Optics- Polarisation of Light

  • Definition: Polarisation of light refers to the phenomenon in which the oscillations of the electromagnetic waves are restricted to a specific plane.
  • Polariser: A polariser is a device that can selectively transmit light vibrations in a particular plane, while absorbing or reflecting those in other planes.
  • Types of Polarisation:
    • Linear Polarisation
    • Circular Polarisation
    • Elliptical Polarisation
  • Mechanism of Polarisation:
    • By reflection
    • By scattering
    • By absorption
    • By double refraction

Polarisation by Reflection

  • When an unpolarised light wave is incident on a smooth surface at a particular angle (known as the polarising angle), the reflected light is partially or completely polarised.
  • The reflected light is polarised because the electric field vector of the incident wave perpendicular to the plane of incidence gets partially or completely cancelled due to reflection.
  • The polarising angle can be given by the formula: θ_p = arctan(μ), where μ is the refractive index of the medium.
  • The amount of polarisation at the polarising angle is maximum.

Polarisation by Scattering

  • When light waves are incident on small particles or molecules, such as in the Earth’s atmosphere, the light gets scattered.
  • In the scattered light, the vibrations in certain planes (perpendicular to the direction of propagation) are preferentially transmitted, while others are absorbed or scattered.
  • This phenomenon is known as polarisation by scattering.
  • Examples: Polarisation of sunlight in the sky, creating the blue color of the sky and the colours of a rainbow.

Polarisation by Absorption

  • Certain materials, like tourmaline, can absorb or transmit light vibrations in specific planes.
  • In such materials, the absorbed component is preferentially absorbed, allowing only the vibrations in a specific plane to pass through.
  • This is known as polarisation by absorption.
  • This effect is used in polarising sunglasses to reduce glare and improve visual comfort.

Polarisation by Double Refraction

  • Some crystals, such as calcite, exhibit an optical property called double refraction, where the incident light wave is split into two refracted waves.
  • The refracted waves have different velocities and directions of polarisation.
  • This phenomenon is used in devices called polarisers and analyzers to manipulate and detect polarised light.
  • Double refraction is a result of anisotropy in the structure of crystals.

Malus Law

  • Malus Law states that the intensity of light transmitted through a polariser is directly proportional to the square of the cosine of the angle between the polariser’s transmission axis and the plane of polarization of the incident light.
  • Mathematically, it can be expressed as: I = I₀ cos²θ where I₀ is the initial intensity of the incident light, I is the transmitted intensity, and θ is the angle between the transmission axis and the plane of polarization.
  • This law is applicable for linearly polarised light.

Polarisation by Selective Absorption

  • Some substances, like tourmaline, possess the property of selective absorption, where vibrations of light waves in a particular direction are preferentially absorbed.
  • The absorbed component of light is responsible for causing the other component, transmitted vibrations, to be polarised.
  • This phenomenon is used in the production of polarisers and polarising filters used in various optical devices.

Malus Law - Example

  • Example:
    • Initial intensity of light, I₀ = 1 W/m²
    • Angle between transmission axis and plane of polarization, θ = 60°
    • According to Malus Law, I = I₀ cos²θ
    • Substituting the given values, I = 1 × (cos²60°) = 0.25 W/m²
  • Thus, the transmitted intensity of light is 0.25 W/m².

Applications of Polarisation

  • Polarisation is used in various applications, including:
    1. Polarising sunglasses to reduce glare from reflected light.
    2. LCD (Liquid Crystal Display) screens to control the orientation of light and create images.
    3. 3D glasses for stereoscopic viewing.
    4. Polarised filters to improve visibility in photography.
    5. Communication systems, such as fibre optics, to transmit and receive polarised light.
  • Understanding the principles of polarisation is crucial in understanding the working and design of these devices.

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  • Polarisation by Brewster’s Law:
    • Brewster’s law describes the relationship between the angle of incidence and the angle of reflection for which the reflected light becomes completely polarised.
    • According to Brewster’s law, the tangent of the angle of incidence is equal to the refractive index of the medium.
    • It can be expressed as: tan(θ_p) = μ, where θ_p is the polarising angle and μ is the refractive index.
  • Polarisation by Reflection (Example):
    • Suppose light is incident on a glass surface from air with an angle of incidence of 45°, and the refractive index of glass is 1.5.
    • Using Brewster’s law, the polarising angle can be determined as: θ_p = arctan(1.5) ≈ 56.31°.
    • Therefore, the reflected light will be completely polarised when the angle of incidence is 56.31°.
  • Polarisation by Double Refraction (Example):
    • Consider a calcite crystal that is illuminated by unpolarised white light.
    • The light incident on the crystal is split into two refracted rays with different velocities and directions of polarisation.
    • One of the refracted rays follows the normal laws of refraction, while the other one undergoes extraordinary refraction.
    • As a result, the light emerging from the crystal is composed of two polarised rays that are perpendicular to each other.
    • This phenomenon is used in devices like polarising sheets and Nicol prisms.
  • Polarisation by Double Refraction (Contd.):
    • Nicol Prism:
      • Nicol prism consists of a calcite crystal that is cut and rearranged to form a wedge-like shape.
      • The incident light is split into two rays, one of which undergoes total internal reflection and emerges as linearly polarised light.
      • The Nicol prism acts as a polariser by transmitting only a certain orientation of the electric field.
      • This polarised light can then be further passed through another Nicol prism acting as an analyser to control the transmitted light.
  • Circular Polarisation:
    • Circular polarisation refers to the polarisation of light in which the electric field vector rotates in a circular path while propagating in a certain direction.
    • It can be achieved by superposing two perpendicular linearly polarised waves with equal amplitudes and a phase difference of 90°.
    • The resulting motion of the electric field vector traces out a circular path.
    • Circular polarisation is used in applications such as 3D glasses and optical communication systems.
  • Elliptical Polarisation:
    • Elliptical polarisation is a combination of linear and circular polarisations, where the electric field vector follows an elliptical path instead of a circle.
    • This occurs when two linearly polarised waves with different amplitudes and a phase difference other than 0° or 90° are superposed.
    • The resulting motion of the electric field vector creates an elliptical path.
    • Elliptical polarisation is observed in certain natural and artificial optical phenomena.
  • Analyser:
    • An analyser is a device that can transmit or block specific orientations of polarised light.
    • It is used in conjunction with a polariser to analyze and detect the polarisation of light.
    • The transmission axis of the analyser determines the orientation of the polarised light that can pass through.
    • When the transmission axes of the polariser and analyser are parallel, maximum intensity of light is transmitted.
    • When the transmission axes are perpendicular, minimum or no light is transmitted.
  • Polaroids and Polarising Sheets:
    • Polaroids are sheets made of long-chain polymer molecules aligned in a specific direction during the manufacturing process.
    • These sheets act as polarisers by preferentially transmitting light vibrations in a particular plane.
    • Polarising sheets are often used in optical devices and photography to control the transmission of light and reduce glare.
    • They can be rotated to change the orientation of the transmitted polarised light.
  • Applications of Polarising Sheets:
    • Polarising sheets have numerous applications, including:
      • Increasing contrast in LCD screens by controlling the polarisation of light.
      • Improving visibility and reducing reflections in photography.
      • Reducing glare and improving visual comfort in polarising sunglasses.
      • Polarising filters in scientific experiments to control the orientation of light.
      • An essential component in optical devices like microscopes, cameras, and projectors.
  • 3D Glasses:
    • In stereoscopic 3D technology, two images taken from slightly different perspectives are presented separately to each eye.
    • To perceive the depth of the 3D image, different images must be seen by each eye.
    • 3D glasses use polarisation to achieve this effect.
    • One lens of the 3D glasses contains a vertical polarising filter, and the other lens contains a horizontal polarising filter.
    • Combined with a 3D screen that emits vertically and horizontally polarised light, each eye sees a different image, creating the perception of depth.
  • Polarisation in Communication Systems:
    • Optical communication systems, such as fibre optics, use polarised light to transmit and receive signals.
    • The light source, typically an LED or laser, emits polarised light that is guided through optical fibres.
    • Polarisation maintaining (PM) fibres are used to maintain the polarisation of the transmitted light.
    • At the receiving end, polarisers or photodetectors are used to accurately detect and interpret the polarised signals.
    • Polarisation in communication systems helps improve signal quality, reduce interference, and increase transmission distances.
  • Polarisation in Photography:
    • Polarising filters are commonly used in photography to control the polarisation of light and improve image quality.
    • These filters can selectively transmit or block certain orientations of polarised light.
    • By reducing reflections and glare, polarising filters enhance color saturation and contrast.
    • Photographers often use polarising filters when shooting landscapes, water surfaces, or in bright sunlight conditions.
    • Adjusting the orientation of the filter can vary the intensity and direction of the polarised light, allowing for creative effects.
  • Polarisation in LCD Screens:
    • Liquid Crystal Displays (LCDs) use polarisation to control the orientation of light and create images.
    • An LCD screen consists of a backlight that emits unpolarised light, which passes through a polariser to become polarised in a specific direction.
    • The polarised light travels through liquid crystal cells, which can be manipulated by an electric field to either block or transmit the light.
    • The variation in light transmission through different liquid crystal cells produces the desired image on the screen.
    • Finally, another polariser aligned perpendicular to the initial polariser filters the transmitted light, resulting in the display of different colors and shades.
  • Polarisation in Fiber Optics:
    • Fiber optics is a communication technology that uses polarised light to transmit information through thin strands of glass or plastic fibers.
    • Polarisation maintaining (PM) fibers are used to maintain the polarisation of the transmitted light.
    • This ensures that the transmitted signal retains its integrity and can be accurately received at the other end.
    • PM fibers have a unique structure that preserves the polarisation of light by reducing cross-coupling between different polarisation modes.
    • Fiber optic communication allows for high-speed, long-distance data transmission and is widely used in telecommunications and internet networks.
  • Polarisation in Medical Imaging:
    • Polarised light is utilized in certain medical imaging techniques to enhance visualization and diagnostics.
    • For example, polarised light microscopy is employed in pathology to detect and analyze birefringent materials like crystals or collagen fibers in tissues.
    • Polarised light can also be used in dermatology to observe polarisation patterns of skin lesions, aiding in the diagnosis of conditions like skin cancer.
    • Additionally, polarising filters are used in ophthalmology to examine the cornea and detect irregularities or abnormalities.
  • Polarisation in Astronomy:
    • In astronomy, the polarisation of light from distant celestial objects provides valuable information about their properties and composition.
    • By measuring the degree and angle of polarisation, astronomers can study the magnetic fields, scattering properties, and physical processes occurring in astronomical objects.
    • Polarimetric observations are used to study various phenomena, such as the polarization of light from stars, exoplanets, nebulae, and interstellar dust.
    • Polarisation also plays a crucial role in studying the cosmic microwave background radiation, which provides insights into the early universe.
  • Polarisation and 3D Movies:
    • The perception of depth and three-dimensionality in movies can be achieved using polarisation techniques.
    • In movie theaters, 3D glasses with polarising filters are distributed to viewers.
    • The projection system uses dual projectors, each polarising light in a different orientation.
    • The left eye lens of the glasses filters out light polarised in the same orientation as the right projector, and vice versa.
    • This ensures that each eye sees a slightly different image, creating the illusion of depth and a three-dimensional viewing experience.
  • Polarisation and Holography:
    • Holography is a technique that uses the interference patterns of laser light to create three-dimensional images.
    • During holography, a laser beam is split into two parts: the object beam and the reference beam.
    • The object beam reflects off the object and interacts with the reference beam.
    • The interference pattern created by the two beams is recorded on a photosensitive medium.
    • When illuminated with coherent light, the recorded interference pattern reconstructs the original object, including depth information, resulting in a holographic image.
  • Quantum Nature of Light and Polarisation:
    • From a quantum mechanical perspective, light can be understood as consisting of individual particles called photons.
    • Each photon carries a certain amount of energy and exhibits wave-particle duality.
    • The polarisation of light is related to the orientation of the oscillating electric field vector associated with the photons.
    • Quantum mechanics describes the probabilistic nature of photon polarisation and its measurement through observables like polarisers.
    • Understanding the quantum nature of light is crucial for advanced applications such as quantum information processing and quantum communication.
  • Polarisation in Nature:
    • Polarised light is observed in various natural phenomena and environments.
    • For instance, polarisation contributes to the vibrant colors seen in butterfly wings or the iridescence of certain bird feathers.
    • Polarised light is also utilized by some species for navigation, detection of prey, or signaling purposes.
    • The natural world provides numerous examples where polarisation plays a role in the behaviors and adaptations of organisms.
  • Polarised Light and Scientific Research:
    • The study of polarised light has significant applications in scientific research across various disciplines.
    • It is used in fields such as material science, crystallography, chemistry, biology, and atmospheric research.
    • Polarised light helps in investigating molecular structures, understanding the behavior of materials, determining the composition of substances, and studying environmental processes.
    • Scientists use polarisation techniques to unravel fundamental properties of matter and explore the interaction of light with different systems.
    • Polarised light is a valuable tool for making scientific advancements and enhancing our understanding of the natural world.