Understanding Polarisation of Light: Complete Guide for Students

Polarisation of light is a fundamental concept in wave optics that describes the restriction of the vibration directions of the electric field in light waves. This property establishes that light is a transverse electromagnetic wave and is crucial in understanding several optical phenomena and practical applications.

Meaning and Definition: Polarisation of Light

Polarisation of light refers to the phenomenon where the vibrations of the electric field vector are limited to a single plane perpendicular to the direction of wave propagation. Unpolarised light contains vibrations in all possible perpendicular planes, while polarised light has vibrations confined to one specific plane.

Types of Polarisation of Light

Light can be polarised in different ways based on the alignment and variation of the electric field. These forms include linear, circular, and elliptical polarisation, each described by the motion of the electric field vector's tip as the wave propagates.

Polarised and Unpolarised Light: Distinction

Unpolarised light has electric field vibrations distributed randomly in all perpendicular planes, as observed in sunlight and most artificial sources. In contrast, polarised light exhibits vibration in a single specified plane. This distinction highlights the transverse wave nature of light, as only transverse waves can be polarised.

Methods of Producing Polarised Light

Polarisation can be achieved using various physical phenomena or devices. Each method results in light with electric field restricted to one direction or a specific pattern. The main methods include polarisation by reflection, scattering, and transmission through polarising materials.

• Reflection at Brewster’s angle produces fully polarised light
• Scattering by small particles causes partial polarisation
• Transmission through polaroid filters produces plane polarised light

Polarisation by reflection occurs when unpolarised light strikes a transparent surface at a specific angle, known as Brewster’s angle. At this angle, the reflected light is completely polarised perpendicular to the plane of incidence.

Brewster’s angle, $\theta_B$, is given by the relation:

$\tan\theta_B = \dfrac{n_2}{n_1}$

Here, $n_1$ and $n_2$ are the refractive indices of the two media at the interface. This relation is important for solving numerical problems on Reflection And Refraction of light.

Polarisation by scattering is observed in phenomena such as the blue colour of the sky. When sunlight passes through the atmosphere, small particles scatter the light, resulting in partial polarisation, especially at $90^\circ$ from the sun.

Transmission through polaroid filters restricts the vibrations of light to a single plane, producing linear polarisation. This principle is based on the selective absorption of light vibrating in certain directions, commonly used in sunglasses and optical instruments.

Laws and Formulae Related to Polarisation

The quantitative study of polarisation involves Malus' Law, Brewster’s Law, and the degree of polarisation. These provide relationships between intensity and angles of polarisers, or refractive indices and angles of incidence.

• Malus’ Law: $I = I_0 \cos^2\theta$
• Brewster’s Law: $\tan\theta_B = n_2 / n_1$
• Degree of Polarisation: $P = \dfrac{I_{max} - I_{min}}{I_{max} + I_{min}}$

In Malus' Law, $I_0$ is the initial intensity of polarised light, $\theta$ is the angle between the transmission axes of the polariser and analyser, and $I$ is the transmitted intensity. These relationships are important for Light Waves problems in physics exams.

Proving the Transverse Nature of Light

The phenomenon of polarisation only occurs in transverse waves, not in longitudinal waves. The ability of light waves to exhibit polarisation confirms their transverse electromagnetic nature. This provides clear distinction compared to waves such as sound, which cannot be polarised. Refer to the detailed Difference Between Light Waves And Sound Waves.

Applications of Polarisation of Light

Polarisation of light has several important applications in optical devices and day-to-day life. It is used to reduce glare in sunglasses, enhance contrast in photography, and control displays in LCD screens. In scientific fields, polarisation is used for stress analysis in materials and in optical communications. Advanced details are available on the Polarisation Of Light page.

• Glare reduction by polarised sunglasses
• Contrast improvement in camera filters
• LCD displays using controlled polarisation
• Stress analysis in plastics and glass

Sample Numerical: Brewster’s Angle Calculation

Light is incident on a glass surface with refractive index $n = 1.5$. To find the Brewster angle, use the formula:

$\tan \theta_B = n$

$\Rightarrow \theta_B = \arctan(1.5) \approx 56.3^\circ$

Key Points on Polarisation of Light

• Polarisation proves the transverse wave nature of light
• Methods include reflection, scattering, and transmission
• Brewster’s Law gives the polarising angle for a medium
• Malus' Law relates transmitted intensity to polariser alignment

For an in-depth understanding of the properties of light, students should refer to the section on Properties Of Light which covers additional optical concepts relevant for exams.