The notion of ray is used in the field of geometry.
Before proceeding to exhaustively analyze the concept of ray, it is necessary that we first establish what its etymological origin is. Specifically, it is clear that this is found in Latin where we observe that it is made up of two clearly differentiated parts: the prefix semi - which translates as "middle" and the word rectus which can be defined as "straight".
The concept of ray is used in geometry to identify each of the fragments in which any straight line can be divided by any of the points that compose it. It is important to keep in mind that the correct way to write this word is with two "r"s and not semi-straight (with a single R).
What is a ray
The ray, therefore, can be presented as the portion of a straight line that is composed of all the points that are located towards one of the sides of a certain fixed point that is taken as a reference: this means that a ray has a origin (the point that starts it) but extends towards infinity. The straight line, on the other hand, has neither beginning nor end.
In other words, the line is an infinite sequence of points. The ray begins at a point of origin and, from there, it is formed by infinite points. The ray can also be called a closed half line , since it includes the point of origin but extends towards infinity.
The bijective function
When considering the bijective function between a line and real numbers, it can be said that positive real numbers correspond to one ray, while negative real numbers correspond to another ray. Zero , for its part, is the border point (origin) of both semilines.
A ray has an origin but no end.
To understand what a ray is, we could imagine a rule that begins with the number 0 and extends in a natural order to infinity ( 0 , 1 , 2 , 3 , 4 , 5 , etc.).
Differences between the ray and the segment
It is important that we keep in mind that within geometry, sometimes there is confusion between what is a ray and what are other very common concepts in said science. However, its differentiation must be made clear with respect to what is, for example, a segment or the semiplane.
In this sense, we must make it clear that, while a ray is the result of dividing a line by a point, a segment is the part of the line that is included between two specific points. This means that that element has both a beginning and an end.
Therefore, it is clear that the main difference between the ray, also known by the name ray, and the segment is that while the first has an infinite length, the second is the opposite, it is finite. It can be said that a segment is comprised by the intersection of the set of points of two rays (it has a beginning and an end).