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### Course: Class 12 Physics (India)>Unit 9

Lesson 7: Some natural phenomena

# Why is the sky blue?

Let's explore why the sky is blue, why the sunset/sunrise is red, why are clouds white. It's all due to scattering of sunlight by the molecules of our atmosphere. Created by Mahesh Shenoy.

## Want to join the conversation?

• at , Mahesh said, after hitting an electron the photon will glow the colour which is scattered by the particle. My question is, according to Compton Effect, after hitting the particle the frequency of the electron must be decreased hence the wavelength would increase. So why it should the photon scatter yellow, shouldn't it emit colours with larger wavelength than yellow ?
(5 votes)
• at the end, when talking about the clouds, it's said that if the molecular size is smaller than the wavelength, it's more likely to scatter blue color; if the size of molecule is larger than the wavelength of visible spectrum, it's more likely to scatter all the wavelength.

Molecule Diameter (nm)
N_2 0.37
O_2 0.12
H_2 O 0.275

Sorry, I can't understand this part, could anyone explain a little bit more please.
(1 vote)
• The clouds are made of condensed (liquid) water, so when clouds scatter light, the scattering happens by refraction through whole water droplets! The individual water molecules in the droplet of course have a similar size to the diatomic molecules in the gaseous atmosphere - but in a droplet the water molecules are close enough together to cause their own collective scattering effect. The droplet is made of a huge number of molecules, and therefore is larger than the wavelength of blue light and will not scatter white light in the same way as the gas particles.
(6 votes)
• At , due to the fact that all the blue light has been scattered away, we see the yellowish glow. Does it mean that in the higher parts of the atmosphere, it would look a little greenish yellow and in the lower/deeper parts of the atmosphere, it would look reddish yellow?
(2 votes)
• Being higher or lower in the atmosphere from that point will not be enough to scatter more light and change the colour.

The phenomenon is quite rare but the sky can be greenish at times
(2 votes)
• Hello! Nice video. And one question: While normally the clouds are white, why are some other clouds gray or dark, for example, those occur before a storm? Thanks!
(2 votes)
• A cloud gets thicker and denser as it gathers more water droplets and ice crystals — the thicker it gets, the more light it scatters, resulting in less light penetrating all the way through it. The particles on the underside of the rain cloud don't have a lot of light to scatter to your eyes, so the base appears gray as you look on from the ground below. So they appear gray!
(1 vote)
• What is the relation between the size of particles that scatters light of particular wavelength and scatters light of all wavelength(white light)?
(1 vote)
• The wavelength-dependent scattering (blue light getting scattered by oxygen and nitrogen) is called Rayleigh scattering. The one non-dependent on the wavelength (light scattered by the water molecules in the clouds, causing their white color) is called Mie scattering. You can read more about this here: http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c2.
(1 vote)
• Why does the colour of the sky too change during sunset sometimes?
(1 vote)
• Same reason. As the sun moves moves away, The light has to travel a longer distance to reach us. Thus, most of the light coming to our eyes is usually of longer wavelength such as red. The blue light on the other hand get easily scattered and Thus, doesn't enter our eyes. Thus the sky too sometimes appear red during sunset and sunrise
(1 vote)

## Video transcript

- Have you ever wondered why the sky is blue? Why is it that the sun, which is actually white in color, looks yellow to us? Why does the sunlight usually look yellow to us? And why is it that the sunrise or the sunsets are usually red in color? Well, the short answer for this is because the molecules of our atmosphere, like the nitrogen molecules or the oxygen molecules, tend to scatter blue light more than red. So let's explore this in a little bit more detail. We've talked about scattering of light in previous videos. Basically, when light hits a tiny particle, it reflects light in all the direction, and that's what we call a scattering. When this scattered light enters our eyes, we see that particle glowing the same color that it ends up scattering. So if it scatters, let's say yellow light, then the particle will glow yellow to us. Now, the important thing is that the molecules of our atmosphere don't scatter all the colors of light equally. Now do you understand why that's a little complicated? Because the physics of scattering is a little bit complicated. But in short, what's happening is that you see, our white light is actually made up of seven colors: "VIBGYOR," the seven colors of the rainbow. The reason we even see these different colors, is actually because light is a wave, or it can be thought of as a wave, like a wave on a string. And then if you were to look at these waves, then it turns out that the shorter wavelength then to hit our eyes, we perceive that as violet. The longer wavelength then to hit our eyes, we tend to perceive it as red. So as the wavelength becomes longer, the color changes from violet towards the red. And when we do the physics of scattering, it turns out that when we're dealing with particles of the atmosphere which are much smaller than the wavelength of light, they always tend to scatter the shorter wavelength more compared to the longer wavelength. Okay? We're now going to see why that happens. As I said, that's a little difficult, well that's not little, that's actually pretty complicated. But it turns out that the shorter wavelength, which we see as violet, so this shorter wavelength, let's write that down somewhere over here. So this shorter wavelength scatters the most, and the longer red wavelength scatters the least. Therefore, if white light were to come and hit one of these atmospheric molecules, then they would tend to scatter blue light much more than the red light because blue has a shorter wavelength, somewhere over here, compared to the red. Now I'm pretty sure we might be wondering, "Why blue? Why not the violet itself?" Because that is an even shorter wavelength, isn't it? Well, it turns out that the sun doesn't produce enough of violet in the first place. So there isn't much violet light in the incoming sunlight, and therefore there won't be much violet in the scattered light. Another reason why we don't see violet is because our eyes turn out to be not so sensitive to violet at all. Combined with these results, we don't tend to see violet, but we tend to see this indigo blue, which together we tend to usually call it as blue. So that's the blue light that gets scattered the most, and therefore the atmospheric molecules tend to glow, they tend to glow blue in color, when seen from any direction. Now we are ready to answer all of our questions. So let's get rid of these pictures. First, let's look at what we would have seen if there wasn't an atmosphere. So let's say it's the daytime, that means the sun is right above us. Something like afternoon. Then, the rays of light would be coming straight down like this. Now if we were to look in the direction of the sun, in this direction, then we would see the white sun. But if we were to look in any other direction, then there's no light coming towards us from any of the direction because all of the light is just falling downwards. So we would see nothing. Therefore, all you would be seeing without an atmosphere, is the white sun and everything else would just look dark. And you know what? Let's perform an experiment side by side. All you would need is a flashlight, a tank of water, and some milk. When we add milk to this water, the milk particles are going to represent our atmosphere. So if you look at the flashlight directly, without any milk particles in between, then all we see is the white light. And look at the sky, the whole thing has become dark. Exactly like what we would see over here without an atmosphere. But now let's bring in the atmosphere. When we bring in the atmosphere, the sunlight strikes all this atmospheric particles and like we discussed, they'll end up scattering blue light the most. They do scatter all the other colors, but blue light gets scattered the most. And as a result, almost all these atmospheric particles will end up glowing blue. So now, in whichever direction we look, all we would see is the blue light. And now in our experiment, if we bring the tank of water and start adding milk, the milk particles are going to mimic our atmosphere. Just like our atmospheric particles, they are scattering blue light the most. And as a result the whole tank is glowing blue. Because everywhere, the milk particles are mostly throwing blue light towards us. Beautiful, isn't it? But did you notice, as the sky became blue, the sun turned yellow? Why did it turn yellow? Well, if you come back over here, the initial incoming rays are white. But once it hit the atmospheric particles, they start scattering blue light. So from the incoming light, blue got scattered away. So if you look at the colors that are remaining now in this incoming light, violet was never there in the first place, not much, indigo blue got scattered away, so now the incoming rays only contain green, yellow, orange, and red. Even these are being scattered, but not as much as blue. So when we combine these colors together, we get that yellowish glow. And that's why the sunlight now, once it has entered the atmosphere, only retained that yellowish glow. Because most of the blue has been scattered away. That's why when we look in the direction of the sun, the sun starts looking yellow to us. The same thing is happening in our experiment as well. All right. Finally, what happens during the sunset or the sunrise? Well, the effect or the concept is pretty much the same. The only difference now is that the sun is near the horizon. So let's say the sun is somewhere over here. Then the rays of sunlight makes its way through the atmosphere reaching us. Notice now it's passing through a much longer part of an atmosphere compared to before. And so by the time this light reaches our eyes, not only is the blue light being scattered away, but pretty much the green and yellow is also gone because it's hitting so many more atmospheric molecules. So the only color that survives are the long wavelengths which get scattered the least, that is orangeish-red. Therefore, by the time this light reaches us, it's going to look pretty much orangeish-red. We can see the same thing in our experiment as well. If we look from the top, we can see the light is passing through the short part of the tank, only the short part of the atmosphere. So what we can do, is we can take that tank and we can turn it so that we make the light pass through the larger part, the longer part of the tank. Now if we see it from the front, notice the sun turns red, or at least reddish-orange. Beautiful, isn't it? Also notice that the light that is reaching us is pretty dull, we can hardly see the flashlight now. That's because most of the light has been scattered away from us. That's what makes the sunrise and the sunset so beautiful, because we can see it directly without hurting our eyes. To quickly summarize, the reason we see the blue sky and the red sunset is because when light, or white light hits these atmospheric particles, they scatter blue light more compared to all the other colors. One last thing I want to talk about before we wind up, is why are the clouds white in color? What I mean is that clouds are made up of tiny drops of water. So when sunlight hits those drops of water, shouldn't they also scatter blue light in all the direction? That means shouldn't the clouds appear blue in color? Well, it turns out that only those particles whose size is smaller than the wavelength of light, only those particles can scatter blue light more compared to red; or shorter wavelengths more compared to the longer ones. They're oxygen, nitrogen molecules. Now the molecules of the atmosphere and the milk particles that we find in water, pretty much fall into that category. But if the particles become much larger than the wavelength of light, in such cases it turns out that they will scatter all the colors equally. So drops of water are actually, which are found in clouds, are much larger than the wavelengths of light. So when white light falls on them, they scatter all the colors equally. As a result they end up scattering just white light. It is for that reason we will see clouds to be white. Even fog or mist also fall into that category. Even they appear pretty white to us.