Mirage
In the desert, a lost person was short of water and food. Suddenly he saw an oasis, but when he got closer, he found that it was just a mirage. He was overjoyed. It is a mirage, a magic trick played by nature on us. It is caused by light being refracted in the air, coupled with total internal reflection.
If you want to understand the cause of mirages, you must first understand why light is refracted in the air. It turns out that air at different temperatures has different refractive indices, just like many different media. Air near the ground is hotter and has a lower refractive index. We can imagine air as many layers of medium, and each layer has a different refractive index. The closer to the ground, the lower the refractive index.
On the other hand, we also need to know what total internal reflection is. If light enters the air from the glass at a slight angle, part of the light will be reflected back, and part of the light will be refracted and come out of the glass. Since the refractive index of glass is higher than that of air, the refraction angle is always greater than the incident angle. As the incident angle becomes larger and larger, the refracted light will get closer and closer to the interface between air and glass. Until the incident angle is greater than the critical angle, the light will only be reflected instead of refracted. This phenomenon is called total internal reflection.
Suppose there is an oasis. The light it emits at point A is refracted by the air and takes a curved path. At point B, total internal emission of the light occurs, causing the light to travel upward. After that, the light is refracted by the air again, and finally the light will enter the eyes of the observer standing at point C, causing him to form the illusion that the oasis is very close to him! Humans discovered total internal reflection a long time ago and applied this phenomenon. For example, optical fibers, single-lens reflex cameras and binoculars all use the principle of total internal reflection.