Optics and Electromagnetic Waves

Electromagnetic Nature of Light

  • Light is an electromagnetic (EM) wave.
  • EM spectrum consists of various wavelengths including visible light, infrared, ultraviolet, X-rays, etc.

EM Spectrum

  • Electromagnetic waves are categorized based on their wavelengths and frequencies:
    • Radio waves
    • Microwaves
    • Infrared light
    • Visible light (small part of EM spectrum)
    • Ultraviolet light
    • X-rays
    • Gamma rays

Photometry

  • Photometry: the science of measuring light in terms of perceived brightness to the human eye, distinct from radiometry (which measures radiant energy).
  • Uses two types of quantities:
    • Radiometric (characterizing energy of EM waves)
    • Photometric (determining physiological effects on the eye)

Geometrical Optics

  • Key laws and principles:
    • Law of Straight Line Propagation: Light travels in straight lines in a homogeneous medium with a uniform refractive index.
    • Law of Independent Propagation: Multiple light rays can travel in various directions independently in a uniform medium.
    • Law of Reflection: Angle of reflection equals angle of incidence (θr = θi).
    • Law of Refraction (Snell's Law): Relation of refractive angles based on the refractive indices of media; n1 * sin(θ1) = n2 * sin(θ2).

Light Polarization

  • Polarization refers to the direction of the electric vector oscillation of a light wave.
  • Types of polarization:
    • Linear
    • Circular
    • Elliptical
  • Degree of Polarization (DOP): measure of the portion of an electromagnetic wave that is polarized.

Dispersion and Absorption of Light

  • Dispersion: phenomenon of a beam of light splitting into constituent colors when passed through a medium (e.g., a prism).
    • Chromatic dispersion is a result of wavelength-dependent refractive index.
  • Absorption: occurs when substances selectively absorb light frequencies, defining colors in objects.
    • Bouguer-Lambert Law describes the relation of light attenuation to properties of materials.

Scattering of Light

  • Rayleigh scattering explains the color of the sky, where shorter wavelengths (blue light) are scattered more than longer wavelengths (red light).
  • Mie theory addresses scattering by larger particles, where sizes comparable to or larger than the wavelength scatter light without strong dependency on wavelength.

Optical Lenses

  • Types of lenses:
    • Converging Lenses:
    • Double-convex
    • Plano-convex
    • Positive meniscus
    • Diverging Lenses:
    • Biconcave
    • Plano-concave
    • Negative meniscus
  • Lens parameters: focal length (f), optical power (P = 1/f in meters), and aberrations (e.g., chromatic, spherical).

Aberrations

  • Refers to deviations from expected image quality in optical systems:
    • Defocus
    • Spherical aberration
    • Coma
    • Astigmatism
    • Field curvature
    • Image distortion

The Human Eye as an Optical System

  • Key structures:
    • Cornea: main refractive surface.
    • Lens: adjustable focusing component.
    • Iris/Pupil: controls light entry.
    • Retina: light-sensitive area, contains rods (for low light) and cones (for color vision).
  • Optical power of the eye (59D without accommodation, adjustable with distance).

Optical Microscopy

  • Involves two key components:
    • Objective lens: projects a real image.
    • Eyepiece: further magnifies the image.
  • Total magnification calculated as product of individual magnifications of both components.
  • Resolution affected by lens design, numerical aperture, and light wavelength.

Types of Microscopes

  • Various types include:
    • Bright-field (basic)
    • Phase contrast (for transparent specimens)
    • Dark-field (highlight scattered light)
    • Fluorescence microscopy (uses fluorescent dyes)
    • Confocal microscopy (improves resolution through optical sectioning).

Summary

  • Optics blends physical principles with practical applications to manipulate light behaviors for various technologies, including visual perception and imaging.