Rainbows, Prisms, & Spectra: Page 2.3

More about Thomas Young

So, you've been to see Mr. Newton.  A great man, but I don't agree with him about the nature of light. I am Dr. Thomas Young, and I'm glad you've come to hear my side of the story too.    

Although many people thought Newton must be right, we showed  that light is a wave, like the wave in a plucked  string. We did this with careful experiments, and not just by arguing about our ideas. 

That is very important; in science your ideas should help you to explain or predict what you observe in the real world. So if you have an idea or a theory, and nature does what your idea predicts, that's how you know it's a good idea! 

 I realized that the color and wavelength of light are really the same thing. What does that mean?  If you picture light as a wave, the wavelength is just the distance from one highest point to the next, like this:

picture defining wavelength


Although we don't see the light "waving" up and down, each wavelength interacts with our eyes in a different way, and we see the different wavelengths of light as different colors. It also means that every color of light has its own size: its wavelength.

You probably know that light is energy, and the wavelength of light is also a measure of the energy light carries. Shorter wavelengths carry more energy than longer ones. This is true for lots of kinds of waves; if you make a lot of short wiggly waves in a rope, it takes more energy to keep them going than a long slow wave. Your arm gets tired faster, doesn't it?


  • What colors do you think have the shorter wavelengths? 
  • What colors have the longer wavelength?

Let's think about this: You saw with Mr. Newton that each color of light bends by a different amount when it goes through a glass prism, making a spectrum. This is because the waves carrying more energy bend more. They're like speedy race horses that come to a bend on the inside of the track and get around faster.  So now you know two things:

shorter wavelength = more energy
+ more energy = bends more

Can you tell something more from this?  It's like if a=b, and b=c, then a=c. Right!  Shorter wavelengths will bend more when refracted, and longer wavelengths will bend less. That seems like solid, scientific reasoning.

Question:  How would you try to find out which wavelength is which color? 

To Do:   If you have a prism and light source available, do your own experiment to determine the order of colors in the rainbow according to wavelength.

I have also made up a puzzle for you, which will help you find the order of the colors if you can find the messages in the clues.  It will also test whether you know the vocabulary we're using. Please read these instructions carefully, and record your scores.

Observe and Record: As you determine what colors have the longest and shortest wavelengths, write the order down in your notebook, like this:

color that bent the least Color name longest wavelength
second most, etc.. ... ...
... ... ...
color that bent the most ... shortest wavelength

From now on, you'll notice that the colors of any spectrum are arranged in this longest-to-shortest wavelength order. The order is actually the same as that of a painter's color wheel, if you split it between red and violet and stretch it out straight.  Can you make a drawing of that for me?  

Next:  Besides the sun, there are many other sources of light. Do you think their spectra would be the same? I've been wondering--especially about what we can learn from the brightness of light.  Will you kindly go to the next page and see if Mr. Azophi has any clever ways to measure it?

BackMain MenuNext