Why is the sky blue? Why are sunsets red?

On a sunny day, you can see the beautiful blue color of the sky. But why does it have this color? Why is it not green, red or yellow?The main reason for this is the way the different molecules in the atmosphere scatter sunlight as it passes through our atmosphere.

What does scattering of light mean?

Basically, atoms or molecules which are exposed to light absorb its energy and re-emit light in different directions with different intensity. This phenomenon is known as scattering.When this scattered light enters in to our eyes, we only see the scattered color of that light.

What are the molecules of the atmosphere that scatter sunlight?

An atmosphere is a layer or a set of layers of gases surrounding a planet. So our plant also covered up with several layers of different gases. The most common gases are 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. All these molecules of the air are much smaller than the wavelengths of visible light. So the light can easily scatter with these molecules.

Why only the blue color scatter in the sky?

The white light which is coming from the sun is made up of seven colors. Those are red, orange, yellow, green, blue, indigo and violet (rainbow colors). The particles of the atmosphere always tend to scatter the shorter wavelength more compared to the longer wavelength. The shorter the light’s wavelength is, and the better it scatters. Among the above colors violet has the shorter wavelength and the red has the longer wavelength. So the shorter violet wavelength scatters the most and the longer red wavelength scatters the least.

Different frequencies of light bend and travel differently in a lens. Credit: Lucas V. Barbosa

So why isn’t the sky violet?

Actually, the sun doesn’t emit as much violet waves when compared to the blue light waves. The sun emits a higher concentration of blue light waves. So when there is not enough violet in the sunlight and there will not have much violet in the scattered light. Other reason is our eyes are not much sensitive to the violet rather than blue.
But we tend to see indigo and blue color in the sunlight which together appears blue in color. When the blue wavelengths scatter in to the atmosphere, the molecules of atmosphere tend to glow blue. So the sky appears blue in color.

Why sunset or sunrise appears in red?

In a sunset or sunrise, the sun is near the horizon. When the sun is near the horizon, sun rays have to take a much longer path through the atmosphere. By the time those rays reach our eyes, violet, indigo and blue light rays have already scattered. Now the remaining colors of the sunlight are green, yellow, orange and red. But when the rays pass through so many atmospheric molecules, green and yellow light also disappear. At the end, only orange and red can survive. This is because the wavelengths of above colors are the longest. Therefore those colors are scattered the least and are left to be seen. So it makes the sky appear in orengeish-red color.

Why the sun looks dimmer in the evening?

Several factors cause for this. The main reason is when the sun is near the horizon, sunlight has to come very long way to reach to us. In that way air has absorbs more sunlight and making the sun less bright. The other factor is the stuff in the atmosphere. Such as dust, water vapor and carbon monoxide.

You may also like...