HOW does Rayleigh scattering make the sky red/blue/whatever?


Okay so there’s lots of particles in the atmosphere, the sun shines white light, the light gets scattered and depending on the angle, the sky gets its color. But like, how?

In: Physics

So light from the sun is all colors combined or white light as you said. When that light hits gas molecules in the atmosphere it causes electrons and protons in the gas molecules to oscillate. These oscillations causes electromagnetic radiation (that’s what light is) of the same frequency but in all directions instead of the straight line that the light was originally traveling in. This is the scattering effect that Rayleigh Scattering refers to.

Now why blue though? Blue light has a shorter wavelength with a higher frequency than the other light, like red. This means blue light (contained in the white light from the sun) causes the air molecules to oscillate more and thus scatter more. Due to some fancy math stuff blue light gets scattered nearly 10 times more than red light so the sky appears blue.

What about sunset and sunrise though? Well what happens here is basically the opposite. Since the light is hitting our perceived sky at an angle, the blue scattered light is deflected away from us making the sky more clear while red light, which isn’t getting scattered nearly at all, gets beamed right at us so sunsets and sunrises look red.

The color is not dependent on the angle. It is the case that air scatter blue light more then red. So light from the sun when it is low in passes through more air so primary red light reaches you.

You can compare the effect to waves and som object in the water. A object in the water scatter waves more if the size of the waves are closer to the same size as the object. A twig has minimal effect on hug waves but a large effect on small ripples on the surface.
The same way in air where molecules are smaller then the wavelength if visible light. Blue light has shorter wavelength then red so it get scattered more.

If you have larger particles then the wavelength of visible light red light will be scattered mor then blu and you get a red sky. the smoke from forests fires is a example and the result is that the sky will be red.

Think of rays of light from the sun as being packets that contain a mix of colors. (This isn’t really how it works but it simplifies the picture.) When one of these packets enters your eye, you see the average of all the colors in the packet.

Packets from the Sun contain all the colors in equal amounts(for our purposes). If it’s high in the sky, looking straight at it will give you all of the colors. So the Sun looks white.

But the Sun isn’t shining only at you. There are countless rays taking countless paths through the sky, and only a tiny fraction of them hit you directly.

Raleigh scattering is an effect that more or less means that as light passes through air, it has a tendency to get knocked off of its straight line course. And it happens to have a preference for knocking blue light more than any other color. For us, we can think of it like our rainbow light packet is slowly “bleeding off” its blue light as it travels through the air. Or like it’s being run past a kind of imaginary sandpaper that prefers to scrape off blue light. The farther the packet goes through air, the more light that gets bled off. That bled off blue light gets thrown out in a random direction.

Since Raleigh scattering is occuring everywhere in the sky, all the time, some of it is bound to randomly fly in your direction and hit your eye. That is why the sky is blue. You’re basically getting some of the blue light shavings of all the light from the Sun that *didn’t* hit you.

But if this “blue light bleeding” behavior affects all light from the Sun, shouldn’t it affect the light that *does* directly hit you, too? Yes! The light streaming into your eye is having the blue shaved out of it just like all the other light. But this effect is very gradual, and related to how far that light had to travel. If the Sun is overhead, light’s path to you takes a relatively short path through air, so not much blue light has managed to escape, so you don’t really notice.

If it’s sunrise or sunset, the light is passing through the atmosphere at a lower angle. That means its path through the air is longer now, meaning more blue light gets scraped away. At some point, so much of the bluer light has been scraped away that only reddish light remains. That redder light scatters too, just not as much as the bluer light, so if you look in the direction of the sun at sunrise or sunset, that light looks red. All the blue light has been scraped out of your sunset to make the sky blue for someone else beyond the horizon.

The only major inaccuracy with this is that there’s no such thing as a “rainbow packet” of light that can have some colors “scraped out of it”. What’s really going on is that there are a zillion rays, each with one unique color, and blue rays tend to get knocked around more often. But pretending like it’s just one packet representing the sum of all colors present means less to keep track of.

Imagine the atmosphere is lots of teeny tiny baseball players. Now imagine the different colours of light are different sized balls. On one end of the colour spectrum you have blue light that’s the size of baseballs, and on the other end you have red light and each ball of red light is the size of a car.

The baseball players in the atmosphere won’t be able to hit the red, car sized pieces of light so they just kinda keep going forward. The blue, baseball sized pieces of light are hit by the baseball players and scattered in random directions. Since we’re on the ground, when we look up we are seeing all the redirected blue light cuz the atmosphere hits it down to us. The red light just kinda keeps going and doesn’t get hit.

This definitely isn’t 100% accurate but it helps illustrate the point.