There’s nothing like intense reds and oranges, pinks and purples filling the sky at the end of a sweltering day.
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Why is the sky so beautiful at sunset? It’s all to do with the angle of the sun, the range of colours that make up white light and the stuff that’s floating about in the sky.
We see sunlight as white but is made up of a spectrum, or range, of colours. Different colours are associated with different wavelengths of light.
When you combine light and a phenomenon called scattering, things get interesting. Scattering happens when a beam of sunlight hits a solid particle in the air and reflects off it.
The Earth’s atmosphere is full of solid particles. Some, like dust and water droplets, are quite large but others, like oxygen and nitrogen, are quite small.
A smaller particle will reflect shorter wavelengths of light, or blue and purple colours. A larger one will reflect longer wavelengths, or the reds and oranges.
The more time the sunlight takes to pass through the atmosphere, the more chance it has of being reflected off one of the larger particles.
In the middle of the day sunlight shines directly down from above, spending only a small amount of time in the Earth’s atmosphere. Shorter wavelengths of light are most likely to be reflected by smaller particles over that distance.
As a result, we will usually see the daytime sky as blue. As the Earth turns and the sun drops towards the horizon the distance that the sunlight must travel through the atmosphere to reach your eyes also grows longer.
More distance means more likelihood that larger molecules like dust, smoke, or water droplets have a chance to reflect the longer wavelengths of red and orange light.
In other words, the sunlight you see in the evening has travelled a long way and has bumped into many things before it reaches you.
The beautiful, romantic sunset is simply the different wavelengths of light being bounced off differently sized molecules in the atmosphere.
The longer the distance, the more chance the light has of encountering a large molecule that is better able to reflect longer wavelengths.
For those lovers of spectacular sunsets, it’s really a case of ‘bigger is better’.