The Nature of Light

Summary

• Light is a transverse, electromagnetic wave that can be seen by the typical human.
  • The wave nature of light was first illustrated through experiments on diffraction and interference.
  • Like all electromagnetic waves, light can travel through a vacuum.
  • The transverse nature of light can be demonstrated through polarization.
  • Light is sometimes also known as visible light to contrast it from "ultraviolet light" and "infrared light".
  • Other forms of electromagnetic radiation that are not visible to humans are sometimes also known informally as "light". This is specially true in astronomy.
• The speed of light in a vacuum…
  • is represented by the symbol c (from the Latin word celeritas, swiftness)
    • The symbol v used for the speed of light in a medium.
  • is equal to 299,792,458 m/s exactly, by definition
    • The value 3.00 × 10⁸ m/s with three significant digits is precise enough for many applications.
  • is the universal speed limit
    • v < c for all meanings of the symbol v.
    • The speed of light in a medium (v) is always slower than the speed of light in a vacuum.
      • The difference is often negligible for low density media like air.
      • Faster than light communication appears to be impossible.
    • The speed of anything with mass (v) is always less than the speed of light in a vacuum.
      • Faster than light travel appears to be impossible.
  • is a universal constant
    • There are no known factors that affect the speed of light in a vacuum.
  • is also the speed of all other electromagnetic waves in a vacuum
• The amplitude of a light wave is related to its intensity.
  • Intensity is the absolute measure of a light wave's power density.
  • Brightness is the relative intensity as perceived by the typical human eye.
• The frequency of a light wave is related to its color.
  • Monochromatic light can be described by only one frequency.
    • Laser light is very nearly monochromatic.
    • There are six simple, named spectral colors in English each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
      • The number of named spectral colors and their range of frequencies vary with culture and person.
      • Whether indigo (a color between blue and violet) should be considered a distinct spectral color is open to debate.
    • Light ranges in frequency from 385 THz on the red end to 790 THz on the violet end, where 1 THz = 1 terahertz = 10¹² hertz.
      • Frequencies lower than 385 THz are said to be infrared (literally "below red").
      • Frequencies higher than 790 THz are said to be ultraviolet (literally "beyond violet").

  Visible light in order of increasing frequency (with wavelength for comparison)

  ↑ 400 THz

  ↑ 500 THz

  ↑ 600 THz

  ↑ 700 THz

  ↑ 700 nm

  ↑ 600 nm

  ↑ 500 nm

  ↑ 400 nm

• Polychromatic light is composed of multiple frequencies.
  • Polychromatic light is more common in everyday experience.
  • Polychromatic mixtures of light waves with special color names include…
    • Grayscale mixtures: white (high intensity), gray (medium intensity), black (low intensity)
    • Grayscale averaged with spectral colors: pink (white + red), brown (black + orange), etc.
    • Grayscale minus spectral colors: (white − red), magenta (white − green), yellow (white − blue)
    • Purple is the color name assigned to combinations of red and violet light waves.
• Color is such a complex topic that it has its own section in this book.
• The wavelength of a light wave is inversely proportional to its frequency.
  • Light is more often described by its wavelength in a vacuum than by its frequency.
  • Light ranges in wavelength from 380 nm on the violet end to 780 nm on the red end, where 1 nm = 1 nanometer = 10⁻⁹ m.

  Visible light in order of increasing wavelength (with frequency for comparison)

  ↑ 400 nm

  ↑ 500 nm

  ↑ 600 nm

  ↑ 700 nm

  ↑ 700 THz

  ↑ 600 THz

  ↑ 500 THz

  ↑ 400 THz

• Light is produced by one of two methods.
  • Incandescence is the emission of light from "hot" matter (T ≳ 800 K, 500 °C, 1,000 °F).
  • Luminescence is the emission of light when bound electrons experience a drop in energy level.
• A graph of relative intensity vs. frequency is called a spectrum (plural: spectra). Although frequently associated with light, the term can be applied to many phenomena.
  • A continuous spectrum is one in which every frequency is present within some range.
    • Incandescent sources emit a continuous spectrum.
  • A discrete spectrum is one in which only a set of well defined and isolated frequencies are present. (A discrete spectrum is a finite collection of monochromatic light waves.)
    • Luminescent sources emit a discrete spectrum.

• The interference of light waves is best illustrated with a coherent light source.
  • A coherent light source is one where all the rays have the same frequency and are in phase (e.g., light from a laser).
  • An incoherent light source is one where all the rays are not in phase, and may not even have the same frequency (e.g., light from everything that is not a laser).