Why is the sunset red?

The light that we see looks white, but if you look through a prism you will see that light is actually divided up into many colors. These are the colors of the light ‘spectrum’ and are the same ones that we see in a rainbow. We see all of the colors, with the exception of ‘indigo’ with our eyes: ROY G BIV: red, orange, yellow, blue, indigo, violet. This is important, because everything that we look at seems to have color, including a sunset.


Each color that we can see has a ‘wavelength’. The longer wavelengths are found in colors such as red, green, yellow and orange and the shorter wavelengths are the blue colors. The longest wavelength color is red.

When light flows through space from the sun it is going in a straight line and has all of the colors of the spectrum. Each color is traveling at its own wavelength. Some are slow and some are fast. By the time the light reaches the earth’s atmosphere it may encounter water molecules, dust and ice. Since the visible light waves are incredibly small (smaller than one millionth of a meter) the light waves will interact with even the tiny gas molecules that are in the air. They begin to bounce off of the particles in the same way you might bounce around in a busy school hallway in between classes. This bouncing process is called ‘scattering’ and where they go depends on how large the particle was that they hit when compared to the wavelength size.

Nice Sunset

Small particles (compared to the wavelength) will scatter the blue light a lot more strongly than red light. Since the earth’s atmosphere is mostly made up of oxygen and nitrogen, the blues will scatter sunlight in more directions than the red. This creates the ‘blue sky’ that we see. The red light waves have the least amount of scattering due to the interaction with the gas molecules. So when you are outside at sunrise or sunset the sunlight is actually travelling a longer path, through the atmosphere and finally to your eyes where you can see it. The blue light has been almost completely removed, but that still leaves the red and yellow light.

This is why when you see a sunset, you can see the beautiful colors of reds, oranges and yellows. If there are any clouds, they will reflect these colors, creating an even prettier picture. The colors are being reflected from the water and ice particles in the clouds and the molecules also absorb the wavelength colors as well. We then wait until full daylight, when we can see the same process, but now all of the blue colors are being scattered and with so much more blue, we don’t see the reds, oranges or yellows any more

There actually are a lot more ‘colors’ – but as humans, we can’t see them. There are specialized equipment that scientists use to ‘see’ these colors and this helps them in various scientific studies.