Optics - Polarisation of Light - Polaroid Sheet (or Sheet Polariser)

  • Polarisation
    • Definition: process of restricting or confining the vibrations of a transverse wave to a single plane
    • Transverse wave: oscillation is perpendicular to the direction of energy transfer
  • Natural light
    • Contains waves oscillating in all possible transverse directions
    • Vibrations are in all planes perpendicular to the direction of propagation
  • Polarised light
    • Contains waves oscillating in a single plane perpendicular to the direction of propagation
    • Vibrations are confined to a specific plane
  • Polaroid sheet (or polariser)
    • Device used to polarise light
    • Consists of long chain polymer molecules aligned in a single direction
    • Only allows vibrations in one direction to pass through while blocking vibrations in other directions
    • Unpolarised light passing through a polaroid sheet becomes polarised
  1. Properties of Polarised Light
  • Polarised light has the following properties:
    • Oscillates in a single plane perpendicular to the direction of propagation
    • Vibrations are in a fixed direction
    • Vibrations cannot be changed by passing through additional polarisers
    • Can be transmitted, absorbed, or reflected based on the angle of incidence and orientation of the polariser
  1. Malus’s Law
  • Malus’s Law relates the intensity of polarised light transmitted through a polariser to the angle between the polariser and the plane of polarisation of the incident light.
  • The equation for Malus’s Law is:
    • I = I₀cos²θ
    • Where I is the intensity of the transmitted light, I₀ is the initial intensity of the incident light, and θ is the angle between the polariser and the plane of polarisation.
  1. Analyzing Polarised Light
  • Polaroid sheets can be used to analyze the properties of polarised light.
  • When two polaroids are aligned parallel to each other, maximum intensity of light is transmitted.
  • When the second polaroid is rotated with respect to the first, the intensity of transmitted light decreases.
  • The transmitted intensity is zero when the two polaroids are perpendicular to each other.
  1. Applications of Polaroid Sheets
  • Polaroid sheets have various applications in everyday life and scientific research, including:
    • Sunglasses: Reduce glare and improve visual comfort by blocking polarised light reflected from shiny surfaces.
    • 3D Glasses: Separate images for each eye by using polarisers for an enhanced 3D viewing experience.
    • LCD Displays: Use polarisers to control the orientation and block light in liquid crystal displays.
    • Photography: Polarising filters reduce reflections and enhance color saturation in photography.
    • Microscopy: Polarised light microscopy allows for the observation of birefringence in materials.
  1. Optic Axis and Birefringence
  • Birefringence is the property of certain materials to have different refractive indices for different polarisation states of light.
  • These materials have an optic axis, a direction along which the refractive index is the same regardless of the polarisation of light.
  • When polarised light passes through a birefringent material, it splits into two components with different velocities and directions of vibration.
  • This phenomenon is known as double refraction.
  1. Retardation and Phase Difference
  • When polarised light passes through a birefringent material, the two components experience different phase shifts.
  • The phase difference between the two components is called retardation.
  • The retardation depends on the thickness of the material and the difference in refractive indices for the two polarisation states.
  • The phase difference can be calculated using the equation:
    • Δφ = (2π/λ) * (ne - no) * d
    • Where Δφ is the phase difference, λ is the wavelength of light, ne and no are the refractive indices, and d is the thickness of the material.
  1. Polarisation by Scattering
  • Light can be polarised through the process of scattering.
  • When light is scattered by small particles, the scattered light becomes partially polarised.
  • This is because the electric field of the incident light induces oscillations in the charged particles, causing the scattered light to vibrate in a specific plane.
  • The polarised light is usually oriented perpendicular to the direction of propagation of the original light.
  1. Brewster’s Angle
  • Brewster’s angle is the incident angle at which light is completely polarised when it is reflected off a surface.
  • At the Brewster’s angle, the reflected light is completely polarised parallel to the surface.
  • The angle of incidence and the angle of reflection are complementary to each other.
  • The equation for Brewster’s angle is:
    • tanθB = n₂/n₁
    • Where θB is the Brewster’s angle, n₁ is the refractive index of the incident medium, and n₂ is the refractive index of the medium it is entering.
  1. Polarisation by Reflection
  • When light is incident on a transparent medium at an angle greater than the critical angle, it undergoes total internal reflection.
  • In this case, the reflected light becomes partially polarised parallel to the surface.
  • This can be explained by Brewster’s angle and the fact that light incident at the critical angle undergoes a phase shift of 180 degrees upon reflection.
  1. Circular and Elliptical Polarisation
  • Light can also exhibit circular or elliptical polarisation.
  • Circular polarisation occurs when the electric field vector rotates in a circle perpendicular to the direction of propagation.
  • Elliptical polarisation occurs when the electric field vector traces out an ellipse perpendicular to the direction of propagation.
  • Circular and elliptical polarisation can be produced by passing unpolarised light through certain optical devices, such as quarter-wave plates or birefringent crystals.
  1. Quarter-Wave Plates
  • Quarter-wave plates are optical devices that can convert linearly polarised light into circularly or elliptically polarised light.
  • They are made of birefringent materials, typically crystals or films.
  • When linearly polarised light passes through a quarter-wave plate, one component undergoes a phase shift of 90 degrees while the other component remains unchanged.
  • The resulting light is circularly or elliptically polarised, depending on the relative amplitudes of the two components.
  1. Applications of Quarter-Wave Plates
  • Quarter-wave plates have various applications in optics, including:
    • 3D Glasses: Used to produce circularly polarised light for 3D movie viewing.
    • Polarimetry: Used in polarimeters to measure the polarisation state of light.
    • Optical Communication: Used in fibre-optic systems to maintain the polarisation state of light during transmission.
    • Microscopy: Used in polarised light microscopy to observe birefringent materials.
    • Optical Isolators: Used to block reflected light in lasers and prevent feedback-induced instabilities.
  1. Optics of Anisotropic Materials
  • Anisotropic materials exhibit different optical properties in different directions.
  • Birefringent materials are an example of anisotropic materials.
  • In birefringent materials, the refractive index varies with the direction of polarisation.
  • This leads to different velocities and phase shifts for different polarisation states of light passing through the material.
  1. Circular Dichroism
  • Circular dichroism is a phenomenon where the absorption of right-handed circularly polarised light differs from that of left-handed circularly polarised light.
  • It occurs in optically active materials, such as chiral molecules.
  • Circular dichroism can be used to study the structure and composition of molecules in various fields, including chemistry, biology, and pharmaceuticals.
  • It can provide important information about the chirality and conformation of molecules.
  1. Polarisation Filters
  • Polarisation filters are devices that can selectively filter out specific polarisation states of light.
  • They are typically made of aligned polaroid sheets or other materials with similar properties.
  • Polarisation filters can be used to block unwanted polarised light and enhance desired polarisation states.
  • Examples of polarisation filters include polarising sunglasses, camera filters, and LCD displays.
  1. Dichroic Polarisation Filters
  • Dichroic polarisation filters are optical filters that selectively transmit or reflect light based on its polarisation state and wavelength.
  • They are made of materials that have different absorption or reflection properties for different polarisation states.
  • Dichroic polarisation filters are commonly used in photography, spectroscopy, and optical systems where selective filtering of polarised light is required.
  1. Polarisation of Electromagnetic Waves
  • Electromagnetic waves, including light, can exhibit polarisation.
  • The polarisation of an electromagnetic wave refers to the orientation of its electric field vector.
  • Electromagnetic waves can be classified as linearly polarised, circularly polarised, or elliptically polarised based on the orientation of the electric field vector over time.
  1. Polarisation of X-Rays
  • X-rays are a form of electromagnetic radiation that can also exhibit polarisation.
  • X-rays can be linearly polarised if they are scattered by electrons or other charged particles.
  • The polarisation of X-rays can be used to study the structure and composition of materials in X-ray crystallography, diffraction, and imaging techniques.
  1. Applications of Polarisation in Medicine
  • Polarisation techniques have various applications in the field of medicine, including:
    • Polarised Light Imaging: Used in ophthalmology to examine the cornea, lens, and retina of the eye.
    • Polarised Light Microscopy: Used in histology to study tissues and cells under polarised light.
    • Polarised Light Dermoscopy: Used in dermatology to assess skin conditions and diagnose skin cancers.
    • Optical Coherence Tomography: Uses polarisation-based techniques to produce high-resolution images of biological tissues.
  1. Review Questions
  • What is polarisation of light?
  • How does a polaroid sheet work as a polariser?
  • What is Malus’s Law and how is it applied?
  • How can polarised light be analysed using polaroid sheets?
  • Explain the applications of polaroid sheets in everyday life.
  • What is birefringence and how does it affect light?
  • Describe the properties and applications of quarter-wave plates.
  • What is circular dichroism and how is it used in scientific research?
  • Explain the concept of polarisation filters and their applications.
  • How can various types of electromagnetic waves exhibit polarisation?