Rainbow Prism with Water
You do not need a prism to split light — a glass of water and a sunny window will paint you a rainbow.
Never look directly at the sun through the glass. Work with the sun at your back and the paper in front of you.
Materials
- A clear, smooth glass
- Water
- White paper or a white wall
- A sunny window
- A small mirror (optional)
Procedure
Set up glass and paper
Fill the glass nearly to the top with water and set it on a table near a sunny window, with white paper on the floor or wall nearby to catch the light.
Catch the sunlight
Move the glass until sunlight passes through the water and hits the paper. Adjust slowly — the rainbow appears at a specific angle.
Read the spectrum
Once you see the color band, note the order of colors from one edge to the other. Which color bends the most?
Try the mirror boost
Place the small mirror tilted inside the glass so it reflects the beam upward onto the paper. Compare this rainbow's brightness with the first.
Compare with a real rainbow
On a sunny day after rain, look for a real rainbow with your back to the sun and compare its color order to your indoor one.
What to observe
A faint rainbow band appears on the paper near the glass.
The colors always run red-orange-yellow-green-blue-violet in the same order.
Moving the glass changes the rainbow's position and brightness.
Why it happens
White light is a mix of wavelengths, and water bends each wavelength by a slightly different amount — blue light refracts more than red because its refractive index is higher. Passing through the curved glass spreads the colors apart (dispersion), painting a spectrum on the paper. A glass prism does the same more dramatically, and raindrops do it in the sky: every rainbow is sunlight dispersed and reflected inside millions of tiny water prisms.
Try variations
Shine a flashlight through the glass in a dark room — is the spectrum as vivid as with sunlight?
Try a cut-glass object or a CD: which splits light into the brightest spectrum?