A ray line is a portion of a line. It can be defined as a line segment which is extended indefinitely in one direction. It is bounded by a point on one side and is not bounded the other side. This point is called the end point (sometimes referred as beginning point too). The other side which has no bounds extends to infinity in one direction.Ray Line

A ray is always a part of a line having one endpoint and a set of all points on only one side of the endpoint. In general, a ray is named by using two letters. The first letter must be the beginning point (endpoint) and the second letter being any point on the ray. An arrow with an endpoint over the letters is indicated to represent a ray. The above ray can be represented as $\overrightarrow{AB}$ where A is the endpoint and B is any point on the ray.
A light from a flash light is a real life example for a ray. The origin of the light which is the torch becomes the end point and one cannot measure exactly the distance the light travels in the direction pointed to. Sun rays are the best examples for rays starting from the end point Sun and goes in different directions and infinite distances.

Ray line segments can also be defined using betweenness of points like segments. A ray line segment is a part of a ray that is bounded by two points. The points are called the end points of the ray line segment.

Ray Line Segment

Consider a ray. Let A and B be two points on the ray. Then, the ray line segment between A and B, denoted by $\overline{AB}$, contains the end points A and B, every point on the line between A and B. A ray line segment AB is a locus of all points between the points A and B inclusive on the ray containing A and B.

Since a ray can be extended indefinitely on one side, its length can’t be determined. But, a ray line segment has end points and hence a ray line segment has length. The length of a ray line segment can be drawn using a straight edge with marked units of measurement. The length of the ray line segment is denoted by $\overline{CD}$.
Length of a Ray Line Segment

In the given figure, we have $\overline{CD}$ = 6 cm. However, measuring the length of the ray line segment with a straight edge is not accurate and is approximately equal. However, in the coordinates of the end points known, it is possible to measure the length of the ray line segment accurately by using the distance formula.