Main content

## Geometry (all content)

### Course: Geometry (all content) > Unit 1

Lesson 1: Lines, line segments, and rays# Identifying rays

A ray is a shape that starts at one point and extends forever in one direction. To identify a ray in a picture, look for a line that has one endpoint (the point where the ray starts) and an arrow on the other end (to show that it keeps going). Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- Can it be FEA?(63 votes)
- No, because if the points were FEA, it would be an angle, specifically an obtuse angle.(74 votes)

- At0:20; why does a ray need a point on it for it to be a ray? Doesn't it just need an endpoint and then go on forever in one direction?(4 votes)
- I think that if you just had one point then you couldn't know what direction the ray was going. like the sun is the end point but it puts out rays everywhere, so when you have two points you can know in what direction a single ray is going.

This is just how i think of it. I don't know if it'll make sense to anyone else :-/(2 votes)

- Is geometry just reasoning and shapes(3 votes)
- Geometry is the mathematical study of shapes, yes. All math deals with logical reasoning, though it is typically first taught in a formal way in Geometry class.(2 votes)

- isn't the thing in the middle a line? why did he say it was a ray?(3 votes)
- It is a line, but the connected points that make up the line could also result in rays, since a ray only requires a start point and another to specify the direction in which to travel forever. Every combination of points in a line that fulfil these requirements can be used to make a ray.(2 votes)

- So, if more than one ray start at the same point, they are the same ray?

My question being... if there was a question on a test that said:

List all of the rays that can be found on this plane.

Would you just list one for each point? I am a little confused on how to list the rays if some are the same, and if the significance is there.(2 votes)- They are the same ray if they start on the same point and go the same direction.(3 votes)

- What precisely is the symbol called between the rays CE and CF and the rays FE and FC?(2 votes)
- He is using a slash symbol in a grammatical rather than a mathematical sense, to say that ray CF and ray CE are the same ray. He could have also written an "&" to indicate it.(3 votes)

- can you right a ray like YX but with the arrow pointing <----- instead of XY w/ the arrow pointing ----->(2 votes)
- The arrow over the top is a symbol, and doesn't really indicate which way the ray is headed. This could get tricky if the ray was pointing straight up, or not even in the plane the paper was heading!

You correctly reversed X and Y in your example, to indicate that X was the endpoint, but remember that the first identifier in a ray is supposed to be the endpoint, so it would be improper to write "YX" even though you reversed the ray sign.(2 votes)

- Just wondering, but can rays EC and EF be written as line FEC?(0 votes)
- No...mathematicians always try to make things as simple as possible, and I know that sounds like a lie, but some of those complicated things that you're thinking are not easy are generally as easy as they CAN be. Anyway, naming more than 2 points is pointless (yes, pun intended) because two will point you in the right direction (again, intended), any extra points are like saying "well, if I could name them all (which would be impossible, since a line is an infinite set of points) then it would be better."(6 votes)

- Why does a ray have to have two points inside of it?(0 votes)
- At3:32, can we conclude that ray EF is the same length as ray CF?(2 votes)
- Rays do not have finite length as by definition they keep going straight on their indicated direction on to infinity. It is only a line segment that does so. Therefore, both EF and CF have infinite length(1 vote)

## Video transcript

Identify all the rays shown in the image below. and this is a reminder what a ray is. A ray start at some point and then goes on forever in some direction. In order to find a ray you need that point that you're starting off on so let's that point over there is called X and then you need another point that sits on the ray and the ray is just keep going beyond, we will name that point as "Y" and we will call the ray "XY" It starts in "X" and keeps going in the direction of "y" for ever, it crosses "y" and keeps going further we need the second point to specify the direction in which the ray is going. So, lets identify all the rays shown in the image below, we can start anywhere, we will start at point J, the only line segment we have starting from J is JH going up, goes upto H and keeps on going in that direction beyond H, ray JH, starting from J going through H and going beyond it forever now if we go to H, there is no ray HJ as the line ends in J and does not keep going beyond J, there is no ray H as it is just one point, just usiing one point, we cannot say it as a ray. now looking at our diagram the only ray is JH. Now let us look at the other points. If we look at point C, once again there is no point after C to specify it as a ray, we can have a ray CE, starts at C goes through E and goes on for ever, you can also have a ray starting at C, going through F and going on forever, CE & CF are the same rays as F sits on ray CE and E sits of ray CF, so CE & CF are the same rays, now lets think about what we can do from point E, We can start at point E , go in the direction of C and go beyond C, so it is a ray, ray EC you could start in E and go in the direction of A and go beyond A, so EA is a ray, and we can start at E and go in the direction of F and go beyond F, that is ray EF ray EF and ray CF are different as the starting points are different Now lets go to point F. To the left of point F there is no other point. We can look at the right, we can have ray FE, start at F go through E and beyond E, you can have ray FC, start at F go through C and beyond C that is teh same thing as FE ray FE and ray FC are the same as the point E is on ray FC, then finally we have not focussed on point A you may think there is ray AE, but the line does not go beyond E, so it is not a ray, to the top of A there is no other point, so there is no ray there either that is all the rays based on the points specified. If they had given us a point over there, we could have had other rays, There are 6 rays in this problem