At some point, everybody looks up and says, dang. That sky is blue. I wonder why that is. Any old wikipedia page can answer this question, but you’ve chosen to ask me. Which is better. Because my answer comes with Nicola’s Patented Doodles.
You probably already know that it’s something to do with the way the light comes through the atmosphere. You might even have batted around a few words like ‘diffraction’ or ‘scattering’. Awesome. What do they mean?
Well, we know that light generally travels in straight lines. When a light wave encounters an object, it can interact with it in a way that forces the light to continue in a different direction. Diffraction occurs when the path of the light is bent slightly by the object. Scattering occurs when the light wave is bounced off of the object in some random direction. Rayleigh Scattering is a law that specifically tells you how much the path of the light changes. Critically, it tells you that different colours of light get scattered by different amounts.
Let’s take a step back and think about colours.
Light can be coloured, and objects can be coloured.
The colour of light is determined by its wavelength. If you imagine light as a wave; in red light the peaks and troughs are nicely spaced out. In blue light, the peaks and troughs are more bunched up.
Therefore, we say that red light has a longer wavelength (the distance between adjacent peaks), and blue light has a shorter wavelength.
Objects have colour because they reflect a particular colour of light, and absorb all other colours.
White is a special case, because it is made up of all the other colours. The sun emits white light and appears white, but reds, blues, heliotropes; they’re all in there.
Black is a special case because it is the absence of colour. If you were to shine white light on my goth sister, her clothes appear black because they absorb all of the wavelengths of light and reflect nothing. NOTHING!
Sometimes coloured objects can transmit only one wavelengths of light through. If you wear John Lennon’s blue sunglasses, they make the world appear blue. This is because they absorb all of the other colours, and only let the blue light get through into your eyes.
The reason the sky appears blue is not because the atmosphere is blue, or because the atmosphere only reflects blue light. It is a transmission trick. The atmosphere is actually colourless, and allows all the colours through, i.e. it transmits all the visible wavelengths of light. BUT, as the light passes through, the different colours get scattered by different degrees. This is called Rayleigh Scattering, and it goes: the degree of scattering is proportional to 1/(wavelength, to the fourth power). This will make a hell of a lot of difference between red and blue light, because although the difference between these two wavelengths might be small, the wavelength gets multiplied by itself 4 times and then inverted. By comparison, red light doesn’t get scattered much, but blue light gets scattered A LOT.
So, this is reasonably easy to understand, yes? But how does that make the sky blue?
This used to confuse me. If the red light isn’t scattered much, surely it comes through the atmosphere in a relatively straight line and straight into our eyes, making the sky look red? Yes? No.
Right physics, wrong time……
During the day, the sun might be directly above us, and there might well be a few waves of red light coming straight for us, but the atmosphere is so dense with scattering media like gas molecules and dust, there’s blue light coming from every other direction. Our eyes are literally bombarded with it, from all angles. Hence, the overwhelming blue appearance of the sky in the daytime.
At sunset, the oblique angle of the light from the sun means that the red light is travelling almost sideways through the atmosphere. Unlike the blue light, which is scattered earthwards immediately, red light travels a long way before it is scattered sufficiently earthwards and into your eyes, hence the overwhelming red appearance of the sky at sunset.