How to Create a Rainbow with a Glass of Water

Learn how to make a beautiful rainbow at home using just a glass of water and sunlight in this simple science experiment.

  1. What You'll Need. Gather these simple materials: a clear drinking glass, water, a sunny window or bright flashlight, a white piece of paper or white wall, and optionally a small mirror. That's it! You probably have everything you need already at home.
  2. Setting Up Your Rainbow Maker. Fill your glass about three-quarters full with water. Find a sunny spot near a window where bright sunlight is streaming in, or use a strong flashlight in a darkened room. Place your white paper on a table or tape it to a wall where you can easily see it. Position yourself so you can hold the glass between the light source and the white surface.
  3. Creating Your Rainbow. Hold the glass of water between the sunlight and your white paper. Slowly move the glass around until you see colors appear on the paper. Tilt the glass at different angles and try moving it closer to or farther from the paper. The rainbow might appear as a small arc or scattered colors. If you're using a mirror, place it halfway in the water at an angle and position it so sunlight hits the mirror and reflects onto your white surface.
  4. What's Happening. When light passes through water, it bends and separates into all the colors that were hidden inside. White light is actually made up of red, orange, yellow, green, blue, indigo, and violet all mixed together. The water acts like a prism, splitting the light so you can see each color separately. This is exactly how rainbows form in the sky when sunlight shines through water droplets in the air.
  5. Troubleshooting Your Rainbow. If you don't see colors right away, try these adjustments: make sure you have bright, direct light hitting the glass, move the glass slowly in small circles, change the angle of the glass by tilting it different ways, move closer to or farther from the white surface, or try a different glass if yours has thick or textured sides. Sometimes it takes a few minutes of experimenting to find the perfect position.