What does the cause of mirage show?

Figure 1 In air with temperature varying with height, light is refracted out of a curved path.

Fig. 2 Refraction of light entering air from glass. Note that the incident angle i is smaller than the refraction angle r.

Fig. 3 when the incident angle I is greater than the critical angle c, total internal reflection occurs.

Fig. 4 the path of mirage occurrence time.

In the desert, the lost people were short of water and food, and suddenly they saw the oasis, but when they got closer, they found that it was just a mirage. They were so happy. Although this is a common bridge in movies, it is a mirage, and nature is playing with us with the magic of light. It is caused by refraction and total internal reflection of light in air.

To understand the cause of mirage, we must first understand why light refracts in the air. It turns out that the refractive index of air at different temperatures is different, just like many different media. The air near the ground is hot and has a low refractive index. We can think of air as a medium with many layers, and each layer has a different refractive index. The closer to the ground, the lower the refractive index. Therefore, when light propagates in the air, the path is as shown in figure 1.

On the other hand, we also need to know what total internal reflection is. If light enters the air slightly obliquely from the glass, part of the light will be reflected back and the other part will be refracted out of the glass. Because the refractive index of glass is higher than that of air, the refraction angle is always greater than the incident angle (Figure 2). When the incident angle is 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, and the light will only be reflected, not refracted. This phenomenon is called total internal reflection (Figure 3).

Figure 4 shows the path of mirage occurrence time. Suppose there is an oasis, and the light it emits at point A is refracted by the air and takes a curved path. At point B, the light exits completely internally, making the light upward. Then the light is refracted by the air, and finally the light will enter the eyes of the observer standing at point C, giving him the illusion that the oasis is close to him! Long ago, human beings discovered total internal reflection and applied it. For example, optical fiber, single-mirror reflection camera and binoculars all apply the principle of total internal reflection.