Optics- Polarisation of Light - Propagation of linearly Polarised Light through a Polariser

  • Introduction to polarisation of light
  • Understanding polarization
  • Types of polarization
  • Linear polarization
  • Circular polarization
  • Why polarizers are used in various applications

Introduction to polarisation of light

  • Light is an electromagnetic wave
  • Electromagnetic waves vibrate in different planes
  • Polarization refers to the direction of the electric field vector of light waves
  • Light can be unpolarized or polarized
  • In unpolarized light, the electric field vector vibrates randomly in all possible directions

Understanding polarization

  • Polarization of light occurs when the electric field vector of the light waves vibrates predominantly in one plane
  • Transmission axis of a polarizer allows only a specific plane of polarization to pass through, while blocking other orientations
  • When unpolarized light passes through a polarizer, only the polarized component aligned with the transmission axis can pass through

Types of polarization

  • Linear polarization
    • Electric field vector vibrates in a single plane
    • Light is said to be plane polarized
    • Can be achieved by passing unpolarized light through a polarizer or by reflection
  • Circular polarization
    • Electric field vector rotates in a circular motion
    • Consists of two orthogonal components of equal amplitude and 90-degree phase difference

Linear polarization

  • Light waves vibrating in a single plane
  • Electric field vector oscillates in a fixed direction
  • Examples:
    • Light passing through a polarizer
    • Reflection of light from a non-metallic surface
    • Scattering of sunlight by the Earth’s atmosphere

Circular polarization

  • Electric field vector rotates in a circular motion
  • Composed of two orthogonal components with equal amplitudes and 90-degree phase difference
  • Examples:
    • Some types of artificial light
    • Light reflected from a metallic surface
    • Some biological systems

Why polarizers are used in various applications

  • Polarizers are used to control the polarization of light for practical applications
  • Some common applications include:
    • Sunglasses: Polarized sunglasses reduce glare and improve visibility by blocking light waves vibrating in certain planes
    • LCD displays: Polarizers are used to control the intensity and polarization of light transmitted through liquid crystal displays (LCDs)
    • Photography: Polarizing filters are used to reduce glare and unwanted reflections in photographs

Summary

  • Polarization refers to the direction of the electric field vector of light waves
  • Linear polarization occurs when the electric field vector vibrates in a single plane
  • Circular polarization consists of two orthogonal components rotating in a circular motion
  • Polarizers are used in various applications to control the polarization of light

Optics- Polarisation of Light - Propagation of linearly Polarised Light through a Polariser

Propagation of linearly polarised light through a polariser

  • When linearly polarised light passes through a polariser, only the component of light aligned with the transmission axis can pass through
  • The intensity of the transmitted light depends on the angle between the polariser’s transmission axis and the direction of polarisation of the incident light
  • The intensity of the transmitted light can be calculated using Malus’s law
    • I = I₀ * cos²θ
    • I = intensity of the transmitted light
    • I₀ = maximum intensity of the incident light
    • θ = angle between the transmission axis and the direction of polarisation of the incident light

Malus’s law

  • Malus’s law describes the relationship between the intensity of the incident light and the angle between the transmission axis and the direction of polarisation of the incident light
  • Mathematically, Malus’s law is expressed as:
    • I = I₀ * cos²θ
    • I = intensity of the transmitted light
    • I₀ = maximum intensity of the incident light
    • θ = angle between the transmission axis and the direction of polarisation of the incident light

Example 1:

  • Suppose a linearly polarised light with an intensity of 10 W/m² is incident on a polariser with a transmission axis at an angle of 60° with respect to the direction of polarisation.
  • Using Malus’s law, we can calculate the intensity of the transmitted light:
    • I = I₀ * cos²θ
    • I = 10 * cos²(60)
    • I ≈ 10 * 0.25
    • I ≈ 2.5 W/m²

Example 2:

  • Suppose a linearly polarised light with an intensity of 8 W/m² is incident on a polariser with a transmission axis at an angle of 30° with respect to the direction of polarisation.
  • Using Malus’s law, we can calculate the intensity of the transmitted light:
    • I = I₀ * cos²θ
    • I = 8 * cos²(30)
    • I ≈ 8 * 0.75
    • I ≈ 6 W/m²

Applications of polarizers

  • Polarizers have several important applications in various fields, including:
    • Camera filters: Polarizers are used in photography to selectively block unwanted reflections and glare.
    • 3D movies: Polarized glasses are used in cinemas to separate the different polarizations of light used for the left and right-eye images.
    • Optical instruments: Polarizers are used in optical devices like microscopes and telescopes to enhance contrast and reduce glare.

Applications of polarizers (contd.)

  • Other applications of polarizers include:
    • LCD displays: Polarizers are used in LCD screens to control the intensity and polarization of light transmitted through the liquid crystal display.
    • Communication systems: Polarization of light is utilized in various communication systems, including fiber optics and satellite communications.
    • Material testing: Polarized light can be used to analyze and determine the properties of materials, such as stress and strain analysis.

Summary

  • When linearly polarised light passes through a polariser, only the component aligned with the transmission axis can pass through
  • The intensity of the transmitted light can be calculated using Malus’s law, which relates the intensity to the angle between the transmission axis and the direction of polarisation
  • Polarizers have a wide range of applications, including photography, 3D movies, LCD displays, and material testing.

Key Points to Remember

  • Polarisation refers to the direction of the electric field vector of light waves
  • Linear polarization occurs when the electric field vector vibrates in a single plane
  • Circular polarization consists of two orthogonal components rotating in a circular motion
  • Polarizers are used in various applications to control the polarization of light
  • The intensity of the transmitted light through a polariser can be calculated using Malus’s law

Questions for Practice

  1. What is the difference between unpolarized light and linearly polarized light?
  1. How does a polariser work?
  1. State Malus’s law and explain its significance in the context of polarization.
  1. Calculate the intensity of the transmitted light when a linearly polarized light with an intensity of 5 W/m² passes through a polarizer with a transmission axis at an angle of 45° with respect to the direction of polarization.
  1. Explain any two applications of polarizers in detail.

Slide 21:

  • The angle between the transmission axis of the polarizer and the direction of polarization affects the intensity of the transmitted light
  • When the transmission axis and the direction of polarization are parallel (θ = 0°), the transmitted intensity is maximum (I = I₀)
  • When the transmission axis and the direction of polarization are perpendicular (θ = 90°), the transmitted intensity is minimum (I = 0)
  • The intensity of the transmitted light decreases as the angle between the transmission axis and the direction of polarization increases
  • The transmitted light becomes completely blocked when the angle between the transmission axis and the direction of polarization is 90°

Slide 22:

  • Linearly polarized light can be obtained by passing unpolarized light through a polarizer
  • Unpolarized light consists of an equal mixture of light waves vibrating in all possible planes
  • When unpolarized light passes through a polarizer, only the components vibrating in the plane of polarization are transmitted
  • The polarizer selectively filters out certain orientations of the electric field vector and allows only the desired polarization to pass through
  • The transmission axis of the polarizer is aligned with the preferred plane of polarization

Slide 23:

  • The concept of polarization is not limited to light waves; it applies to other electromagnetic waves as well, such as radio waves, microwaves, and X-rays
  • Polarization is also observed in other types of waves, such as water waves and seismic waves
  • By controlling the polarization of waves, we can manipulate their properties, such as the direction of propagation and interference patterns
  • Polarization is an important phenomenon in the field of optics and has wide-ranging applications in various technologies

Slide 24:

  • Polarization can also occur through reflection and scattering of light
  • When light strikes a non-metallic surface at a certain angle, the reflected light becomes partially polarized
  • The angle at which the reflected light becomes completely polarized is called the Brewster’s angle
  • The Brewster’s angle is given by the formula θp = tan^(-1)(n₂/n₁), where n₁ and n₂ are the refractive indices of the medium and the surface, respectively
  • At the Brewster’s angle, the reflected light is 100% polarized in the plane of incidence

Slide 25:

  • Circularly polarized light consists of two orthogonal components with equal amplitudes and a phase difference of 90°
  • Circularly polarized light can be generated by passing linearly polarized light through a quarter-wave plate
  • A quarter-wave plate is an optical device that introduces a phase shift of one-quarter wavelength between two orthogonal components of light
  • The resulting light wave has a circular motion of the electric field vector
  • Circularly polarized light has diverse applications, such as in 3D movie technology and optical communication systems

Slide 26:

  • The polarization state of light can be analyzed using various techniques, such as a polarimeter or a polarizing microscope
  • A polarimeter is an instrument used to measure the angle and state of polarization of light
  • A polarizing microscope is a microscope that uses polarizers to enhance contrast and observe birefringent materials
  • Birefringent materials exhibit different refractive indices for different polarization states of light
  • By analyzing the polarization state of light, we can gain valuable insights into the properties of materials and their behavior under different conditions

Slide 27:

  • The study of polarization has significant applications in areas such as material science, chemistry, biology, and astronomy
  • Polarization techniques are used to investigate the structure and properties of materials, such as liquid crystals, polymers, and biological molecules
  • Polarized light can also reveal information about the composition and physical processes occurring in astronomical objects, such as galaxies and nebulae
  • Understanding polarization is crucial for the development of advanced technologies, such as optical devices, communication systems, and imaging techniques
  • Researchers continue to explore new applications and techniques related to polarization in various scientific disciplines

Slide 28:

  • In summary, polarization refers to the direction of the electric field vector of light waves
  • Linear polarization occurs when the electric field vector vibrates in a single plane
  • Polarizers are used to control and manipulate the polarization of light for various applications
  • The intensity of the transmitted light through a polarizer can be calculated using Malus’s law
  • Circular polarization involves two orthogonal components with a phase difference of 90° and has diverse applications

Summary

  • Polarization refers to the direction of the electric field vector of light waves
  • Linear polarization occurs when the electric field vector vibrates in a single plane
  • Circular polarization consists of two orthogonal components rotating in a circular motion
  • Polarizers are used in various applications to control the polarization of light
  • The intensity of the transmitted light through a polarizer can be calculated using Malus’s law

Questions for Practice

  1. What is the difference between unpolarized light and linearly polarized light?
  1. How does a polarizer work?
  1. State Malus’s law and explain its significance in the context of polarization.
  1. Calculate the intensity of the transmitted light when a linearly polarized light with an intensity of 5 W/m² is incident on a polarizer with a transmission axis at an angle of 45° with respect to the direction of polarization.
  1. Explain any two applications of polarizers in detail.