Students often benefit from scientific demonstrations because the visual evidence gives them another mode for remembering key concepts. This works especially well for intangible concepts like light and light travel. You can explain to students that light is actually made up of a spectrum of colours and talk about how rainbows form and then cement the information with a demonstration. The simplest light demonstrations involve prisms. Prisms are long, clear, triangular crystals usually made of quartz that split the light spectrum into different colours when used properly.
Tack your white paper or canvas to a wall with drawing pins. Make sure the paper or canvas is flat and smooth so it can capture the rainbow perfectly. You can set it up across the room from a sunny window or use a flashlight if windows are unavailable.
Hold your prism up in front of the paper or canvas, making sure to catch the light from the window. If using a flashlight, hold the prism in your non-dominant hand and the flashlight in your dominant hand. Turn it on and hold the prism in the light beam.
Twist and turn the prism in the light source. Light should fall on the canvas or paper. Turn the prism until a corner of the triangle falls into the light beam. Light should refract through the prism and create a rainbow on your white background.