Why Is the Sky Blue?

It is a common misconception that the sky is blue because it reflects the blue of the seas and oceans.In fact, it’s the Earth’s atmosphere and a process called scattering that makes our sky appear blue.

To understand why the sky is blue, we must first understand something about light.Sunlight seems white, but it actually consists of many different colours.

We can see these different colours of light in the rainbow or when white light falls through the prism.

Rayleigh Scattering

There are many small air molecules in the air. These particles are very small, so small that we normally can’t see them with the naked eye, even though the air is full of them, billions and trillions.

When the white light of the sun travels through the Earth’s atmosphere, it collides with air particles.Different colours or wavelengths of light are scattered in different amounts due to these collisions.

  • Blue light (shorter wavelength) is diffused more than red light (longer wavelength).
  • Shorter waves (purple and blue) are the most diffused, so more blue light is diffused in our eyes than in other colours.

You may be wondering why the sky does not look purple because purple light is even more diffused than blue. This is because sunlight is not so purple, and our eyes are much more sensitive to blue.The effect of this and the dispersion of blue is that our eyes see the unique blue sky.

Finally, blue and purple waves of light scattered across the sky get into our eyes and make the sky appear blue.

Exploring the sky at home

We use torches, water and milk as a model of what is happening in the sky. The torch is the sun, and milk droplets act as gas molecules in the atmosphere. It is these particles that scatter sunlight and light from our torch.

If you point the flashlight towards the tank, you can see a beam of light in the water.

Milk particles scatter light. Examine the container from all sides.

If you look at the container from the side, you will see that the light beam of the flashlight is slightly blue and the end of the flashlight is slightly yellow.

We can make this fluid change colour and examine red and pink sunsets by moving the flashlight and changing the position of the light beam.

The sky also appears in a different colour depending on the position of the sun.

What is white light?

The sun emits all visible light colours, which we consider almost white.

As Sir Isaac Newton showed with a triangular prism. This experiment shows that white light consists of approximately the same amount of all colours of visible light.

These different colours have different wavelengths, which affects their interactions with different substances.

How the sky looks on other planets

Different planets, different particles, but the effect is the same: blue light is scattered across the sky. Other colours are also scattered, but not as much as blue.

It depends on the chemical composition of their atmosphere and the state of the components (e.g. There are droplets, crystals or dust). It also depends on the “colour” of the nearby star and the distance to this star, as well as the depth and density of the planet’s atmosphere, which would be functions of the composition and mass of the planet.

What about rainbows?

Like gas molecules in the air, rainbows form when the colours of sunlight are dispersed by drops of water, be it rain, fountain, stream or waterfall.

Light travels more slowly in the water than in the air, so that the light bends when it enters a raindrop and the light splits into a colour spectrum, thus forming what we see as a rainbow.


Sunlight reaches the earth’s atmosphere and is scattered in all directions by all gases and particles in the air. Blue light is scattered more than the other colours because it spreads out as shorter, smaller waves. This is why we see a blue sky most of the time.

The sky turns blue because all white light from the sun is scattered and disturbed by the gases in the air and also by small drops of water in the cloud and atmosphere so that only the blue light is thrown back at us strong enough to make the sky look blue in colour.

We might expect to see a very faint blue sky, but due to the dust floating in the air, the sky on Mars appears more yellow during the day.

This happens because the larger dust particles absorb the short-wave blue light and scatter the remaining colours to create a yellow-like hue over the Martian sky.

Small particles of dust and pollution in the air can contribute to (and sometimes even intensify) these colours, but the main cause of blue skies and orange / red sunsets or sunrises is the scattering from the gas molecules that make up our atmosphere.

The moon’s atmosphere is negligibly thin, essentially a vacuum, so its sky is always black, as is the case with Mercury.

However, the sun is so bright during the day that no stars can be seen unless the viewer is well protected from sunlight (directly or reflected from the ground).

As we get closer to the horizon, the sky fades to a lighter blue or white. This is because the sunlight reaching us from the horizon has let in more air than the sunlight reaching us from above.

The gas molecules have scattered the blue light back so often in so many directions that less blue light reaches us.

Sunlight reaches the earth’s atmosphere and is scattered in all directions by all gases and particles in the air. When this happens, blue light gets scattered more than the other colours from the sun, because it spreads out as shorter, smaller waves. That is why we see a blue sky most of the time.

The reason the sky appears blue during the winter is that the “cones” in our eyes that recognize violet and blue are much more sensitive to blue light and let us see a blue sky, particularly as in winter the sun is no longer above us -due to the earth’s movement.

First, the sun emits energy as rays of light, which are electromagnetic waves.

As the light beam approaches Earth, the most harmful components of the beam, which have the shortest wavelengths and the highest frequency (gamma, x-ray and ultraviolet waves), are prevented from passing through the stratosphere through the ozone layer.

The ozone layer still transmits radio and visible light waves that have a longer wavelength, hence the blue looking sky

When the light comes to you from the sun, part of the blue scatters (i.e. it doesn’t reach the destination, you).

So the light you receive is “blue washed-out” and your brain recognizes this by showing the remaining colours – red, yellow, oranges.

Dust, pollution, water drops and cloud formation “can also affect the colours of the sky.

Occasionally, pink and purple appear more often than red and orange, due in part to “the optical illusion of the pink wavelengths that illuminate the base of the cloud (due to the low angle of the sun’s rays), and these pink clouds overlap a dark blue sky.

The combination of pink and dark blue can make the sky appear deep purple.

Air molecules, mainly nitrogen and oxygen, are small. They interact very weakly with visible light, but due to their enormous number in the atmosphere, we see the effects.

Air molecules scatter the sunlight individually in all directions.

Blue light is scattered much more than longer wavelengths and the air above us looks blue from this scattered sunlight. At the risk of disappointing poets, this colour is not pure blue but a variant.

All other colours are also scattered, but are increasingly weak and tend to lean towards red.

This is because the sun emits a higher concentration of blue light waves compared to violet.

Since our eyes are sensitive to blue rather than violet, this means for us that the sky appears blue.

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