In optical fiber communication, the physical principle of optical fiber transmitting optical signals is to use the phenomenon of total reflection of light. In order to make this phenomenon happen, it is necessary to meet the following conditions: light is emitted from an optically dense medium to an optically sparse medium, and the incident angle is equal to or greater than the critical angle.

In order to represent the propagation of light, we usually use a straight line with an arrow to represent the trajectory and direction of light. Such a straight line is called light, and light travels along a straight line in a uniform medium. The linear propagation of light has been widely used in ancient astronomical calendars in China. Our ancestors made watches and sundials to measure the length and direction of the sun's shadow.

Determine the time, winter solstice and summer solstice; Install peepholes on astronomical instruments to observe the sky and measure the positions of stars. In addition, our country has long used this characteristic of light to invent shadow play. In the early Han Dynasty, people and things cut by Qi Shaoweng were performed behind a white screen, and the images of people and things were reflected on the white screen with light, so people outside the screen could see the performance of the images.

Shadow play was very popular in the Song Dynasty, and later spread to the West, causing a sensation. A small hole was opened in the Chaoyang wall of a dark hut. People stood outside the house facing the hole, and an inverted figure appeared on the wall opposite the room. Light passing through a small hole is like archery. It travels in a straight line.

A person's head blocks the light above, forming a shadow below, and a person's feet block the light below, forming an upside-down shadow. This is the first scientific explanation for the straight-line propagation of light.