Why is the sky blue?
Light travels in waves of different lengths. The wavelengths of visible light are very short—about one ten-thousandth to one one-thousandth of a millimeter. The longest waves in the familiar spectrum of visible light are red and the shortest are violet. The sun produces light across the entire span of the spectrum. When all of the wavelengths of the visible spectrum are seen at once, the light appears white. That’s why the sun looks white to an observer on the space station.
When light reaches Earth, it has to pass through the atmosphere, the layer of gases that surrounds the solid and liquid parts of the planet. The atmosphere is full of dust particles and droplets of water vapor. When light hits these particles, it reflects and changes direction. As the light passes through the atmosphere, it is reflected over and over. That’s why sunlight can reach places that are not in a direct line to the sun.
Water droplets are larger than the light waves so they tend to reflect every wavelength. That explains why a cloud looks white—the droplets reflect all colors of light and the combination looks white. Most of the atmosphere consists of molecules of nitrogen and oxygen, which are much smaller than the wavelength of light. When light strikes these molecules, it does not reflect in the same way that it reflects from water droplets. However, when light strikes them, the molecules occasionally absorb the energy of the light wave. After a while (a very tiny fraction of a second, generally), the molecules emit light at the same wavelength. While all of the light from the sun is traveling in the same direction, the light emitted by a gas molecule can travel in any direction in a straight line away from the molecule. The light is scattered in all directions. This is called
Rayleigh scattering, named after Lord John Rayleigh, who first described it.
So how does this explain the color of the sky? It turns out that the shorter the wavelength of the light, the more likely it is to be scattered by Rayleigh scattering. Long wavelengths, such as red and yellow, are much less likely to be scattered than short wavelengths, such as violet and blue. If there were no atmosphere the sky would look black, just as it does in space. What you see when you look at a blue sky is blue sunlight
that has bounced around in the air until it is coming at you from all directions.
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Filed Under: Science
