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Why the sky actually is not blue

According to physicists, the sky ought to be violet.

The great Isaac Newton was not always right. He assumed that water vapor or water droplets in the atmosphere color the sky blue. Today, physicists have learned that the air molecules themselves are responsible for the coloring of the sky. Mistakenly, however, their theories predict a violet coloring.  With the understanding of the human color perception it is explained why the color of the sky is perceived blue by the human being.

Although Newton was wrong, he made an important contribution to describe the color of the sky. He was the first to examine the already known dispersion of the sun light into the colors of the rainbow by means of a prism. In addition, his water vapor theory was not completely wrong.  It is indeed proven that light travelling through a several meter thick water or ice layer, appears blue, as water absorbs the red portion of the white light.

However, the water vapor quantity in the atmosphere is by far not sufficient for the blue coloring of the sky. If Newton was right, the sky should not appear blue when the air is dry. However, exactly the opposite is true: In dry air the blue of the sky shines much more intense than in highly humid air.

John Tyndall (1820 – 1893) from Ireland came much closer to the truth. In 1869, he discovered that water scatter more blue light than red light after adding some milk or soap to water. According to Tyndall’s theory, the same occurs in the atmosphere: There dust particles scatter the blue of the sun light more than the red portion. Conclusion: A major part of the red light travels straight through the atmosphere. The blue light is scattered in all directions and thus also downwards to the earth surface where we can see it.

By the way, this also explains the red of the sunrise and sunset: When the sun is only slightly above the horizon, the sun light has to travel a longer way through the atmosphere to reach our eyes. Therefore, this light is missing more of the blue portion than the light around noontime. Consequently, it appears red.

Lord Rayleigh (1842 – 1919) backed up Tyndall’s observations with theoretical calculations. As light is an electromagnetic wave, one can calculate how light effects the electrons of a molecule. The light starts the electrons oscillating causing light reflection from the electrons. The absorption and reflection of light takes place in an “elastic stage”. That means, the molecule absorbs as much energy as it reflects. In simple words: In this process, today known as Raleigh Scattering, light maintains its color.

Raleigh discovered in his calculations that the blue portion of the sun light is more scattered than the red portion. In more details: The intensity of the scattered light is reverse proportional to the fourth power of the light wavelength. That means, violet light is scattered much stronger than red light with its twice as big wavelength. As a rule of thumb, violet is scattered two to the power of four, i.e. the scattering is 16 times stronger than red.

Except for the questionable circumstance that the sky should be much more violet than blue, Rayleigh confirmed Tyndall’s observations with his calculations. However, there is another limitation: Raleigh’s formula is only valid for the scattering on parts, which are small in comparison to the light’s wavelength.  This is just true for small foreign particels in the air, but most of all for the oxygen and nitrogen molecules of the air. Therefore, Tyndall’s assumption, dust particles must be in the air to explain its blue color, was wrong. The air particles themselves do the job as well.

One problem remains: Why is the sky not violet? The answer can be found in the functionality of the human color perception. The human eye is less sensitive for the color violet than for blue. Additionally, all other colors from green to yellow up to red are sparsely scattered and mix with blue and violet. But this information is not transferred to our brain. The retina already “calculates” a final color based on the various, simultaneously present color components of the light. In case of the sky color this result can only be blue due to the violet weakness of our eyes.

Additionally, in higher atmospheres part of the violet portion is filtered out of the sun light and thus, missing. Furthermore, the intensity of violet in sun light is much lower than of other colors. And any rainbow shows that the sun light which reaches us has enough violet parts.

Source: and Brillux