# Rays - Concept

###### Explanation

A ray is part of a line, has one fixed endpoint, and extends infinitely along the line from the endpoint. Opposite **math rays** are rays with a common endpoint, extending in opposite directions and forming a line.

###### Transcript

If we had a line so it extends infinitely

in either direction, and I picked

an end point somewhere, and I erased everything

that extended beyond that

end point, what I've just

created is a ray.

So a ray has one end point and it extends

infinitely from that end point.

But how do you label a ray?

Well, you start off by saying

what is your end point?

My end point of this ray is A because that's

where it starts, and it extends

through point B. So I'm going to

label this ray A, B as a line but

I only have one arrow.

So the arrow is going

to be over the B.

Because my end point is A and notice I

do not have an arrow over the A, which

tells the geometry student or the geometry

teacher that this ray starts

at A and passes through B.

Now, you can also have opposite rays,

and opposite rays share

a common end point.

So if you look at this line right here,

containing X, Y and Z where X, Y and

Z are all co-linear.

We have opposite rays

if I pick point Y.

So I could say that the ray Y, Z -- so

again I'm saying from point Y through

point Z. So go YZ, and notice how

I level this where the arrow is

over the Z because it starts at Y and

the opposite one would be the ray

starting at Y passing

through X.

So I could label this as ray YX.

So opposite rays share an end point and

rays in general have one end point and

extend infinitely from

that end point.