27.8 Polarization

Electromagnetic waves have a hidden property called polarization. It's all about how the electric field wiggles as the wave moves. Understanding this helps us see why sunglasses work and how LCD screens light up.

We can make and use polarized light in cool ways. Filters can create it, reflections can produce it, and even the sky scatters it. This knowledge lets us build better sunglasses, sharper displays, and tools to measure stress in materials.

Polarization of Electromagnetic Waves

Polarization of electromagnetic waves

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• Polarization describes the orientation of the electric field vector in an electromagnetic wave
  • In a polarized wave, the electric field oscillates in a specific direction perpendicular to the wave propagation direction
  • The magnetic field is always perpendicular to both the electric field and the wave propagation direction
• Types of polarization include linear, circular, and elliptical
  • Linear polarization occurs when the electric field oscillates in a single plane (vertical or horizontal)
  • Circular polarization involves the electric field vector rotating in a circular motion, either right-handed (clockwise) or left-handed (counterclockwise)
  • Elliptical polarization occurs when the electric field vector traces an elliptical path
• Unpolarized light consists of waves with random polarization directions (sunlight, incandescent bulbs)
• Polarization is a property of transverse waves, which include electromagnetic waves across the electromagnetic spectrum

Methods of producing polarized light

• Polarizing filters selectively transmit light with a specific polarization
  • Dichroic polarizers absorb one polarization component while transmitting the other (Polaroid filters made of polyvinyl alcohol with embedded iodine crystals)
  • Birefringent polarizers split light into two orthogonally polarized components (calcite crystals, Nicol prisms)
• Reflection can produce polarized light when light reflects off a surface at Brewster's angle
  • Brewster's angle is given by $\tan \theta_B = \frac{n_2}{n_1}$, where $n_1$ and $n_2$ are the refractive indices of the two media (air and water)
• Scattering by small particles (Rayleigh scattering) can partially polarize light
  • Blue sky light is partially polarized due to scattering by air molecules
• Anisotropic materials, which have direction-dependent optical properties, can produce polarized light through birefringence

Applications of polarization principles

• Polaroid sunglasses use polarizing filters to block glare from reflective surfaces
  • The filters are oriented to absorb horizontally polarized light, which is the main component of glare (water, snow, roads)
• Liquid Crystal Displays (LCDs) use polarizing filters and liquid crystal molecules to control light transmission
  • Liquid crystal molecules change orientation in response to an applied electric field, altering the polarization of light passing through them
  • This allows pixels to be turned on or off by controlling the transmission of polarized light (smartphones, computer monitors, televisions)
• Optical activity in materials can rotate the plane of polarization of linearly polarized light
  • Some materials, such as solutions of chiral molecules, exhibit this property (sugar solutions, amino acids, certain crystals)
  • The angle of rotation depends on the material's specific optical rotatory power and the path length of light through the material (glucose concentration measurement in medical applications)
• Photoelasticity uses polarized light to analyze stress distributions in transparent materials

Polarization and wave interactions

• Wave superposition principles apply to polarized waves, affecting their interference patterns
• Malus's law describes the intensity of linearly polarized light transmitted through a polarizer as a function of the angle between the polarization axis and the polarizer axis