Plane-polarized light

The polarisation of light has become an important aspect due to its application in optical fields. Optical design is a subject mainly focusing on the wavelength and the intensity of light while often neglecting polarisation. The latter, however, is an important aspect that affects even optical systems that do not measure it explicitly. The polarisation of light affects the laser beams’ focus, cut-off wavelengths of filters, and prevents unwanted reflections. It’s used in many meteorological applications, including stress analysis in glass and plastic, pharmaceutical analysis, and biology. There are different absorption levels for further polarisation of light, like LCD screens, 3D movies, and glass-reducing sunglasses.

In Physics, Polarisation is termed as a phenomenon that is caused due to the wave nature of electromagnetic radiation. Sunlight has to pass through a vacuum to reach Earth, which is another example of electromagnetic waves. They are called electromagnetic waves because they are formed when electric fields interact with a magnetic field. 

What is polarisation?

Light comes under the category of an electromagnetic wave, and the electric field of this wave oscillates in a perpendicular direction to the direction of its propagation. Light is termed unpolarised if this electric field’s focus fluctuates randomly. Some examples of unpolarised light include sunlight, halogen lighting, LEDs, and incandescent bulbs. However, when the direction of this electric field is defined, it is called plane-polarised light. The most common example of this kind is laser lights and devices.

What are transverse waves?

To understand plane polarisation light better, let us know the concept of transverse waves. They are produced when the movement of particles in the wave is perpendicular to the direction of the wave’s motion. Ripples in water and the motion of sound waves could be an example of transverse waves. Longitudinal waves are referred to as particles when they travel in the direction of the motion of the waves.

Light is the result of the interaction of electrical and magnetic fields that travel through space. The electrical and magnetic vibrations of a light wave happen in perpendicular directions. They move in a perpendicular direction to one another.

Understanding Polarisation.

Let us perform an experiment where we take two planes. The magnetic field occupies one plane while the electrical field occupies the other, and both of them are placed perpendicular to one another. These vibrations can occur in numerous planes. A light wave that vibrates in more than one plane is unpolarised light. The other kind is the polarised light in which the vibrations will occur in a single plane. Plane-polarised light will consist of waves with the same direction of vibrations. The process involving transforming unpolarized light into a polarised light is known as polarisation.

From the figure, you can see how a light beam comes out in the same direction after passing through a polariser. Before the light beam passes through a polarizer, its path keeps varying but as soon as it passes through a polariser, it gets aligned in the same direction.

Types of Polarisation

Depending on the motion of the transverse and longitudinal waves, polarisation can be classified into three types:

• Linear Polarisation
• Circular Polarisation
• Elliptical Polarisation

Linear Polarisation:

In this type of polarisation, the light’s electric field travels in propagation and is limited to a single plane. Electric field of linearly polarised light will be confined to a single plane. In this case, the electric field remains limited to a single plane, the x-y plane. It is linear polarization.

Circular Polarisation:

In this type of polarisation, the electric field of the light consists of two components. They are perpendicular to each other and have an equal amplitude but a phase difference of π/2. As a result, the electric field rotates in a circle around the direction of prorogation. It is known as circular polarisation. The electrical field of circularly polarized light travels along with a phase difference of π/2 or 90 degrees. Depending on the direction of rotation, they are further classified into left-hand circularly polarised and right-hand circularly polarized light.

 Elliptical Polarisation

This case can be seen when an electric field follows an elliptical propagation. It happens due to two linear components that have different amplitudes and phase difference that is not 90 degrees.

What are Polarisers?

Polarisers are used to achieve a particular polarization of light. They can be divided into reflective, dichroic, and birefringent polarisers. They act as an optical filter that allows light waves to pass through it while blocking undesirable light waves of other kinds. Another definition suggests it is an optical device used to convert an unpolarised light into a polarised plane light.

Reflective Polarisers: They help transmit the desired light waves while reflecting the rest of it. Grid wires are a suitable example of this. The light that is to be polarised is reflected through the grid wires, while the light that is to be transmitted is polarised perpendicular to these wires.

Dichroic Polarisers: They help absorb a specific polarization of light and transmit the rest. Nanoparticle polarisers are an example of this.

Birefringent Polarizers: They depend on the refractive index of the polarisation of light. Different plane-polarised light can be refracted at different angles, which can be used to select a desired polarisation of light.

What are waveplates?

Polarisers are used to attain certain polarisations of light while discarding the rest. But in the case of waveplates, they modify polarisations without discarding them or even deviating or displacing the beam. It can be done by delaying one polarisation component against its orthogonal component. They are mostly used to convert polarisation states and linear polarisations into circularly polarised light.

Conclusion

Polarisation is used in various imaging applications where they are placed over a light source or lens to discard excess light, improve contrast, and eliminate hot spots from reflective objects. It is effectively used in sunglasses to reduce glare. The television screen that we watch at home, the LCDs use this concept for its modulating properties of liquid crystals because they do not emit light directly. They are even used to improve contrast and colour effects. When a polariser is used, it filters out some light and reflects it, such that it darkens the perceived colours of these images.