Why are sunsets red?
When we ask this question we actually mean, "why does the sky go red just before the sun goes down?"
To understand why the sky goes red just before the sun goes down we also need to understand why the sky is blue for the rest of the time between sunrise and sunset. We are familiar with the sky usually being blue so we tend to think that red sunsets must be caused by 'something else', and accept that the sky 'just is' blue.Red and blue are words we use to describe different colours (or wavelengths) of light, so that gives us a clue; at different times of the day we are seeing different wavelengths of sun light. When the sun is low down on the horizon (rising and setting) the sky is red, and when the sun is higher in the sky we see blue light.
So, what changes between the sun being low down and high up in the sky? We know that the sun radiates the same wavelengths of light all the time, so we know it can't be the sun only producing red light at dawn and dusk and blue light all the rest of the day.
In fact, the thing that causes us to see different colours is the atmosphere. Apart from being made of different gasses, the atmosphere also contains billions of dust particles and water droplets. Sunlight is scattered as it encounters these particles and molecules, with short wavelengths being more efficiently scattered than long wavelengths. This is called Ray leigh Scattering.
Blue light has a shorter wavelength than red light and, at sunset when the sun is low on the horizon, the sunlight has to pass through more atmosphere than it does when the sun is directly overhead. This means that as the sun sets the blue light has been filtered out by the thicker layer of atmosphere, leaving only the red, orange and yellow light for us to see.
And, of course, for the rest of the day when the sun is overhead it has to pass through less atmosphere before we see it, so the blue light hasn't been scattered - and we can still see it!
When the sun is overhead the light passes through the least thickness of atmosphere before we see it. The blue light hasn't been scattered so we can still see it.
When the sun is low on the horizon the light has to travel through a thicker layer of atmosphere so the blue light is scattered and lost. We see only the longer wavelengths such as red, orange and yellow.