Have you ever wondered why the sky is blue during day-time, and not yellow or any other colour? Some people think the sky is blue because of the water in the Earth’s atmosphere. But the sky is blue in the desert which is an extremely dry place. Others think the sky is blue because of sunlight that is reflected off the ocean and back to the sky. But the sky is blue in areas that are far from the sea, like Johannesburg, so that doesn’t make sense.

The correct answer to this question lies in the nature of sunlight and how it interacts with the gas molecules that make up the Earth’s atmosphere; and in how human vision works.

Sir Isaac Newton (1643 – 1727), an English Mathematician and Physicist, proved that sunlight, which appears white to the human eye, is a mixture of all the colours of the rainbow: red, orange, yellow, green, blue, indigo and violet. He discovered this by darkening his room and making a hole in his window shutter. He let a single beam of sunlight into the room and placed a glass prism in the sunbeam. The sunlight separated into all its colours when the light shone through the prism. This formed a spectrum of colours on the wall.

Light energy travels in waves. Different colours have different wavelengths – some longer, some shorter. The colours of light differ from each other because of their different wavelengths.

All light moves in a straight line unless something stands in the way. A mirror will reflect it and a prism will bend the light. The molecules in the atmosphere will scatter it. When sunlight reaches the Earth’s atmosphere it comes into contact with gases and particles in the air which scatters it. The sunlight is made up of many colours but the blue light is scattered more than the red light because blue light has a shorter wavelength than red light. This is the reason why the sky appears blue most of the time.

So why does the sky not appear violet or indigo as violet has the shortest visible wavelength? One of the reasons is because of how human vision works. We have three types of receptors in the retina of our eyes. These receptors are: red, blue and green because of their strong response to light that travels at those wavelengths. The human eye is less sensitive to the colour violet than it is to, for example, blue. People who are colour blind have a difference in how one or more of the light sensitive cells in the retina respond to certain colours.

Why does the sunset not appear blue but rather appears yellow if the sky is clear? When the sky is clear at sunset the sunset is yellow. But when the air is polluted with small particles it is a shade of red.

By the time the sun gets lower in the sky the colour changes from blue. This because it has passed a longer distance through air and some of the blue light has been scattered away already.

So next time you look up at the beautiful blue sky you’ll know why it is blue.

Tell us: Did you know sunlight is a mixture of all the colours of the rainbow?