A rainbow is a colorful half-circle shape. It is formed when light hits water and is refracted, or bent. Light that appears white (like light from the sun) is actually made up of several colors! The colors that make up white light are the same colors that make a rainbow, they are red, orange, yellow, green, blue, indigo, and violet. Here's how refraction works to make a rainbow:
You can see a rainbow when the sun is low in the sky behind you and there is rain off in the distance in front of you. Beams of light from the sun shine towards the rain in the air and when the light goes into the raindrops, it is bent (refracted). When the light bends, it breaks into all of its colors (the colors of the rainbow). When the light hits the back of the rain drop, it is reflectedand bounces back in the opposite direction (back towards you). Each color leaves the raindrop at its own angle, different from all the others. The colors of light bounce back to your eyes and form a half-circle shape, because of their different angles, and you see a rainbow of all the colors!
The colors of the rainbow always appear in the same order because each color always bends at the same angle. The red angle is reflected into your eye at the top, violet at the bottom, and the others at their specific place in between.
If we could see a rainbow from above the horizon, we would see that it actually forms a perfect circle! The reason it appears to be a half-circle is because the horizon blocks the other half of it from our view when we are on the ground. If you were able to get up above the horizon, the place where the ground and the sky appear to meet in the distance (even though they don't really meet), you might be able to see a full circular rainbow.
When light is reflected two times in the same water drop, a second rainbow will show up right above the first one. The colors of the second rainbow (called secondary) will look a little different. They will be in the opposite order since they are a reflection of the rainbow, so red will be on the inside and violet will be at the top! The colors will look much lighter than the colors of the main rainbow. Have you ever looked into a lake or other pool of water and seen a reflection of yourself? If you remember, you probably looked a lot lighter and backwards in your reflection. To learn more about how a reflection works, try this: write your name in big letters on a piece of paper. Stand in front of a mirror and hold up the paper. What happens? The letters look backwards in the mirror, because you are not seeing a picture taken from the front of you, you are seeing a reflection of yourself and the letters on the paper. The reason a reflection of a rainbow looks lighter is because there is less light to reach your eyes since it has already gone through the raindrop twice by the time you see it.
A rainbow is just one type of optical wonder in the sky that is created by light. Here are a few others.
Moonbow - a moonbow or lunar rainbow is created by light from the moon instead of the sun. Moonbows are usually harder to see since the moon doesn't give off as much light. They are common around waterfalls where there are lots of drops of water in the air, even when it hasn't been raining. Here is a picture of a moonbow over Yosemite Falls in California.
Halo - a halo can appear around either the sun or the moon and usually happens when there are ice crystals in clouds very high up in the sky. Light is refracted and also reflected by the crystals of ice and a glowing ring, called a halo, is formed around the source of the light. Here is a picture of a sun halo.
Mirage - a mirage works sort of like a mirror. It is a reflection of a part of the sky and it happens when light moves from air of one temperature to a very different temperature (such as cool air to very warm air). The rays of light bend (or refract) in a way that causes an image of a different part of the sky than what is normal. A mirage can appear on the ground or above the ground. Have you ever ridden in a car on a hot summer day and thought that the road ahead of you was wet? As you got closer the "puddle" you thought you saw probably disappeared. That is a common road mirage, like in this picture.
Have you ever noticed all the pretty colors in soap bubbles? Bubbles have two thin layers of soap that reflect light, so when a ray of light hits a bubble, it separates into different colors, then the colors re-combine with each other on their way to your eye. However, since different colors of light bend at different angles, the colors don't match up perfectly when they combine. This is called interference and is what causes rainbow colors to appear on the surface of the bubble. You can learn more about bubbles here. A very similar thing happens when light hits the surface of certain types of oil (like the kind that gasoline comes from) in water that has spread out into a thin layer.
Red, yellow, and blue are called the primary colors. When those colors are mixed, lots of new colors can be made. Try this quick activity to let kids discover how colors mix: pour some whole milk into a bowl and put a few drops of red food coloring near the edge of the bowl. Add some yellow and blue spaced evenly away from the red (the three colors should form a triangle). Let kids dip a cotton swab in dish soap and then into the middle of the milk and watch the colors mix! Red and yellow combine to make orange, yellow and blue make green, and blue and red make purple (violet). These are all the colors of the rainbow except for indigo - indigo is made when there is more blue and violet is made when there is more red.
Refraction- when light bends because it passes through a different material like when it goes from air into glass or water.
Reflection- when light hits an object and bounces back in the opposite direction. A reflection could also mean an image, such as a reflection of yourself in a mirror or a puddle of water.