Why is the sky blue during the daytime? And red at sunset?

Have you ever wondered why is the sky blue? Few people think the sky is blue because oceans reflect the sunlight back into the sky. Probably, you must be thinking that is not the correct answer. If you have not found the answer yet, I hope you will be clear after going through this article.

Why is the sky blue?

Sun is the extreme bright source of light. The visible spectrum of sunlight contains seven colors i.e. VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, and Red). If you pass light through a prism, you can see the light is split into seven colors. This theory was demonstrated by Isaac Newton.

The wavelengths of the visible part of the sunlight vary from 720 nm to 380 nm. Red light is having the highest wavelength of 720 nm whereas the wavelength of violet is 380 nm.

When sunlight falls on Earth’s atmosphere, it gets scattered in all directions after colliding on the gas and water particles. When sunlight collides with large particles, the scattering does not matter as all the light are scatter almost equally. Light is expected to scatter most after colliding with an object of around 1/10th of its wave height. As our atmosphere contains mostly Nitrogen and Oxygen, having a size 1/10th of the smaller wavelengths, the colors with small wavelengths are scattered most than red, orange, yellow and green.

The blue color is scattered more than other colors in the atmosphere, hence our sky looks blue.

why is the sky blue explained
A simple illustration of scattering of sunlight.

A question arises here, then why does not our sky look violet as its wavelength is even less than blue? The answer can be well explained by Rayleigh scattering theory. First of all, when sunlight enters into the top layer of our atmosphere, violet and indigo are absorbed and scattered most, which result in not enough amount of these colors in the lower atmosphere.

On top of this, the color receptors inside the retina of the human eye respond to red, green and blue strongly as compared to other colors.

Even, the blue color is so bright that it blocks other stars in the daytime.

Read also: Why is the ocean blue?

Why is the sky red at sunset and sunrise?

Now, if you are wondering about the red sky at sunset, you got the concept right.
This is because the sunlight has to cover more distance in the thicker layer of the atmosphere. In the way, blue and green light are almost scattered away leaving red, orange and yellow. So, people see the remaining colors during the sunset.

why the sky is red at sunset
Red sky at sunset

If the atmosphere is dusty or polluted, the sun even appears more reddish as a part of yellow color can scatter and fade away, leaving behind red and orange.

Why is the Mars sky red?

In mars, there are often storms which cause reddish dust (mostly comprising of iron oxides) to float in the air. When the sunlight reflects from reddish dust, the atmosphere looks like red. If there is no storm in Mars, the sky will look blue. The same concept is applicable to Mars as well.

What is the actual color of the sky?

The sky has no color as it consists of mostly colorless gases Nitrogen and Oxygen. The color of the sky varies depending on the time of the day. During daytime, it appears blue and at sunset, it appears red due to scattering of light. During the night time, the sky appears dark due to lack of sunlight and no star is nearby enough to produce a bright source of light.

Hope you understand all the theory behind the blue color sky. The topic why is the sky blue is explained in a simple manner for kids.



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