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
- is represented by the symbol c (from the Latin word celeritas, swiftness)
- 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 - Monochromatic light can be described by only one frequency.
- 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.
- A continuous spectrum is one in which every frequency is present within some range.
condition | description | spectrum | example |
---|---|---|---|
objects hotter than red hot | incandescent | continuous | fire, the Sun, incandescent light bulb |
bound electrons dropping in energy level | luminescent | discrete | fireflies, LED display, fluorescent tube |
- 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).