How Many Colors in a Rainbow?
Have you seen a rainbow in the sky after a rain shower? How many colors were there? A rainbow is often described as having seven colors: red, orange, yellow, green, blue, indigo and violet, but you might spot a rainbow and see fewer colors. Or you might see more than seven! It can be tricky to count the colors in a rainbow, because they all blend together so seamlessly.
Want to see for yourself? In this activity you will explore the physics of light and create your own mini-rainbows. How many colors will your rainbows have?
In addition to being beautiful, rainbows serve as a wonderful example of light dispersion and the spectrum of wavelengths that make up visible light. Visible light travels to us in the form of a light wave, much like sound waves and ocean waves. Similar to how we hear different sounds, we see a spectrum of colors, from red to blue. And just like sound waves have different wavelengths, each color of light has a different wavelength, which is how we can pick a yellow jellybean out of bowl full of different colored candy. The colors of the visible light spectrum are often referred to as ROYGBIV, which stands for red, orange, yellow, green, blue, indigo, violet. This is the order of the wavelengths of visible light, starting with red (the longest wavelength) and ending with violet (the shortest wavelength). It is also the order that colors appear in a rainbow!
Rainbows appear when sunlight is shining through water droplets suspended in the atmosphere. Each water droplet behaves like a small prism, both 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 that light bends as it enters different mediums. You can observe light refraction first hand by filling a clear glass with water, and then looking through the glass. Everything on the other side of the glass appears slightly misshaped. This is due to the bending of light as it enters the far side of the glass, travels through the water and the other side of the glass, and reaches your eyes.
Different wavelengths of light bend differently. Shorter wavelengths (like blue and violet) bend slightly more than longer wavelengths (like red and orange). When white light (which contains all colors of light) enters a drop of water, and then leaves the water droplet 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!
- A shallow glass baking pan
- A white piece of paper
- A red piece of paper
- A blue piece of paper
- An outdoor table, chair, or helper to hold the pan for you
- Fill your pan about halfway full with water.
- Take your pan outside and place it on the table so that one end is sticking out over the table edge. Be careful not to let the pan tip over! You can use another pan or your helper to anchor the pan in place.
- 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 angle the paper until a small rainbow forms. Experiment with 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 your 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 paper. What do you notice about the rainbow on the blue paper? Does it get easier to see it, or more difficult?
- Switch back and forth between the white paper and the blue paper, and notice how the rainbow changes. Which colors are easiest to see with the blue paper? Which colors are more difficult to see? Which colors are easiest to see with the white paper? Which colors are more difficult to see? Test each paper 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 paper in place so you can still see your rainbow. Gently place the red paper on top of the white paper. What do you notice about the rainbow on the red paper? Does it get easier to see it, or more difficult?
- Switch back and forth between the white paper and the red paper, 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 colors are more difficult to see? Test each paper and note whether it is easier to see colors on the top of the rainbow, or the bottom?
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 paper. 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: reflection and refraction of light.
We know that 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. Since white light (like sunlight) is made up of all of the visible colors of light, this means that all of those different colors bend. However, since 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 that the light you observed in your rainbow changed depending on the color of your construction paper. Note: the changes you observed depend on the shade of blue and red papers you used. If you found slightly different results than ours, that’s ok! Try experimenting with other colors of paper to see what other results you find.
With the white 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 actually 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 may have observed that some of the colors appeared a little crisper. If you had an easier time seeing your rainbow with the blue paper, this may have been because you didn’t see as much of the water reflected on the blue sheet as you did on the white sheet. 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 light, and reflects other colors. Generally, the blue dye on the paper absorbs red light, allowing some green and all blue light to be reflected. Since 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 may have looked similar to those you observed on the white paper.
However, when you used the red paper you may have observed a significantly different rainbow. The red dye on the red paper 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 rainbow. You probably couldn’t see many colors except for some orange and red at the very bottom of your rainbow.