Rainbows

How Rainbows Work 

Light Bends

Refraction is the "bending" of light.  This is what is happening to light when rainbows are formed.  Light bends or changes direction when it goes from one medium to another because light travels at different speeds in different mediums.  

In order to picture this process, imagine pushing a shopping cart through a parking lot.  The pavement is a medium that it is traveling through. Now, imagine you go off the pavement and onto the grass and the cart with simply slow down.  The medium has changed.  The amount of force you place on the cart depends on the medium in which it is travelling through.  The grass medium has more resistence so it takes more energy to move the cart.  

But if you push the cart onto the grass at an angle, something else will happen.  If the right wheel hits the grass first then that wheel will slow down while the left wheel is till on the pavement.  Since the left wheel is briefly moving faster than the right wheel, the shopping cart will naturally turn to the right as it moves onto the grass. (See visual).  

This is what happens to a beam of light when it enters a glass prism. A simplified explanation is that one side of the light wave slows down before the other, so as the light moves from the air to the glass, the beam turns.  

Not only is the light bent, but the white light is also separated into different colors.  Different colors of light have different frequencies.  These frequencies cause them to travel at different speeds as they move through matter.  

The slower the light is travelling, the sharer it will bend.  This is because when the light moves slowly, the speed difference is more severe.  Dispersion is when the glass bends the light twice  like in a prism, and you can see the separated colors more easily. 

Now, if you are like me, you aren't very familiar with the concept of making rainbows through prisms.  Most of us, as well as most of our students, will think of rainbows in the sky.  They will relate rain with rainbows. So, let's look at how rain causes rainbows

Making a Rainbow

An individual raindrop effects light in a similar way that the glass prism did.  When white sunlight hits a collection of raindrops at a low angle, you can see the different colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet.  To keep it simple the diagrams only show red and violet, the end colors of the spectrum.  

Each single raindrop disperses white sunlight into its component colors.  So why do we see a wide band of colors? Because we only see one color from each raindrop.  For example, when raindrop A disperses light, only the red light exits at the correct angle to travel to the observer's eye.  The other colored beams exit at a lower angle, so the observer doesn't see them.  The surrounding drops will be hit by sunlight in the same way, so they will all bounce red light onto the observer.  

However, raindrop B is lower in the sky, so violet light exits to the observer, not red.  The surrounding drops also turn the light violet.  The raindrops between A and B all bounce different colors of light, so the observer sees the whole color spectrum.  

If we stood above the rain, we would see the rainbow as a full circle because the light would bounce back from all around you, but we see the arc of the rainbow that is visible above the horizon.

A View from Above

Sometimes, we see a double rainbow which is a sharp rainbow with a fainter rainbow on top of it.  This happens because the light is reflected twice instead of once.  If you look carefully, you will notice that the colors in the second rainbow are in reverse order. 

A Double Rainbow: Can you see the reverse order?

Rainbows in the Elementary Classroom

Now, I think we can all agree that the idea of rainbows is very abstract.  This is a hard concept to explain to students, especially elementary students.  I don't think that we should go as in depth as the article did, but it is good to know the information yourself to feel comfortable with the concept.  There are many things that you can do with rainbows to show students that it is simply the bending of light.  

Below is a good introductory video that can be shown to students before a lesson on rainbows.  

http://www.youtube.com/watch?v=K0S3RlHTPSI

• A good chance for students to see a rainbow is to simply take them in the sunlight and allow them to hold a CD with the back of it toward the light.  The reflection will make a rainbow appear.

Rainbow in a Box

This activity also involves using a CD.  It is important that students understand that the CD seperates white light into all the colors that make up the rainbow.  When light waves reflect off the CD, they overlap and interfere with each other.  Sometimes the waves ad together to make a rainbow. 

Materials:

1. CD
2. Box
3. Flashlight

Directions:

• Shine a flashlight on an old CD in the bottom of a box turned on it's side.
• Turn off the lights, and move the flashlight across the CD to make the rainbows dance. 

Cross-Curricular Themes

The neat thing about rainbows is that they can be implemented into many different subject areas.   By following the link below, you can find many of these activities.  There is rainbow addition,  eat the rainbow (a health lesson), and rainbow symmetry.  Rainbows can also easily be implemented into art, reading, and music. 

Click Here for More Rainbow Activities

References:

Harris, T. (2002). How rainbows work. HowStuffWorks.com. <http://science.howstuffworks.com/nature/climate-weather/storms/rainbow2.htm>. 12 April 2013.

J Ross. (2012, June 6). Rainbows day 5: What makes a rainbow. Retrieved from http://mylearningboys.blogspot.com/2012/06/rainbows-day-5.html