How to Create a Rainbow with a Glass of Water

Turn a simple glass of water into a magical rainbow experiment that delights children and creates wonder-filled moments together.

  1. What You'll Need. Gather a clear glass filled with water, a small mirror (a pocket mirror works perfectly), a flashlight or smartphone light, and a white wall or piece of paper to catch your rainbow. You'll also want a slightly darkened room—not pitch black, but dim enough that your light source stands out. A sunny day near a window can work too, though artificial light gives you more control.
  2. Setting Up Your Rainbow. Place the glass of water on a stable surface near your white wall or paper. Submerge about half of the small mirror in the water, angling it so it's tilted toward where you want your rainbow to appear. The mirror should lean against the side of the glass, creating roughly a 45-degree angle. Turn on your flashlight and shine it through the water, aiming for the submerged mirror. Move the light slowly until you see colors start to appear on your wall. The rainbow might be faint at first—adjust the angle of the mirror and the position of the light until the colors become clear and bright.
  3. Making the Magic Happen. The rainbow appears because white light is actually made up of all the colors—red, orange, yellow, green, blue, indigo, and violet. When light passes through water and hits the mirror, it bends and separates into these individual colors, just like what happens in the sky after rain. If your rainbow isn't appearing, try adjusting the mirror angle or moving your light source closer or farther away. Sometimes rotating the glass slightly or changing the height of your light makes all the difference. The room doesn't need to be completely dark, but dimming overhead lights will make your rainbow more visible.
  4. Exploring Together. Once you've created your rainbow, invite your child to experiment. What happens if you move the light? Can you make the rainbow bigger or smaller? Try using different light sources—a phone flashlight versus a regular flashlight—and notice how the colors might change. This is a wonderful time to let children's natural curiosity lead. They might want to try catching the rainbow with their hands, or see if they can project it onto different surfaces. Follow their interests and questions rather than turning it into a formal lesson.