# Why is the sky blue

At some point in our childhood we may have asked ourselves why we see the sky blue and why not black like outer space or transparent like air. Although this may seem like a simple question, the answer is not. To come up with an explanation, we must review some notions of physics and why we perceive certain colors as we perceive them.

Light and wavelength

We define light as an electromagnetic wave that travels at a constant speed and that can vibrate with a certain frequency. Wavelength is the distance that a wave is able to travel to complete one cycle and depends on its frequency.

Since the spectrum of light is continuous, there is light at any wavelength. That is, light does not only exist in the spectrum visible to the human eye, but there are waves that we do not perceive as radio waves (longer wavelengths), microwaves (shorter lengths) or ultraviolet and gamma radiation (shorter lengths).

The objects that we see do not emit light by themselves , but it is necessary that they interact with the light to see them in certain colors. Depending on the characteristics of the atoms that form them, light interacts with them or not, since there is an infinite number of frequencies.

The part of the light that cannot interact with the atoms is absorbed or reflected. Of those that are absorbed, a part does interact. This phenomenon of absorption, alters or excites the atoms, which when they return to their natural state (when they lose their excitation) emit a light that has the same characteristics of the light they absorbed. Well, this is the light that our eyes perceive when we see an object . In other words, objects emit light, but only when they receive it from the outside and at certain frequencies. For example, a fruit like strawberry absorbs at any frequency, but only re-emits red light. We perceive a remission of light and not a reflection.

## Why do we see the sky blue

The source of light emitted in this case is the sun. The sun emits some types of light, the combination of which we perceive as white light . This emitted light passes through space and approximately eight minutes later, reaches the layers of the atmosphere.

In the atmosphere there are various molecules (which, like atoms, also absorb and re-emit light). Of all these molecules, the main one is the nitrogen molecule. This molecule has the characteristic of receiving a certain wavelength and re-emitting it in another direction (a phenomenon known as Rayleigh scattering), but in a different way depending on their wavelengths. Thus, blue light (small wavelength) is more scattered than red light (large wavelength).

The process begins with the entry of light rays into the atmosphere. These rays are made up of light in various wavelengths. The longitudes that correspond to red, yellow and green enter the atmosphere, but do not disperse, but impact against the surface or return to space. While the blue lengths are scattered and reflected by particles in the atmosphere, which increases their scattering. Thus, they completely dye the sky blue. In short, blue light remains in the sky , while the others pass through the atmosphere.

As can be seen at times, during sunset it is easier to perceive certain shades of red in the sky. The reason must be sought in the different position of the sun with respect to the day, which determines that the rays strike in a more parallel way against the atmosphere, so that they travel a greater distance and are more dispersed than during other times, that is why we see it .

Thus, when there are many clouds or the sky is polluted we cannot perceive the color blue well, but when it is clear we can.

This phenomenon is only present on those planets where there is an atmosphere (it even depends on its composition). For example, the planet Mars has an atmosphere so thin that blue light cannot be scattered into the interior of the planet, which is why a red atmosphere is perceived . However, during sunrise on this planet, a bluer atmosphere can be seen, since the thickness of the atmosphere is greater during sunsets and therefore allows a greater scattering of blue light.