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Why is the Sky Blue?

Why is the sky blue? It is a classic question and one of the most frequently asked by kids.

Here is a science experiment that simulates how white light turns a white medium into blue. It resembles how sunlight passes through the atmosphere and turns a clear sky into a blue sky.

The science behind the blue sky is quite awesome.

What you need for the blue sky experiment

  • a clear glass
  • water
  • A little bit of white soap, milk, or milk powder should also work)
  • a flashlight that emits white light (LED)


  1. Fill the glass with water.
  2. Dissolve a little bit of soap in the water to get a cloudy solution.

    If you use a bar soap bar, submerge it into the water and rub the surface a little bit to get a white solution. This solution will be the blue sky for your experiment.If you use milk powder, dissolve one teaspoon at a time until you have a white cloudy solution.

  3. In a dark room, point the flashlight at the cloudy solution from the side.
  4. Observe the solution to see a hint of the color sky blue.

Simple explanation for the color of the sky

White light from the flashlight is a mixture of all colors of the rainbow.

Similar to the flashlight, the light from the sun also comprises rainbow colors. Rainbows appear when sunlight is refracted by the water droplets in the air. When the white flashlight passes through the fluid holding small soap particles, blue is scattered by the particles more than most other color.

When the white flashlight passes through the fluid holding small soap particles, blue is scattered by the particles more than most other colors.

As a result, we see a hint of the sky blue color in the solution.

The soap particles suspending in water represents dust particles suspending in the air.

In the air, blue is also scattered more than most other colors in the sunlight. Therefore, when we look at the sky on a clear sunny day, the color of the sky is blue.

Advanced Explanation

Sunlight coming through the atmosphere is scattered by molecules in the air, including air molecules and various dust particles.

These particles are very small, so small that we can’t normally see them with our bare eyes even though the air is full of them, trillions and billions.

This scattering effect is called Rayleigh scattering.

The spectrum of light that makes up the sunlight contains color lights of different wavelengths.

Scattering is more effective with the shorter wavelengths than with the longer ones.

Blue and violet are the shorter wavelength visible lights and so they are scattered more.

Although violet is supposed to be scattered more than blue because its wavelength is shorter, our eyes are less sensitive to violet than to blue.

This is why the sky is blue during the day.


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