How are rainbows formed?

What are your favorite things to do when it rains? For me personally, what I enjoy the most is laying down on the couch snuggling with my dogs while looking through the window at the falling rain. My favorite moment is when the rain stops and the sun comes out along with a beautiful rainbow. On one of these  rainy days, as I was having the usual science conversation with my dogs, I tried to explain to them what makes a rainbow. The explanation went something like this.

Rainbows are formed because of the interaction between sunlight and water droplets. Sunlight is a bundle of all different colors of light, including the ones we see in a rainbow, even though it appears white. There are two main physical interactions of light to produce a rainbow, reflection and refraction. Reflection happens when light bounces back off the surface of a certain medium, such as light reflecting off a mirror. Refraction happens when light penetrates through one medium to another, causing light rays to bend since light travels at different speeds in different media. When sunlight enters a waterdrop, it is refracted on the surface and enters inside the spherical droplet. When the light hits the back of the droplet, some of it will reflect off to the surface and exit the waterdrop. The reason why we see the colors in a rainbow is because different colors of lights are bent at different degrees.  When sunlight hits the water droplets, different colors of lights exit the droplets at different angles, separating sunlight into the colors we see in rainbows.

Now we know where the colors of the rainbows come from, but you may still wonder why we don’t see many rainbows as there are countless waterdrops? This is because we can only observe one color from each waterdrop, even though the same droplet is able to disperse the light into different colors. From our previous article (How Does the Eye Work?), we know that our eyes observe objects by capturing the reflected light. In fact, only the light that exits the waterdrops at the correct angle can travel directly to our eyes. Take a look at figure 1 below, only the red light exiting the waterdrop on the top will be observed while only the violet color from the waterdrop at the bottom will be captured by the human eyes. In other words, the water on top will appear red while the bottom one will appear violet according to our eyes. The same goes to the rest of the colors in between and that’s why we see the rainbows as they are, my puppies.

By Yanyu Wu