This article gives a practical definition of “light ray,” from a technical perspective. 

Scenic forest of fresh green deciduous trees framed by leaves, with the sun casting its warm rays through the foliage with the rays highlighted by red arrows
Rays extend from the sun in straight lines.

In the previous article I defined photons:

Photons are sub-atomic particles that are the fundamental building blocks of light in the same way that electrons are the fundamental building blocks of negative electric charge.

When I proposed that definition, though, I gave the caveat that there is still considerable debate about how to define a photon. That ambiguity arises because light acts in different ways under different conditions. Specifically, light can behave like a particle (photon), a ray, or a wave. The terminology “ray” and “wave” is probably already familiar because “light ray” and “light wave” are common phrases that people use interchangeably. There are significant technical differences, though, between a ray and a wave, so it is important to define them clearly

Sun rays, like in the photograph to the left, are probably the first things that come to mind when you think of a light ray. The rays of light extend outward from the sun in straight lines. That is the first feature of light rays: Light rays travel in straight lines.

The second feature of light is that it is bright. I know that sentence is excruciatingly obvious, but the critical point to grasp is that light is a form of energy moving through space, and the brightness of light corresponds to its energy.

The third feature of light that is also excruciatingly obvious is that light has a color. Human eyes can see a range of colors that run from red through violet. There are colors that humans can’t see, and you have probably heard of two of them: infrared and ultraviolet. Those topics will show up in the upcoming article about light waves.

The fourth feature of light is its speed. In empty, airless space, light travels at roughly 300,000,000 meters per second, regardless of color.That particular speed has a name, “c,” (and, yes, that is the same “c” in the equation E=mc2). When light travels through materials, such as glass, it slows down, but more about that in upcoming articles as well.

We can stitch together those four features of light to construct a practical definition of a light ray:

A light ray is a form of energy that has the feature of color and brightness and which travels through space in straight lines at 300,000,000 meters-per-second.

In the next article, I’ll define a light wave and explain the important differences between rays and waves.

Forest photograph copyright: smileus / 123RF Stock Photo

3 thoughts on “Four Obvious But Important Features of Light Rays

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