Transcript for: Understanding the Science of Rainbows
This looks like an ordinary gray board but from a different point of view through this camera you can see a beautiful rainbow isn't it very nice rainbows usually appear only when it rains but instead of raindrops this board is covered with thousands and thousands of tiny glass beads and they the beads make the rainbow now we're chasing rainbows an answer to a question sent in by Mrs. W.A. Lynn of Gibbon, Nebraska who writes why is a rainbow curved? Now she actually speculates it might be the pot of gold that bends it toward the Earth interesting guess here with a more scientific answer is Jack Netland High School physics teacher from Osseo, Minnesota. Welcome to the program Jack good to see you again. - Thank you Ira nice to be here. - Are you ready to tackle the question why is a rainbow curved? - Oh yes I am. In order to talk about rainbows you're talking raindrops. Okay? - All right. - What we have here is a model here of a raindrop pretty good sized raindrop but it's round like a raindrop - Beaker filled with water. - If you would turn on the sun over there. - You've capture the sun in here. - Yes indeed. And take the little screen there and place it in front of here I think you will see a rather distinct spreading out of colors. - Oh look at that you've created our own little rainbow right here. - The light from the box is actually going into the glass with the water in it and coming back out. - Yes it is. - And creating a rainbow. - Spread into it's colors. - Well let's talk about that what's happening inside of our rainbow. - Let's take a look at another raindrop here. - This is a big raindrop. - Big raindrop cut in half the light from the sun comes in like so it enters this side of the raindrop and it starts to bend into it's colors right here it bounces off the back of the raindrop goes out here bends again and spreads out even further into the colors. What that means is that it makes a very definite angle here between the incoming light and the refracted or our spread out light and that angle is about 42 degrees roughly. - So you have to have at least that angle or that angle to create this bending and bouncing to come out like this. - And all of the raindrops do it. - Now let me ask this question if this is happening all the time during a rainstorm - It is. - How come you don't see rainbows everywhere it's only a rare occasion that you see a rainbow. - Sure well the rainbows are appearing everywhere they are everywhere - They are? - But you can't see them unless you look directly where the light is coming the light must get right into your eye from that color. - I see. - Now that makes it necessary that the sun is low in the sky usually either the morning or the evening and the sun has to be behind you and the raindrop in front of you in order to see that. - And they go create this angle here. - Yes. - I see. - Now in order to understand it a little better we've got another arrangement back here. - Oh lots of rainbows. - Lots so we got this represents the billions and billions we've got rain falling here. - So each one of those is a raindrop? - Well yes and it can represent lots of raindrops. You'll notice from the top one here we get a band of red that comes to that point - Right. - From this one we get a green band and from this one we get a band of purple or blue. Now if you put your eye down there now the light from all of them into your eye which is necessary for you to see them. - But I'm only seeing the red from the top one and the green from the middle so even though it's producing all those colors as you say I'm only seeing a particular color from a band of raindrops. - That's correct the rest of the colors for example here those are going off to somebody else and he is seeing his rainbow over there. - He's seeing a different rainbow than I'm seeing? Everyone sees different rainbows? - You're very own rainbow. - I didn't know that that is fascinating. - It is interesting and it makes it kind of special in a way. - Absolutely. Now let me ask you this question. You have explained very well Jack what rainbows are but you haven't explained the question that's been asked how come rainbows are curved? - Yeah I think we have enough information now we can go back over here to our little model of this person walking in the hill and let's call him Ira. Ira's out there walking. - Okay he's even got a mustache. - I think so yes. He's walking out in the Hillandale and he sees a bunch of rain over here this is a sheet of a billions and billions of raindrops and they're coming down this way and you can imagine then that here again is our angle our 42 degree angle the light from the sun coming from over there okay? - Right. - Now I want you to take that and hold it so that the light comes to Ira down there. - Okay. - In that way. Okay. - Okay. - Now move it some place. - All right ill move it around here. - Now you'll notice that's not pointing anywhere. - Oh wait no the lights not reaching him there. - No not from there. - If I move it here the rainbows not reaching him. - Not from there. - So the only place it's going to hit him like this is if I rotate it. - If you rotate it like that you're going to get an area that is swept out like this. - We sweep out the rainbow. - The rainbow that's why it's curved. - That's very very interesting now if the land weren't here if I could sweep it even further - Yes. - I would get a circle. - A complete circle. - A complete circular rainbow. - Yes. - Can you ever see a circular rainbow? - Sure if you're high enough in the air an airplane for example and the sun is at the right angle you can see a complete circle of a rainbow yes. - One more question about this Jack because I'm really interested now A lot of times you see double rainbows you not only see this one band but you see another band on top of it. - Yes like this right here a beautiful picture of a double rainbow. What we have is this is a of the primary rainbow here and then here is the secondary rainbow. Now this one is caused from two bounces inside the raindrop. - Instead of the one that causes this one. - Right this gives you a 42 degree angle this because of the extra bounce gives you a 51 degree angle it appears at a different place. - That's fascinating and it's not the pot of gold on the end of the rainbow that's curving it down. - Oh no it's not that at all but if you could get to the end of the rainbow I can guarantee you a pot of gold. - That's a big boast Jack. - Well. - It's putting your reputation on the line. - Its because you can't get there. - Why can't you get there? - If you go to where you think the rainbow is remember the angles gotta stay the same rainbow moves - The rainbow's gonna move. So I keep going towards the rainbow and the rainbow keeps getting further away from me. - But if you could get to there the pot of gold would probably look like this. - Well thank you very much Jack for this colorful and now very enriching explanation. We'll have more in a minute.