From MariachiWiki

Introduction

What is the best way to create a rainbow? The demo on the left, created by Tara Falcone, show one way to do it. A beaker filled with water is put on top of an old fashioned overhead projector. Light from the projector diffracts and a nice circular rainbow is projected onto the ceiling. The rainbow is used to show where the spectrum light from plastic scintillators falls in the rainbow.

How to do it

Place a 1000 ml beaker on top of a old fashioned overhead projector. The projector is your source of light. Below the beaker, make a circular colimator. Place a disk at the bottom of the beaker to block upward going light, and put another piece of cardboard with a hole in the midle as shown in the image to the left. Fill with water up to 1000ml. While doing this observe the rainbow slowly being formed as water is being poured. In this setup light will hit the beaker from corners and produce a nice round rainbow in the ceiling or walls. A dark room is best for viewing. This demonstration won't work well in illuminated rooms. The results are very nice. They show how the white light from the projector is actually a combination of many different colors. It is interesting to note that this experiment doesn't work if the beaker is not 3/4 full. Smaller beakers will also work with different degrees of success.