The Nature of Light

Summary

Light is a transverse, electromagnetic wave that can be seen by humans.
- 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"
Light is produced by one of two methods.
- Incandescence is the emission of light from "hot" matter (T ≳ 800 K).
- Luminescence is the emission of light when bound electrons fall to lower energy levels.
The speed of light depends upon the medium through which it travels.
- The speed of light in a vacuum is a universal constant in all reference frames.
- All electromagnetic waves propagate at the speed of light in a vacuum.
- The speed of light in a medium is always slower the speed of light in a vacuum.
  (The difference is usually negligible when the medium is air.)
- The speed of anything with mass is always less than the speed of light in a vacuum.
  (The speed of light in a vacuum is the universal speed limit.)
- The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
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 average human eye.

The frequency of a light wave is related to its color.

Color is such a complex topic that it has its own section in this book.
Monochromatic light can be described by only one frequency.
- Laser light is very nearly monochromatic.
- There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
Polychromatic light is compused of multiple frequencies.
- Every light source is essentially polychromatic.
- White light is very polychromatic.
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.
  - Blackbody radiators 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.)
  - The excited electrons in a gas emit a discrete spectrum.


condition	description	spectrum
hotter than red hot	incandescent	continuous
excited electrons	luminous	discrete

The wavelength of a light wave is inversely proportional to its frequency.
- Light is often described by it's wavelength in a vacuum.
- Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.
  - Wavelengths slightly shorter than 400 nm are said to be ultraviolet.
    (They are "beyond violet" in terms of frequency.)
  - Wavelengths slightly longer than 700 nm are said to be infrared.
    (They are "below red" in terms of frequency.)
Phase differences between light waves can produce visible interference effects.
(There are several sections in this book on interference phenomena and light.)

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