The Visual System: Learn It 2—Light Waves

Amplitude and Wavelength

Light travels in waves, and understanding a few basic properties of those waves helps explain how we perceive color and brightness. Two of the most important characteristics of any wave are its amplitude and wavelength.

amplitude and wavelength

The amplitude of a wave is the distance from the center line to the top point of the crest or the bottom point of the trough. Wavelength refers to the length of a wave from one peak to the next.

A diagram illustrates the basic parts of a wave. Moving from left to right, the wavelength line begins above a straight horizontal line and falls and rises equally above and below that line. One of the areas where the wavelength line reaches its highest point is labeled “Peak.” A horizontal bracket, labeled “Wavelength,” extends from this area to the next peak. One of the areas where the wavelength reaches its lowest point is labeled “Trough.” A vertical bracket, labeled “Amplitude,” extends from a “Peak” to a “Trough.”
Figure 1. The amplitude or height of a wave is measured from the peak to the trough. The wavelength is measured from peak to peak.

Wavelength is directly related to the frequency of a given waveform.

frequency

Frequency refers to the number of waves that pass a given point in a given time period and is often expressed in terms of hertz (Hz), or cycles per second. Longer wavelengths will have lower frequencies, and shorter wavelengths will have higher frequencies.

Stacked vertically are 5 waves of different colors and wavelengths. The top wave is red with a long wavelengths, which indicate a low frequency. Moving downward, the color of each wave is different: orange, yellow, green, and blue. Also moving downward, the wavelengths become shorter as the frequencies increase.
Figure 2. This figure illustrates waves of differing wavelengths/frequencies. At the top of the figure, the red wave has a long wavelength/short frequency. Moving from top to bottom, the wavelengths decrease and frequencies increase.

Light Waves

The visible spectrum is the portion of the larger electromagnetic spectrum that we can see. As Figure 3 shows, the electromagnetic spectrum encompasses all of the electromagnetic radiation that occurs in our environment and includes gamma rays, x-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves.

The visible spectrum in humans is associated with wavelengths that range from 380 to 740 nm—a very small distance since a nanometer (nm) is one-billionth of a meter. Other species can detect other portions of the electromagnetic spectrum. For instance, honeybees can see light in the ultraviolet range (Wakakuwa et al., 2007), and some snakes can detect infrared radiation in addition to more traditional visual light cues (Chen et al., 2012; Hartline et al., 1978).

This illustration shows the wavelength, frequency, and size of objects across the electromagnetic spectrum.. At the top, various wavelengths are given in sequence from small to large, with a parallel illustration of a wave with increasing frequency. These are the provided wavelengths, measured in meters: “Gamma ray 10 to the negative twelfth power,” “x-ray 10 to the negative tenth power,” ultraviolet 10 to the negative eighth power,” “visible .5 times 10 to the negative sixth power,” “infrared 10 to the negative fifth power,” microwave 10 to the negative second power,” and “radio 10 cubed.”Another section is labeled “About the size of” and lists from left to right: “Atomic nuclei,” “Atoms,” “Molecules,” “Protozoans,” “Pinpoints,” “Honeybees,” “Humans,” and “Buildings” with an illustration of each . At the bottom is a line labeled “Frequency” with the following measurements in hertz: 10 to the powers of 20, 18, 16, 15, 12, 8, and 4. From left to right the line changes in color from purple to red with the remaining colors of the visible spectrum in between, occurring roughly between 10 to the power of 15 and 10 to the power of 12.
Figure 3. Light that is visible to humans makes up only a small portion of the electromagnetic spectrum.

Within the visible spectrum:

  • Longer wavelengths appear as reds and oranges
  • Medium wavelengths appear as greens and yellows
  • Shorter wavelengths appear as blues and violets
A helpful way to remember the color order is the mnemonic ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Amplitude and Brightness

While wavelength determines color, amplitude affects how bright that color appears. Larger amplitudes = brighter or more intense light; smaller amplitudes = dimmer light.

Wavelengths from low to high as measured in nanometers. Below the visible spectrum, in increasing order, are “Cosmic radiation,” “Gamma rays,” “X-rays,” and “Ultraviolet,”. The visible wavelengths of light are between 400 and 700 nanometers. Wavelengths above the visible spectrum, in increasing order, are “Infrared,” “Terahertz radiation,” “Radar,” “Television and radio broadcasting,” and “AC circuits.”
Figure 4. Different wavelengths of light are associated with our perception of different colors. (credit: modification of work by Johannes Ahlmann)