The different colours of the sky

The blue sky... we use this phrase when refering to the sky and its beauty. However, have you ever wondered why the sky is blue? Many other smart people have also thought about it, just as you did. And it took a long time to figure out how it really happens! Have you noticed that the light from the Sun seems white? However, you must know that it is really made up of all the colours of the rainbow.

Do you know what a prism is? A prism is a specially shaped crystal through which, white light is

 separated into all its colours. The light you see is just one tiny bit of all the kinds of light energy beaming around the Universe and around you!

Like energy passing through the ocean, light energy also travels in waves. Some light travels in short "broken up" waves. Other light travels in long, lazy waves. Blue light waves are shorter than red light waves. All light travels in a straight line unless something gets in the way to reflect it (like a mirror), bend it (like a prism), or scatter it (like molecules of the gases in the atmosphere).

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

When closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead.

As the sunlight has passed through all this air, the air molecules have scattered the blue light many times in many directions. In addition, the surface of the Earth has reflected and scattered the light. All this scattering mixes the colours together again so we see whiter and less blue.

What makes a red sunset?

As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.

Sometimes, the completely western sky seems to glow. The sky appears red because larger particles of dust, pollution, and water vapour in the atmosphere reflect and scatter more of the reds and yellows.

Why does scattering matter?

How much of the Sun's light is bounced around in the Earth's atmosphere and how much is reflected back into space? How much light do land and water, asphalt freeways and sunburnt surfers, soak up? How much light do water and clouds reflect back into space?

And why do we care? Sunlight carries the energy that heats Earth and powers all life on Earth. Our climate is affected by how sunlight is scattered by forests, deserts, snow- and ice-covered surfaces, and different types of clouds, smoke from forest fires, and other pollutants in the air.

Compiled by Janani Amarasekara