What is the difference between unpolarised and plane-polarised light?

Unpolarised light, like that from the sun or a typical bulb, has its electric field vectors oscillating randomly in all possible planes perpendicular to the direction of propagation. There's no preferred direction for the vibrations. Plane-polarised light, on the other hand, has its electric field vectors oscillating along a single, fixed direction in the plane perpendicular to the direction of propagation. All vibrations are confined to one specific plane, known as the plane of polarisation. This ordered vibration is the key distinction.

How does a Polaroid sheet work to polarise light?

A Polaroid sheet works based on the principle of selective absorption, also known as dichroism. It contains long-chain polymer molecules that are stretched and aligned in a particular direction. When unpolarised light passes through it, the electric field components vibrating parallel to the alignment of these molecules are strongly absorbed. The components vibrating perpendicular to this alignment, however, are transmitted. The direction perpendicular to the absorption axis is called the 'pass axis' or 'transmission axis'. Thus, the light emerging from a Polaroid is plane-polarised with its electric field vibrations parallel to the pass axis.

What is Brewster's angle and why is it important?

Brewster's angle ($i_p$) is a specific angle of incidence at which unpolarised light, when reflected from a transparent dielectric surface (like glass or water), becomes completely plane-polarised. At this angle, the reflected light's electric field vibrations are entirely perpendicular to the plane of incidence. It's important because it provides a method to obtain completely plane-polarised light through reflection, and it's related to the refractive index ($mu$) of the medium by the simple relation $ an i_p = mu$. Also, at Brewster's angle, the reflected and refracted rays are perpendicular to each other.

Can sound waves be polarised? Why or why not?

No, sound waves cannot be polarised. Polarisation is a phenomenon exclusive to transverse waves, where the oscillations are perpendicular to the direction of wave propagation. Light waves are transverse electromagnetic waves, so they can be polarised. Sound waves, however, are longitudinal waves. In longitudinal waves, the particles of the medium oscillate parallel to the direction of wave propagation. Since there's only one direction of oscillation relative to propagation, there's no 'plane' to restrict the vibrations to, hence polarisation is not possible for sound waves.

What is Malus's Law and when is it applied?

Malus's Law describes the intensity of plane-polarised light after it passes through an analyser (a second polariser). If plane-polarised light of intensity $I_0$ is incident on an analyser whose transmission axis makes an angle $ heta$ with the plane of polarisation of the incident light, the transmitted intensity $I$ is given by $I = I_0 cos^2 heta$. This law is applied whenever plane-polarised light passes through a polarising filter, allowing us to calculate the reduction in intensity based on the relative orientation of the light's polarisation plane and the filter's transmission axis. It's fundamental for problems involving multiple polarisers.

Polarisation

Physics

NEET UG

Version 1Updated 22 Mar 2026

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Polarisation refers to the phenomenon where the vibrations of a transverse wave are restricted to a single plane or a specific pattern. For light, which is an electromagnetic wave, polarisation describes the orientation of its electric field oscillations. While unpolarised light consists of electric field vibrations occurring in all possible planes perpendicular to the direction of propagation, po…

Quick Summary

Polarisation is the phenomenon where the vibrations of a transverse wave, specifically the electric field oscillations of light, are restricted to a single plane or a specific pattern. Unpolarised light has electric field vibrations in all directions perpendicular to propagation, while plane-polarised light has vibrations confined to one plane.

Key methods of achieving polarisation include selective absorption (using Polaroid sheets, governed by Malus's Law: $I = I_0 cos^2 \theta$ ), reflection (at Brewster's angle, where $an i_p = mu$ ), refraction (double refraction in birefringent crystals like calcite, producing O-ray and E-ray), and scattering (e.

g., light from the sky). Applications range from glare-reducing sunglasses and LCD screens to 3D movies and chemical analysis. Understanding these methods and their associated laws is crucial for NEET, focusing on conceptual clarity and problem-solving.

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Key Concepts

Malus's Law Application

Malus's Law, $I = I_0 cos^2 \theta$ , is fundamental for calculating light intensity after passing through a…

Brewster's Angle Calculation

Brewster's Law, $an i_p = mu$ , allows us to determine the angle of incidence for complete polarisation by…

Polarisation by Double Refraction

In birefringent crystals like calcite, unpolarised light splits into two plane-polarised rays: the Ordinary…

Unpolarised Light: — E-field vibrates in all planes.

Plane-Polarised Light: — E-field vibrates in one plane.

Polariser: — Converts unpolarised to plane-polarised light.

Analyser: — Detects plane-polarised light.

Malus's Law: —

I = I_0 cos^2 \theta

(for plane-polarised light through analyser).

Unpolarised through Polariser: —

I = I_u/2

Brewster's Law (Reflection): —

an i_p = mu

. Reflected and refracted rays are perpendicular (

i_p + r' = 90^circ

Double Refraction (Birefringence): — Splits light into O-ray and E-ray (mutually perpendicular polarisation).

Polarisation by Scattering: — Scattered light at

90^circ

is partially/completely polarised.

Confirms: — Transverse nature of light.

Polarisation Really Always Demonstrates Super Transverse Moves.

Polarisation

Reflection (Brewster's Law)

Absorption (Malus's Law)

Double Refraction

Scattering

Transverse (confirms transverse nature)

Malus's Law:

I = I_0 cos^2 \theta