What Makes a Rainbow?

 

 

Student:    Hey Professor, this might sound like a childish question, but where do rainbows come from?  And can anyone ever actually find the end of the rainbow?  Sometimes I see rainbows when I look at my reflection in my CD’s and DVD’s, and sometimes I see a rainbow when I use the garden sprayer in my back yard.

Professor:  Heh, not so childish at all.  I wondered that same thing when I was a kid.  I never really heard a good explanation until I got my degree in physics.  The answer to your second question is no – it is basically impossible to get to the end of a rainbow – at least an “atmospheric rainbow.”  I suppose you could get to the end of the rainbow you see in your DVD when you look at it at the right angle.

Student:    So what causes a rainbow?

Professor:  Well, we need to talk about color first, and how it works.  You see, “white” is not actually a color in the ordinary sense.

Student:    White is not a color?!  What do you mean?

Professor:  I’m not saying that there isn’t anything out there that looks white.  I’m just saying that white is a color of a different nature from red, orange, yellow, green, blue, and violet.

Student:    Oh, yeah!  Those are the colors of the rainbow – except that you forgot the color indigo.

Professor:  Right.  Some people include the color “indigo” if they’re wanting to be poetic – it’s not always necessary to say “indigo” unless you’re writing a poem or something.  And I’m going to tell you something equally surprising – the colors of the rainbow are the only true colors that there really are.

Student:    Well, you seem to have forgotten about brown, pink, and peach, and teal.

Professor:  Okay, okay.  I’ll get to all of that in a minute.  But first, we need to talk about the visible spectrum of light.  You see, scientists aren’t really sure exactly what light is made out of, but we know that in certain situations it behaves like a wave.  And this example is one of those situations.  As I’m sure you know, or can at least imagine, waves have a wavelength and a frequency.

Student:    Okay, I’m with you.

Professor:  The different colors you see are because of the fact that light can have different wavelengths.  The different shades of red have wavelengths between about 620 nanometers and 720 nanometers.  The different shades of blue fall between 420 and 495 nanometers.  One nanometer is one billionth of a meter, to a scientist.  The spectrum of visible light looks like this:

Student:    Oh, hey!  That’s the rainbow.

Professor:  Right!  The rainbow is just all the colors lined up in order of increasing wavelength.

Student:    But where’s teal and pink, my two favorite colors?

Professor:  Well, teal is right around 505 to 510 nanometers on the rainbow.  But pink is not a real color.

Student:    Man!  How can you say that pink is not a real color.  I see the color pink all the time!

Professor:  No – you think you see the color pink all the time.

Student:    Huh?

Professor:  Pink is actually white light with an excessive amount of red in it.  And white light is not really a color unto it’s own.  White light is actually all of the colors mixed in equal amounts. 

Student:    Oh, so I think I’m starting to see what you’re saying about white not being a real color.  White is just a mix of all the colors.  The sensation that we humans call “white” is actually just an illusion of sorts.

Professor:  Exactly!  The only true colors are those of the rainbow.  Everything else is a mix of some other color.  Pink is a mix of red and white, and white isn’t even a real color itself to start with.  So pink is not a real color.  It’s just an illusion.

Student:    Well, wait, something’s not right here.  My art teacher told me that green is a mix of yellow and blue.  What’s green doing on the rainbow if it’s really a mix of two other colors?  Seems to me like green is not a real color either.

Professor:  That old phrase “yellow and blue makes green” is actually just a trick for mixing paint that looks green when you don’t have any truly green paint.  The green paint that you mixed in art class from yellow and blue paint is just paint that has the illusion of being green paint.  A neodymium laser, on the other hand, shines a truly green laser beam at 532 nanometers.

Student:    So what does all this have to do with rainbows?

Professor:  Oh, well, a rainbow happens anytime that white light (which is made of all the colors) is separated into individual colors.

Student:    Yeah, like a prism or a crystal ball.

Professor:  Exactly.

Student:    So how does a rainbow form in the sky?

Professor:  Well, you just brought up the fact that a prism and a crystal ball can both break white sunlight into individual colors.  It turns out that drops of water can do that as well.  The difference between water drops and prisms is that a prism lets the sunlight pass through it and continue on in the same direction after the light gets broken up.  But the water droplets will reflect the light after they break it up, if the light is bright enough:

 

Student:    So I see that a rainbow in the sky has to do with light being reflected by raindrops, but how do the raindrops actually add up to a rainbow.

Professor:  You get a rainbow when the sun is shining low in the sky (like in the morning or the evening) and there is rain on the other side of the sky.  The sunlight shines past you, hits the raindrops on the other sides of the sky, and then gets reflected back to you in the form of a rainbow.

Student:    So why is the rainbow round?

Professor:  Because the sun is round:

 

 

Student:    Oh! I see.  And I meant to ask you what’s up with that second little rainbow I see sometimes when it’s a really bright rainbow.

Professor:  That second rainbow occurs when the light bounces off the back of the rainbow not once, but twice.  The light coming out isn’t as intense, so the secondary bow is usually fainter, and comes out of the raindrop at a different angle, so you see it in a different place.

Student:    So can I ever get to the end of a rainbow?

Professor:  Sorry to disappoint you, but no not really – at least not an atmospheric rainbow.  Every time you move towards the rainbow, you are looking at the sunlight being reflected off of a different set of drops.  It’s like chasing a moving image.  I guess the closest you can get to finding the end of a rainbow is the rainbow in the spray of the garden sprayer or the DVD.

Student:    Oh yeah, and what causes the rainbow in the DVD?

Professor:  The microscopic grooves that hold the data in the back of a DVD or CD can break white light into its colors just like a raindrop can.

Student:    Cool, I guess I’ll never look at a rainbow the same way again!

Professor:  Awesome!  Catch the rainbow!

 

Interesting Links:

www.photocentric.net -- An interesting explanation of rainbows from a photographer's point of view (the author of Socratic Dialogues is also a professional photographer, among other things). Very accurately explained, along with rainbow photography.