Why so blue?

Why is the sky blue?

This might be the question that has appeared in many little heads. It might be a very common question, but do you actually know why?

I have came across this question for many times, and for the first time now, I am going to get to the end of it!

The light from the Sun is white, but it is made up of all the colours of the rainbow - a spectrum. The experiment that probably all of us did some at some parts of our life - the prism experiment proves exactly this. The white light entering the prism is diffracted and as all the colours in the white light has different wavelenghts, they diffract at different angles; the white light is separated into red, orange, yellow, green, blue, indigo and violet.

When sunlight enters the Earth's atmosphere, it collides with all the gas and dust particles in the atmosphere causing it to scatter or deflect in all directions. Blue light has a shorter wavelength than red and so is scattered the most. The scattered blue light is refected into our eyes and that is why we see a blue sky most of the time.

The light that are scattered somethings collide back together to make white light again. So we actually see a lot of white light as well as blue. This is why the sky isn't a matt blue colour, but a sky blue.

At sunsets, the sun is further away from the earth and so, the light from the Sun has to pass through even more gas and dust particles in the atmosphere; so the blue light scatter even more allowing the red and yellow lights to past through straight to your eyes.

At polluted areas on Earth, the whole sky might seem red because there will be more and larger dust and water particles present in the atmosphere.

Now wait a minute.

Back to blue. If the colour with the shortest wavelength scatter the most in the Earth's atmosphere, then why isn't the sky violet?

This is explained by how the eye perceives colour. The human eye is sensitive to light between 380 and 740 nm. In the retina of the eye, there are cones for detecting colour. Each cone contains pigments that restrict the range of wavelength that it responds to - long, medium and short wavelengths. The peak of each responses are at 570 nm (yellow), long, 543 nm (green), medium and 442nm (between blue and violet) for long. However, the three cones are sensitive over broad, overlapping wavelength ranges - meaning that two different spectra can cause the same response in a set of cones. 

Now think about different colours of light. Red light and green light makes yellow light; and this is due to the overlapping of wavelengths. This is exactly what is happening in the sky. The blue and violet wavelengths elicits the same cone response as blue light and white light. So in other animals, the sky colour is undoubtedly different.