- 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"
- 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.
Types of visible spectra
|hotter than red hot
- 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.)