This article will define a light wave in terms of three characteristics and compare that definition to the features of a light ray.
I previously wrote that light behaves differently depending on the circumstances. Usually it is easiest to think of light as a ray of energy that travels in straight lines; however, in some instances, that simple behavior breaks down. This change in behavior is particularly noticeable for laser light or for light passing through very small openings. In those situations, light interacts with itself in ways that are best understood by thinking of light as a wave rippling through space. As an analogy, consider waves in water.
Waves are defined by three features:
Speed is the rate at which one peak of the wave moves.
Amplitude is the height of the wave. It is usually measured as one half the distance between the highest point of the wave and the lowest point. The amplitude corresponds to the energy that the wave carries—the higher the amplitude, the more energy.
Wavelength is the distance between two peaks of the wave, and it is usually denoted by the Greek letter “λ.”
These three defining features of a light wave correspond to three features that define a light ray—speed, brightness, and color. With regard to speed, light travels at 300 million meters-per-second through empty space, regardless of whether one is talking about rays or waves. The amplitude of a light wave is equivalent to how bright it appears. Finally, the wavelength of a light wave is equivalent to the color of a light. Although color sensitivity varies from person-to-person, humans see a spectrum of light that starts at roughly 400 nm (violet) and ends around 700 nm (red), where “nm” means nanometer—one-billionth of a meter. Wavelengths just short of the visible spectrum are “ultraviolet” (UV), and wavelengths somewhat longer than the visible spectrum are “infrared” (IR) both of which are invisible to humans. In a future article I will talk about wavelengths shorter than UV and longer than IR.
An alternative way to describe the color of a light wave is frequency, rather than wavelength. The frequency of a wave is how many wave peaks pass a fixed point in one second. It is indicated by the Greek letter “ν,” and it is measured in Hertz (Hz). There is a simple relationship between wavelength, frequency, and speed:
If one knows a wavelength, then one can calculate frequency and vice versa. Wavelength and frequency have an inverse relationship, meaning that the larger the wavelength, the shorter the frequency. Humans can see light of frequencies between 430 THz (red) and 770 THz (violet), where “THz” means terahertz, which is one trillion repetitions per second.
This information results in the following definition of a light wave:
A light wave is a form of energy that ripples through space at a speed of 300,000,000 meters-per-second and is defined by its amplitude (energy) and its wavelength or frequency (color).
wave photograph copyright: willyambradberry / 123RF Stock Photo
ADDENDUM: Given the discussion about the human visual spectrum that this post prompted, I thought it would be helpful to include a graphic of how the human eye responds to light. I will write more extensively about the human eye in a future article. In brief, though, the back side of the eye (the retina) contains three kinds of light-sensitive cells (cones) that respond to light waves in different, but overlapping, ways. The graph at left shows the strength of each cell’s reaction to various wavelengths of light. The level of sensitivity is different for every person, but the graph at left is the accepted standard.
“Simplified human cone response” attributed to Vanessaezekowitz at en.wikipedia based on data from Stockman, MacLeod & Johnson (1993) Journal of the Optical Society of America A, 10, 2491-2521d.