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Physics: Why is the sky blue?

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Physics: Why is the sky blue?

And sometimes, like, red or orange or pink or…

One of the best things about studying Physics is learning the answers to questions that you might not have even thought to ask yourself, like how do cats land so gracefully? Or why is the sky blue? Well, we can help with the second one. You’re on your own with that first question.

Let’s start with the visible spectrum

The forces of evolution equipped us with three different types of cone cells in our eyes, with each one being better at seeing a certain type of colour – red, green and blue. We call these primary colours and they can be mixed together to create all the colours of the rainbow… or brown if we’re mixing it together with paint. This is our visible spectrum.

Moving onto the power of the sun

The sun is rapid-firing parallel rays of white light towards the Earth, which is made up of all the wavelengths of the visible spectrum at equal intensity. The colours themselves vary in wavelength, from 750 nanometres (which is one billionth of a metre) on the warmer end of the spectrum, to 450 nanometres on the cooler side. These concentrated beams of light penetrate through the Earth’s atmosphere and bounce off various molecules.

Right, now we’re talking Rayleigh Scattering

Our atmosphere is made up of tiny molecules, mostly oxygen and nitrogen. When the beams of light enter the atmosphere, they collide with these molecules and scatter – this is called Rayleigh Scattering. These light rays then scatter at varying rates depending on the size of the wavelength, with larger wavelengths scattering much less. Basically, blue light rays will scatter much more than red ones, which is why more blue-coloured light rays end up in our vision when we look at the sky. As the light rays scatter, they lose their intensity, which explains how more red and green beams sneak into the mix and sometimes add a reddish-orange tinge to the sky.

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