 Rainbows are beautiful but how exactly do they form and why is always the red color on the outside and the violet on the inside and how far are these rainbows formed? These are some of the questions we'll try and answer in this video. So imagine I'm standing here it's a beautiful day and I want to see a rainbow. What do I need? Well you might know we need sunlight you don't see rainbows at night so sunlight is important let's say here's our sunlight we also need raindrops it can be any drops in fact like it could be from the sprinklers or it can be from the fountain but let's put raindrops. Now you might say raindrops are not round yeah I've just considered them to be round for simplicity you'll see it'll help us when it comes to drawing ray diagrams later. Okay but how does raindrops and sunlight give me rainbows? We need one more thing to answer this question. We need to remember that rainbows are always formed on the opposite direction of the Sun. If the Sun is to the right of me the rainbows will be on the left. If the Sun is behind me the rainbows will always be in front of me so over here the Sun Sun is behind me so over here I would see the rainbow in front of me. So with this info that rainbows are always on the opposite direction of the Sun. Let's see if we can figure out how rainbows are formed at least the basics but here's how I like to think about it. Let's start from the rainbow. If I'm looking at the rainbow I know that the light from the rainbow must be reaching my eyes the red light the blue light the yellow all the colors must be reaching my eyes so let me first draw those. Okay the blue light is a little hard to see let me just reduce the background okay that's good. Alright now comes the next question where does that light come from? Where did that light come from? Well we might guess that since rainbows are formed due to sunlight maybe that's coming from the sunlight but in what direction is sunlight? Sunlight is coming this way so let's go ahead and draw that as well. Nice now with the rays drawn we can start predicting what must be happening over here. You see the incoming light is white something happens and then we have colors being thrown at me so what phenomena must be happening over here? What must be happening to convert that white light into the colors towards me? Can you pause and think about this? You may have already learned this before. Okay so the first thing I'm thinking about is how is this white light come turning into colors? Have we seen this before? Yes we have. We've seen this happening in prism. When you throw a white light at the prism we see colors happening colors coming out and why does that happen? Well we saw before that white light itself is made of all the colors of the rainbow all the seven colors. If they are traveling together but when they go from one medium to another different colors bend differently. Red bends the least while it bends the most and as a result all the colors separate out. We gave a name to this phenomena we call this dispersion. You can guess something maybe similar must be happening maybe the white light is separating out giving us colors so that's one thing that's happening over here but that's not all something else. You see just like in the case of the prism the light should have just traveled in the forward direction the light the colors must have just for traveled in the forward direction but no the colors are turning and coming back towards me the colors seem to be bouncing off of something. That means the colors are reflecting right? So it's kind of like the colors are hitting and then get reflected back. So these are the two things that must be happening. Let's go ahead and write that down first. So the two things that must be happening over here is one separation which we call dispersion, dispersion of light that must be happening over here and what else after dispersion after the colors got separated they must be reflected back so there must be reflection happening as well. So after dispersion reflection must be happening and with these two we are getting our rainbow. Okay but who's causing that? Who's making this dispersion and reflection happen? There are no prisms in the air. Any guesses who's doing that? You might guess that is the raindrop. So the raindrops are taking the white light separating them and then reflecting them back to give us the rainbow. Isn't this amazing? Just like how scientists do we work backwards to figure out or at least make a guess as to how rainbows are formed. But this might start raising more questions, more interesting questions. For example, if each raindrop is giving me colors why do I just see one single rainbow? Shouldn't each raindrops be giving me rainbows? It should be a complete mess of colors right? Why do I see this nice pattern? Okay interesting question now maybe to answer this we need to dig a little bit deeper and see what's exactly happening inside those raindrops. So you know what? Let me get rid of this rainbow for now and zoom into this particular raindrop. So now let's see if we can figure out what happens inside this raindrop. Alright so here's the white light entering the raindrop. Let me draw that. There are a lot of rays of light. Let me just draw one ray of light that's entering my drop. Okay here is the white light entering. What's gonna happen to that? Well I want you to try and pause this video and see if you can draw this yourself. It's important because you already know the rules of reflection, refraction. You've already done ray diagrams before so it'll be a great idea to pause and you know see if you can do this yourself. Alright hopefully you've tried. Let's zoom in and see what happens. We have white light. Let me zoom in even more. Okay we have white light that is going from a rare medium to denser medium. So it's going to some part of the light will enter into the drop. It will refract and some of the light will reflect. Okay I don't care about the reflection. I'm worried about what happens inside the drop so let's consider the refraction case. So to draw the refraction case I need to first drop in normal and how do we draw normals for curved surfaces? Just like in the case of mirrors we'll draw it from the center. Center of curvature over there. Here it'll be the center of this drop itself. Alright and in fact this is the reason why we consider the raindrops to be spherical. In reality the raindrops are not exactly the shape but you know we are approximating it so that it becomes easier for us to draw. Okay so what happened to this white light? It's going from rarer to denser so it's gonna bend towards the normal but remember not all colors bend equally. Red bends the least. So here's my red bending towards the normal. It's gonna bend a little bit let's say and then blue or violet I'm just gonna draw it as blue it's easier to see. So violet or blue that bends a little bit more so I'm just gonna make it bend a little bit more. What happens next? Now this is important because there are some misconceptions that exist over here. What do you think happens? Well it's a raindrop which is transparent that means the light will just exit and that's true. In fact most of this light will just exit the raindrop this way and it will not reach our eyes in this direction but some of that light will reflect. Like I mean you know right in water some light gets reflected and motion light goes through same thing's gonna happen over here. Some of the light is gonna get reflected. Now the misconception is a lot of people think and even in some books it's written that the light gets total internally reflected meaning all the light gets reflected. That's not true. Okay just saying that that's not true only some part of light gets reflected just like usual. Like how usually water reflects some of the light some part of light gets reflected and that's what I'm interested in. And so I'll not draw the right that goes out. So let me draw the light that is reflected. To do that again I have to draw normals right. So let me draw one normal for blue and let me draw one normal for red. With this now let's draw the reflected light. For red here's the angle of incidence so let's try to make the angle of reflection equal to the angle of incidence. Not like this that's too small this is too big. Somewhere over here approximately. This is the same thing for blue. Notice for blue the angle of incidence is a little smaller compared to that of red. I hope you can see that and so we need to try and match that. It's not gonna be like this. It's not gonna be like this. Somewhat like this. I always had troubles drawing this actually. And as you can see drawing normals actually help. Okay what happens next? Well again most of this light will exit the raindrop and some of the light will get reflected again. But now I'm no longer worried about the light that gets reflected. Let's draw the case of exiting. So again I have to draw normals. So one normal for red. One normal for blue. And now we can draw the light that exits. So the blue light it's bending from it's going from denser to rarer it bends away from the normal. Away from the normal. And even the red light going from denser to rarer it bends away from the normal. Let's quickly include the arrow marks. Great with this hard work we'll now be able to answer our question. So let me just zoom back out. And so we are seeing that each raindrop is gonna throw all the colors at me. Red at the bottom blue at the top. But what's important is only one of that color is reaching my eye. In this particular case notice that only the red from this raindrop is reaching my eye. And therefore that raindrop is gonna look red to me. Let me draw that. This raindrop is gonna look only red to me. If somebody else was standing on top of me and looking at that raindrop maybe that would look blue to him or her. So for each person that each raindrop is gonna look a specific color. Hopefully that makes sense. And also one more important thing you might be wondering you might be saying hey you said that only a little bit of light reaches my eye because most of the light goes out. Only a little bit of light is getting reflected right. So then is that little bit of light enough for me to see it? Absolutely not. A single raindrop is actually not enough. But there are many raindrops. So let me make a copy of this. What I'm trying to say is let's take another raindrop which is in the same line over here. That's also throwing red light at me. Another raindrop over here is also throwing raindrop the red light at me. So if you look at all the raindrops in this line all the all the raindrops in that line they're all throwing red light towards me. And all of that combined is powerful enough so that I can see the light. In fact this is the reason why we need strong sunlight. Without the strong sunlight the light will not be powerful enough and I won't be able to see rainbow. So it's powerful light you need a lot of raindrops only then you'll be able to get enough light to see red. Okay what will I see for other raindrops or how do I see other colors? Well let's see. Let's see if I want to see violet. Where should that raindrop be? Well now I can just move this around. If I want violet or blue notice that raindrop has to be a little bit below. So now if I look at this raindrop or if I look at this angle a little bit below. So let me just draw that again. See this is my horizon. If I look a little bit above I will see red. If I look a little bit below I will see blue. So this raindrop looks blue and all the raindrops over here are going to look blue to me. And so what's important is that each raindrop doesn't give me a rainbow they're all forming a part of the rainbow and there'll be all the raindrops in between over here and all these will give me other colors. What about the raindrops on top of this? They will not give me any color. Let me show you that. If there's a raindrop on top over here none of the colors reach my eye. So they will not contribute to rainbows. If there's any raindrop below again none of those colors reach my eye again they will not contribute to rainbows. And that's why the rainbows are only contributed by those specific raindrops which are forming over here giving me that beautiful pattern. So here's one last question. How far is a rainbow? Well at first we might think well it depends upon which water droplet the light is coming from right because that's what gives us a rainbow. But we just learned that rainbows are not formed due to a single water droplet. They're formed due to all lots and lots of drops. Drops which are far away and drops which are close to us. All of them combine to give us a single rainbow. So are rainbows far? No they're not just formed by those drops. Are they close? No they're not just formed by those drops. They're formed by a combination. So where is the rainbow? It's everywhere. Isn't that weird? I mean clearly rainbows look like they're at a certain distance but we just saw they're not. They are everywhere. Everywhere from close by to far far away. Everywhere is a rainbow. Amazing right? It's not a thing. That's a thing and rainbow is not a thing. It just looks like it's a thing but it's not. It's not at a particular place. And of course we didn't explore why the rainbow is a bow shaped. That's something we'll explore in the future video but that's it for this one. So let's summarize what we just learned. So let's recall some important things from the video. Can you first of all recall what are the two important phenomena that has to happen to our light in order for us to see rainbows? Secondly can you draw a ray diagram showing refraction inside the raindrop? Then using that ray diagram can you explain the color sequence of the rainbow? Why is it that we always get red on the outside and the violet or the blue on the inside? And finally can you explain maybe to your friends or your to your family how far are the rainbows? If you have troubles answering this question don't worry you can always go back and revisit the parts of the video.