Key concepts

Physics

Wavelength

Refraction

Reflection

Introduction

Did you know that in the U.S. we generally count seven colors in a rainbow whereas elsewhere in the world people only count five? We all see the same rainbows but it can be tricky to count the colors because they all blend together so seamlessly. So next time you see a rainbow in the sky try to count how many distinct colors you see!

If you don’t want to wait that long, you can count the colors in the mini rainbow you create yourself in this activity! With it, we can explore the physics of light that creates these beautiful “rainy arches.”

Background

In addition to being beautiful, rainbows serve as a wonderful example of visible light’s dispersion and its wavelength spectrum. Visible light travels in the form of a wave, much like sound and ocean waves do. Similar to how we hear different sounds depending on their wavelengths, we see different light wavelengths as a spectrum of colors from red to blue, which is how we can pick a yellow jelly bean out of bowl full of different-colored candies. The colors of the visible light spectrum are often referred to as ROYGBIV, which stands for: red, orange, yellow, green, blue, indigo and violet. This is the order of the wavelengths of visible light starting with red (the longest) and ending with violet (the shortest). It is also the order that colors appear in a rainbow!

Rainbows appear when sunlight shines through water droplets suspended in the atmosphere. Each water droplet behaves like a small prism, refracting the light and reflecting it back to our eyes. When sunlight enters a water droplet, it is first refracted (bent as it enters the water droplet) then reflected by the internal surface of the water droplet so that it turns around at (very approximately) a 90-degree angle, and then it is once again refracted when it leaves the water droplet and reenters the air. That’s a lot of bouncing around!

Light refraction is a term for the way light bends as it enters different mediums. You can observe light refraction firsthand by filling a clear glass with water and then looking through the glass. Everything on the other side of the glass appears slightly misshapen. This is due to the bending of light as it enters the far side of the glass, travels through the water and then the other side of the glass before it reaches your eyes.

Different wavelengths of light bend differently. Shorter ones (such as blue and violet) bend slightly more than longer wavelengths (such as red and orange). When white light (which contains all colors of light) enters a water droplet and then leaves it again, each different wavelength of light refracts at a slightly different angle. As a result, the white light is broken up into its component parts and we can see all of the colors in the form of a beautiful rainbow!

Materials

• Shallow glass baking pan
• Water
• White sheet of paper
• Red sheet of paper
• Blue sheet of paper
• Sunlight
• Outdoor table, chair or helper to hold the pan for you

Preparation

• Take your pan outside and place it on the table so one end sticks out over the table edge. Be careful not to let the pan tip over! You can use either another pan or your helper to anchor the pan in place.

Procedure

• Place your white paper on the ground, where the sun is shining through your pan. What do you notice about the appearance of the water on the paper? Do you see any colors?
• Slowly change the angle of the sheet until a small rainbow forms. Test the angle of the paper until you get the largest rainbow possible. What paper position works best for a rainbow? What position makes it most difficult to see a rainbow? Which colors are the clearest in the rainbow? Which are the most difficult to see?
• Notice the order of the colors that appears on the paper. What color is at the bottom of the rainbow? What color is at the top?
• While holding your white paper so you can view the rainbow place your blue paper gently on top of the white one. What do you notice about the rainbow on the blue sheet? Does it get easier to see or more difficult?
• Switch back and forth between the white and blue sheets and notice how the rainbow changes. Which colors are easiest to see with the blue paper? Which are more difficult to see? Which colors are easiest to see with the white paper? Which are more difficult to see? Test each sheet and note whether it is easier to see colors on the top of the rainbow or the bottom.
• Remove the blue paper, but keep the white one in place so you can still see your rainbow. Gently place the red paper on top of the white one. What do you notice about the rainbow on the red sheet? Does it get easier to see or more difficult?
• Switch back and forth between the white and red sheets, and notice how the rainbow changes. Which colors are easiest to see with the red paper? Which colors are more difficult to see? Which colors are easiest to see with the white paper? Which are more difficult? Test each paper and note whether it is easier to see colors on the top or bottom of the rainbow.
• Extra: Use colored pencils, crayons or markers to color in the rainbow that you see on your white, blue and red papers. Use a clipboard to hold the paper steady and fill in the colors of the rainbow you see on each sheet. Compare the colors you draw!

Observations and results

In this activity you created your own mini rainbow. Even though yours was much smaller than the ones you see in the sky, both rainbows are created by the same principles: light reflection and refraction.

We know refraction refers to how light bends when it passes through different mediums or materials such as glass, water or plastic. When the sunlight shines through the water of your glass pan the light bends. Because white light (such as sunlight) is made up of all of the visible colors, this means they all bend. Because those different colors of light have different wavelengths, they bend at slightly different angles. The water effectively breaks up the white light into its different colored components. This light reaches your paper and you see all the different colors reflected.

You should have noticed the light you observed in your rainbow changed depending on the color of your construction paper. Note that the changes you observed depend on the shade of blue and red papers you used. If you found slightly different results than these, that’s okay! Try testing other colors of paper to see what other results you find.

With the white sheet of paper you should have been able to see all of the colors clearly. This is because white paper reflects white light—and as we know, white light is made up of the whole spectrum of visible light. Therefore, all of the colors could be observed in your rainbow. When you used the blue paper you might have observed some of the colors appeared a little crisper. If you had an easier time seeing your rainbow with the blue paper, this might have been because you didn’t see as much of the water reflected on the blue sheet as you did on the white one. The water’s reflection on the white paper can make it more difficult to see your rainbow clearly. Instead of reflecting all light, like white paper does, blue paper absorbs some of the colors and reflects others. Generally, the blue dye on the paper absorbs red light, allowing some green and all blue light to be reflected. Because many visible colors in the rainbow are made up of some green and blue light (including some shades of yellow and pink, which have green and blue light in them), the rainbow colors you see on your blue paper might have looked similar to those you observed on the white paper.

When you used the red paper, however, you might have observed a significantly different rainbow. The red dye on the red sheet absorbs blue and green light and only reflects red light. Because green and blue make up a large part of the visible rainbow, removing these colors leaves us with a pretty bland looking version. You probably couldn’t see many colors except for some orange and red at the very bottom of your rainbow.

More to explore

Mixing Light to Make Colors, from Science Buddies

Technicolor Shadows: Lessons in Light and Color, from Science Buddies

Solution Science: Colorful Candy Chromatography, from Scientific American

Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies