X-ray: infrared ray, light emitted by flashlight.

Straight line: A line with several axes and infinite extension.

The differences between lines, rays and line segments are as follows:

A line segment is an inextensible line with endpoints at both ends. The two ends of a zebra crossing cannot extend, but have endpoints, which can be regarded as line segments.

Ray refers to a straight line formed by the infinite extension of one end of a line segment. The light has one and only one endpoint, so the length cannot be measured. The flashlight light in life and the light emitted by car headlights can be approximately regarded as rays.

A straight line consists of countless points. A straight line is a part of a surface and then constitutes a body. There is no end point, extending to both ends indefinitely, and the length cannot be measured. A straight line is an axisymmetric figure. There is no straight line in life that extends infinitely in the strict sense. Mathematically, the number axis and rectangular coordinate system can extend infinitely and can be regarded as straight lines.

reference data

Institute of curriculum and teaching materials. Mathematics in the fourth grade of primary school. Beijing: People's Education Press, 20 16.