What is a Ray in Geometry? - Definition & Examples

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  • 0:01 Ray: Definition & Examples
  • 1:07 Notating & Measuring a Ray
  • 1:59 Lesson Summary
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Lesson Transcript
Instructor: Joseph Vigil
In this lesson, you'll learn what a ray is and see a few examples of rays. You'll also learn why we can't measure rays. Then, you can test your knowledge with a brief quiz.

Ray: Definition and Examples

A ray is a line with an endpoint that extends infinitely in one direction. One of the most obvious examples is a sun's ray of light in space. Let's look at how light from the sun originates and travels:

ray of sunlight

The light originates from the sun, and its path extends infinitely (assuming there's nothing to block it). Why do we refer to the light from the sun as sun rays? Why not sun lines? After all, light does travel in a line. However, unlike a ray of light in space, a line has two endpoints. We call this a sun ray because it has only one endpoint (the sun) and extends infinitely in one direction.

Here's tennis player Tommy Haas about to serve:

If there were no friction, the ball would continue flying through the air after Tommy serves it. The green line indicates the path it would take. This path is another example of a ray. It has an endpoint (in this case, the point where the ball meets the racket), and it would extend infinitely. However, due to friction in the air, the ball will eventually fall to the ground, causing it to travel in an arc.

Notating and Measuring a Ray

Coming back to the first example, the sun is the endpoint for the ray of light, and it's a ray because it extends infinitely in one direction. It other words, there's no second endpoint because a ray never ends. We would draw a ray similarly to how we drew the ray from the sun:


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