Because the sky is blue?

Everyone knows that white light, emitted by the Sun, is actually composed of seven basic colors. They range from violet to red, each with its own frequency.

The air molecules that make up the Earth’s atmosphere, in turn, reflect, absorb, and scatter solar radiation. “Sunlight, also called white light, enters the atmosphere and is scattered by air molecules – mainly nitrogen – in all directions”, says physicist Alexandre Souto Martinez, from USP’s Institute of Physics and Mathematics in Ribeirão Preto .

Blue light has a frequency (wave cycles per second) very close to the resonance of atoms in the atmosphere, unlike red light. Thus, blue light moves electrons in the atomic layers of molecules much more easily than red light. This causes a slight delay in the blue light that is re-emitted in all directions, in a process called Rayleigh scattering (named after the 19th-century English physicist who explained this phenomenon). The red light, which is not scattered but transmitted, continues in its original direction, but when we look at the sky it is the blue light that we see because it is the one that has been scattered the most by the molecules in all directions.

At dawn and dusk, however, light passes through a thicker layer of the atmosphere. The blue spreads out so far that it cannot reach us and so we see the red sky.

Moisture particles present in the atmosphere can also alter this light scattering. That’s why, before or after it rains, we can see the seven colors of the spectrum in the range where light passes through water droplets. It’s called a rainbow. For the same reason, the sky on Mars is also red. As it has many scattered dust particles, the blue light spreads out even more and only red light makes it to the surface.