Why the sky is blue?

Have you ever wondered why the sky is blue? The first steps towards correctly explaining why the sky is blue were taken by John Tyndall in 1859.

The clear cloudless daytime sky is blue because molecules in the air scatter blue light from the sun more than red one. When we look at the sun at sunset, we see red and orange colors because blue light is scattered away from the line of sight.

The white light of the sun is a mixture of all the colors of the rainbow. This was demonstrated by Isaac Newton, who used a prism to separate different colors and form a spectrum. The colors of light vary in wavelength.

The visible spectrum ranges from red light at around 720 nm to violet at around 380 nm with orange, yellow, green, blue and violet in between. Three different types of color receptors in the human retina react most strongly to red, green, and blue wavelengths, giving us our color vision.

Tyndall effect

The first steps towards explaining the color of the sky correctly were taken by John Tyndall in 1859. He found that when light passes through transparent liquid containing fine particles in suspension, the shorter blue waves are scattered more strongly than the red ones.

This can be demonstrated by directing a beam of white light through a container of water with a little milk or soap. From the side, the beam can be seen by the blue light that it scatters; but the light seen directly from the end turns red after it has passed through the reservoir. Scattered light can also be shown polarized with the help of a polarized light filter, just as the sky appears deeper blue through polaroid sunglasses.

This is called the Tyndall effect, but to physicists it is better known as Rayleigh scattering - after Lord Rayleigh, who studied it in more detail a few years later. He showed that the amount of scattered light is inversely proportional to the fourth power of the wavelength for sufficiently small particles. Hence it follows that blue light is scattered more than red, by (700/400) 4 ~ = 10 times.

Why is the sky blue - Dust or molecules?

Tyndall and Rayleigh believed that the blue color of the sky should be due to small dust particles and water vapor droplets in the atmosphere. Even today, people sometimes incorrectly say that this is so. Scientists later realized that if this were true, there would be more changes in the color of the sky depending on humidity or haze conditions than there actually was, so they correctly assumed that there are enough oxygen and nitrogen molecules in the air to explain the scattering.

The case was finally settled by Einstein in 1911 who calculated a detailed formula for the scattering of light by molecules; and this was found to be consistent with experiment. He was even able to use computation as an additional test of Avogadro's number over observation. Molecules are capable of scattering light because the electromagnetic field of light waves induces electric dipole moments in the molecules. This is why the sky is blue.

Why is the sky blue and not purple?

If the shorter wavelengths are scattered the most, the puzzle is why the sky doesn't appear purple - the color with the shortest visible wavelength. The sun's radiation spectrum is unstable at all wavelengths and, moreover, is absorbed by high layers of the atmosphere; therefore, the light is less violet.

Our eyes are also less sensitive to violet. This is part of the answer; yet the rainbow shows that there remains a significant amount of visible light colored blue and violet, in addition to blue. The rest of the answer to this conundrum is how our vision works.

There are three types of color receptors, or cones, in our retina. They are called reds, blues and greens because they react most strongly to light at these wavelengths. Because they are stimulated in different proportions, our visual system constructs the colors we see.

When we look at the sky, red cones react to a small amount of scattered red light and less strongly to orange and yellow waves. Green cones respond to yellow and more strongly to scattered green and green-blue waves.

Blue cones are stimulated by colors close to blue wavelengths, which are highly scattered. If there were no blue and violet in the spectrum, the sky would look blue with a slight green tint. But the most highly scattered waves of blue and violet stimulate the red cones in the same way as blue, so these colors appear blue with the addition of a red tint.

The net effect is that red and green cones are roughly equally stimulated by light from the sky, while blue is more stimulated. This combination gives a pale blue color. It is perhaps no coincidence that our vision is tuned to see the sky as a pure shade. We have evolved to fit in with our environment; and the ability to distinguish natural colors most clearly is probably our survival advantage.

Sunsets

Why are sunsets yellow and red? When the air is clear, the sunset turns yellow because sunlight has traveled a long distance through the air and some of the blue light has scattered. If the air is polluted with small particles of natural or other origin, the sunset will be redder.

Sea sunsets can also be orange or red due to salt particles in the air, which are effective Tyndall's scatterers. The sky around the sun appears reddened, as does the light coming directly from the sun.

This is because all light is relatively well scattered at small angles, but blue light is more likely to be scattered twice or more at large distances, leaving yellow, red, and orange colors behind.