Is an undefined concept, a straight line extends infinitely in two directions, there are countless points on a straight line, and this straight line is straight.

Step 2 light

In Euclidean geometry, a figure formed by a point on a straight line and its edge is called a ray or a semi-straight line.

In geometric optics, light is a curve describing the propagation direction of light or other electromagnetic radiation, which is perpendicular to the wavefront of physical optics.

In most simple cases, the light in a given conductor is a straight line. When light travels from one conductor to another, it will be refracted or totally internally reflected according to Snell's law.

3. Line segment

Two points on a straight line and the part between them are called line segments.

4. Parallel lines:

In the same plane, two lines that do not intersect are called parallel lines, and the parallel relationship is mutual. Two lines perpendicular to the same line are parallel. After a while, there is only one line parallel to this line.

5. Definition of vertical line:

When one of the four angles formed by the intersection of two straight lines is a right angle, it is said that the two straight lines are perpendicular to each other, one of which is called the perpendicular of the other, and their intersection is called the vertical foot.