligned, then no polarized light will pass through the analyzer. This is because the polarized light coming out from the polarizer is oscillating parallelly.