When looking over my lectures notes from our climate dynamics course whilst revising last year, I came across a small section that starts to touch on the optics of the sky and why sunsets are red. Now, if you know me, it comes as no surprise that I love a good sunset. It brings me a lot of jog when the sky turns various hues of gold, red, pink, and purple. There's no better way to end a long day of revision than coming out of the library and seeing a gorgeous sunset, a feat that is guaranteed to make it onto my instagram story because there is no denying, I am just like other girls. So, I thought it would be cool to discuss the science behind the magic, and see why the dipping of the sun beyond the horizon turns the whole sky red.
One of my many sunset pics, this one being the sunset in the desert whilst I was out in Dubai visiting my parents last December.
So for starters we know that the sun emits light and this travels through space to reach us on the Earth. This light is a form of electromagnetic (EM) radiation that is at a wavelength that is visible to our eyes. There are other types of EM radiation such as radiowaves, microwaves, x-rays, and ultraviolet (UV) rays which we can't see but we have other uses for, they are shown in the diagram below. The sun emits EM at other wavelengths that aren't visible to us too, but for the purpose of thinking about sunsets let's focus on the stuff we can see. This light the sun emits is split into different colours based on it's wavelength. This should be familiar to everyone as I'm sure we have all heard of the rainbow. When all the colours of light are together they appear white, but when split out they form the rainbow with blue light having the shortest wavelength and red the longest. All of these colours (different wavelengths) of light are emitted by the sun, and travel through space, before arriving at our atmosphere.
A helpful diagram of the electromagnetic spectrum taken from Wikipedia, note the visible spectrum in the middle and the wavelengths of the different colours of the rainbow.
Space, as you may recall, is a vacuum. This means that it has practically no particles in it, no molecules of air, no gases, just nothingness, empty space. Our atmosphere however is made up if gases, and there particles are all around us. The top of the atmosphere has a lower concentration of gases, and the concentration increases as you get closer to the surface of the Earth. That's why there is more oxygen at sea level than there is at the top of Mt Everest, the concentration of the atmosphere is the highest at sea level because of the pressure of all the other gas on top of it. But that gets a bit complicated and is not what we are here to discuss. The main takeaway is that the atmosphere is full of particles and space is not.
Light (and all EM radiation) can be thought of as traveling as a wave. In the atmosphere, this wave will inevitably collide with some of the particles, it will then 'bounce off' these particles causing it it be scattered in different directions. This does not happen when the light is travelling through space because there are no particles for it to bump into.
Hopefully this drawing helps to visualise the scattering I was talking about. The wiggly lines are the waves of light and the little circles are the particles of the atmosphere. As the waves hit the particles some parts of the light energy bounces off in all directions whilst some continues travelling forwards.
During the day, the sun is right over head and the light from the sun has to travel through the shortest amount of atmosphere to reach our eyes (illustrated below) . As it does this, the blue light with the shortest wavelength gets scattered the most while the red with the longer wavelength gets scattered the least. This is because the shorter wavelength means it essentially hits more particles so is scattered more. This means that there is blue light everywhere in the sky(as it has been scattered all around the place) so during the day the sky looks blue.
As the sun sets or rises however, it is in a different position in the sky. This means the light it is emitting has to travel further through the atmosphere to reach your eyes. As it goes this further distance it interacts with more particles. This means that the blue light gets scattered so much that none, or very little, of it our eyes. The red light however is scattered less so more of it is able to get through the atmosphere to reach our eyes and that is why the sky appears red. It appears the colours of the longer wavelengths of light, red, orange, yellow, pink, that are scattered less by the atmosphere.
Here we have a helpful diagram from my lecture notes to hopefully help you visualise this. The image is not mine, it was taken from slides by Prof. Samar Khatiwala so all credit goes to him. The diagram hopefully shows you how the light has to travel further through the atmosphere as the sun sets than during the day, meaning there is more particles to travel through for the light to be scattered.
So there you have it, that is why the sky looks red at sunrise and sunset. A fun little science fact for you to whip out at your next sunset walk. You're welcome. But in all seriousness this is why I think studying the Earth Sciences is so cool, you get to learn the why behind all the incredible stuff you see going on in the world around you!
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