How does a rainbow form , physicist guide to explain this natural phenonmenon

Rainbow ??…………..

If I ask you a question what is a rainbow?
Then most of you would be able to answer because everyone must have watched the rainbow but simply if ask you a question what is the colour sequence then most probably you won’t be able to answer and if I keep on asking  questions like how many bows are there? Then also you won’t be able to answer because you have looked the rainbow but haven’t seen it.

I, PIYUSH MADAAN – Student of IIT ROORKEE trying to impart you the knowledge about rainbow by writing this article. So before going into the discussion about rainbow let’s first review refraction

So refraction is the phenomena of change in the direction of propagation of wave due to change in transmission medium.

And this wave has been analysed using very famous law called Snell’s Law.

Now let’s see what happen when the same light passes through the water droplet?

So you can get this diagram by spending your 4-5 minutes and you have to use the geometry. Consider this to be the raindrop and in this case a ray is entering the raindrop from the bottom but if you want to consider that ray is entering from the top and coming out from the bottom, yes that is also applicable because of principle of reversibility. Moreover you can apply the snell’s law at every point B, A etc.

So let’s see what happens when sunlight (or white light) enters a water droplet.

Let’s zoom out this image

So what we are getting from this about 42⁰  and 40⁰ . These are nothing but these are the angle which these colours will be going to make with your eye.  And complete spectrum comes under these two angles and what we should see somewhat like given in the image.

Isn’t it ?……………………

Yes that means violet above and red below and other colours in between. So keep this thing in your mind. So from now on if someone will going to ask you about the colour sequence in rainbow then what you will have to say red at the bottom and violet at the top. Is the answer correct? For this let’s see the image.

Isn’t it in this image the red is at the top and violet is at the bottom. That means either this is wrong or else we are wrong. So who is correct? Obviously the picture shown is the correct can’t go wrong, this proves that we are wrong. But how?

Let me tell you how. The image which I am showing will tell you what we actually see.

So if want to conclude the above stated points then how would be able to? So for this look at the image below.

Now I am sure by reading up till here you will be able to answer the question which I raised in the beginning. What is the colour sequence? And this time you will answer it correctly.

Now let me ask you my next question. Why do we call it as the Rain BOW not a rain line not a rain zizag

Why only BOW?

Have you ever noticed? You would not have………..
so the answer to this question is
See you and your eye are the apex of a half huge cone and these ends make the rainbow.

I think you must have got the answer.

Now I want to tell you something else which you would not have noticed ( I can bet) that
Rainbow consists of two bows:-

Primary Rainbow and
Secondary Rainbow
So lets have a look

If its still not clear then look another

And just look into the above image and try to figure out  that sky is darker between bows than otherwise. Can you answer this question? Think about it…………….

Definitely I will answer this question in this article only. Now let’s come first two types of bows ????………………… now your question must be how two?………………

Ok what we have analysed that rainbows are being formed because sunlight when passed through the water droplet gives the rainbow. Can’t this also happen look at the image at right hand side.

That why isn’t that possible that light gets reflected twice inside water droplet. So what is the conclusion?
Primary rainbows are being formed because of the reflection of light only once in a drop while secondary rainbow is being formed due to reflection of light twice inside a drop. Like

Again look into the image and try to figure out that primary bow starts from violet (bottom) to red (top) while the order reversed for the secondary bow. No I think you can answer this question think about it. If not then look at the image below.

Now why the sky between the two bows are more dark than otherwise its very simple as it doesn’t receive any singly or doubly reflected ( or refracted) ray. Therefore more dark.

Now let me tell you the last ( but not the least) thing of this article

Rainbow are polarised. Let’s look at the image

So what is there in the image. What I am trying to show? So here is the person holding a polaroid film ( you can also take sunglasses instead). So what you are looking into the figure that rainbow is visible. But when you rotate this polaroid what you will observe that no rainbow is there. I can show you the final rotated image. Here we go

Now look at this here in this figure the person is rotating (anticlockwise) this film from the top and in the second image his hand comes to the other side and moreover it’s not visible too. That tells us that colours are polarised in rainbow. Moreover you can do this activity by using your sunglasses.

That was all I wanted to discuss with you with the help of this article.
Here you have learnt what you saw………. And also which you didn’t notice.

I have enjoyed writing this article hopefully you must have enjoyed reading this article. So thanks for showing your interest in my article. If you have any queries left with you then you can post your queries on the discussion board here.

So Thanks for reading this article. I am Piyush Madaan member of askIITians family. So we have created the sample papers and study materials for JEE aspirants. So you can go to our website for regarding the same. Here is the link.