Two points on a straight line and the part between them are called line segments, and these two points are called the endpoints of the line segments.

Characteristics of line segments:

(1) is limited in length and can be measured.

(2) There are two endpoints

(3) Symmetry

(4) The length of the line segment between two points is the distance between two points.

Attribute of line segment:

Of all the straight lines connecting two points, the line segment is the shortest. The line segment between two points is the shortest.

Definition of light:

A figure consisting of a point on a straight line and a part next to it is called a ray.

Characteristics of rays:

(1) has only one endpoint and one direction;

(2) immeasurable.

Properties of light:

Ray is an axisymmetric figure, and its axis of symmetry is the straight line in which it lies.

Definition of straight line:

Is the trajectory of a point moving in the same or opposite direction in space.

Characteristics of straight lines:

Without endpoints, it can extend to both ends indefinitely, and the length cannot be measured.

Properties of straight lines:

A straight line is an axisymmetric figure. It has countless axes of symmetry, one of which is itself, and any straight line perpendicular to it. Because a straight line is vertical at any point, it can be regarded as splitting into two rays with opposite directions. If a ray is folded along this vertical line, the two rays will coincide. Therefore, a straight line has countless symmetry axes.