Spring is here! Did you know spring showers make for the perfect opportunity to see colorful rainbows? Raindrops and low sun angles reflect some of the prettiest colors in our spectrum of light.

In this colorful at-home activity, we’re exploring rainbow science and creating our own spectroscope to capture the sun’s rays and reflect them into the coolest rainbows! Follow along in the video, then learn more about rainbows and get step-by-step directions for our DIY spectroscope in the content below.

Rainbow Science

To get the perfect rainbow, you need water droplets and a low sun angle. Physics takes care of the rest!

When the sun’s rays shine through a water droplet, some of the sunshine is reflected out of the droplet, and some of it is refracted or bent in a different direction within the droplet.

As the rays get bent inside the droplet, the droplet itself acts like a prism, separating the light into the color spectrum ROYGBIV.

Since the raindrop is circular, the rainbow reflection it creates is also circular, but we can only see half of the arc since the Earth’s surface gets in the way.

Sometimes, we can even see two rainbows, the second of which is often a bit fainter. That’s because the sunray is reflected two times inside the raindrop. Most of the time there are secondary rainbows, we can’t see them!

To get the best view of a rainbow: the sun should be to your back with rain droplets in front of you. Near sunrise or sunset are also prime times for rainbows because the sun’s angle is 42 degrees above the horizon or smaller! It is much more difficult to see rainbows when the sun is directly overhead, the rays don’t have the right entrance angle into the droplets.

Create a Spectroscope

Age range: Elementary, middle school
Prep time: 5-10 minutes
Learning time: 15 minutes

Materials: 
– Paper towel roll
– Craft knife or sharp scissors
– CD
– Pen or pencil
– Tape
– Index card
– Paint or markers
– Glitter (optional)

Directions:

1. With an adult’s help, use a craft knife or sharp scissors to cut a slit with a 45-degree angle toward the bottom of the paper towel roll.

2. Directly across from the slit, use the same knife or scissors, and cut a small viewing hole with an adult’s help.

3. Trace one end of the paper towel roll onto an index card to make a perfect circle.

4. Cut out the circle. Cut a slit across the middle of the circle.

5. Tape the circle to the top of the paper towel roll.

6. Insert the CD into the 45-degree slit, with the shiny side facing up inside the paper towel roll.

7. Decorate the paper towel roll with colorful paint, markers and/or glitter!

8. Step outside into the sunlight. Face your spectroscope up and take a peek inside your viewing hole. What do you see?