atmospheric refraction

When studying the concepts of optics, we saw that the phenomenon of refraction of light is nothing more than a deviation, that is, it has its propagation speed altered. When the incidence occurs obliquely, refraction occurs with a change in the direction of the light beam.

The refraction of light is responsible for different optical phenomena, such as the fact that a pencil appears broken when immersed in water and that the depth of a swimming pool appears smaller than it really is.

Another phenomenon that we don’t mention on a daily basis, but that we can notice quite often is atmospheric air effraction.. This phenomenon is seen when the Moon or the Sun is very close to the horizon, where we see them with a reddish color. It is at this moment that we ask why this happens only when they are close to the horizon.

In our studies on optics, we have seen that the atmosphere is not considered a homogeneous medium, so the further we move away from the Earth’s surface it becomes more rarefied, so the lower the density, the lower the refractive index. That’s why we see a star in an apparent position P’ different from the real position P when we observe it from the Earth’s surface.

The light that comes from the star, until it reaches the observer on Earth, when it enters the atmosphere passes from less refringent layers to more refringent layers, deviating from its initial direction.

So when the observer receives the light, he has the impression that it comes from a direction tangent to the trajectory described in the atmosphere. The figure below shows an illustration of the phenomenon of atmospheric refraction for an observer on the Earth’s surface.

This phenomenon is also seen in the illusions of the existence of puddles of water on roads on hot and dry days, as well as when they happen in mirages in the desert. All these facts are explained based on the variation of the refractive index of atmospheric air with temperature.