Why the Sky Looks Blue

Why the Sky Looks Blue
A sunny day with a beautiful blue sky may prompt a child to wonder why the sky is blue. It's a good question. After all, the Sun isn't blue, and even if the sky above us looks blue, the air around us doesn't. The answer has to do with sunlight, Earth's atmosphere, and our eyes.

The light we see is a type of electromagnetic radiation. It's the part of the electromagnetic spectrum that human eyes sense.

Sunlight is white light, but it's made up of the colors you can see when they're separated in a rainbow or a prism. These colors form the visible spectrum. Each color has its own wavelength and frequency. Wavelength is the distance from the top of one wave to the next, and the frequency is the number of waves that pass per second. Waves with a short wavelength and high frequency, such as blue light, carry more energy than long waves like red.

Rayleigh scattering
Our atmosphere is made up of gas molecules, mostly nitrogen and oxygen. When light enters the atmosphere the molecules of the air interact differently with the different wavelengths. They have a significant effect only on light with a wavelength less than ten times the size of the air particles. That means that red, orange, yellow and green tend to continue the journey together without disruption.

However the particles interact strongly with the shorter wavelengths such as blue and violet, scattering the light. This means that the light is briefly absorbed by the particles and then cast out in a random directions. It happens many, many times and the blue light fills the sky to give it its blue color. The selective scattering is called Rayleigh scattering, named after 19th-century English physicist Lord Rayleigh who discovered the phenomenon.

The more air the blue light passes through, the more often it gets scattered. If you look at the sky straight overhead you can see that it's a darker blue than elsewhere. This is because the light has traveled to your eyes by the shortest path, and undergone the least scattering. Near the horizon the blue is paler because the light has been scattered many more times in all directions.

Why isn't the sky violet instead of blue?
Violet light has a shorter wavelength than blue, so it should be scattered more. In fact, it is. Yet although the sky should be violet, it isn't. One reason is that some of the violet light is absorbed in the upper atmosphere.

More importantly, our eyes are much more sensitive to blue light than to violet. They have three types of color receptor: one is most sensitive to red, one to green and one to blue. Each of the receptors also has some sensitivity to a few other colors. Our greatest sensitivity is to green, and this receptor also responds to green-blue wavelengths. Indigo and violet stimulate both the red and the blue receptors. Overall the blue is stimulated much more strongly than the rather weak violet.

Total lunar eclipses
Having shown how Rayleigh scattering gives us a blue sky, you may be surprised to learn that it also gives us a red-orange Moon during total lunar eclipses. Even though the Moon can't receive direct sunlight at totality, it's not completely in the dark. There is indirect lighting from sunlight in the Earth's atmosphere. Since the blue light gets scattered, it leaves the redder colors to travel through the atmosphere. Some of this light shines on the Moon, often making it look as though it's covered in blood.

Reflection nebulae
Rayleigh scattering doesn't just happen on Earth. This is the Witch Head Nebula (IC 2118). (Image credit & copyright: Jeff Signorelli) Although nebulae are clouds of gas and dust with no light of their own, this one glows a beautiful blue color. It's reflecting the light from a nearby bright star. The light is blue because of Rayleigh scattering by extremely small grains of carbon dust. Because of their size they they scatter blue light more efficiently than red light. The dust does the job that air molecules do on Earth.

You Should Also Read:
Astronomy ABC – H for Herschel Space Observatory
Lunar Eclipses

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