Distribute materials to make rainbows, either with one or several methods. These methods separate light into its colours.
White light is a mixture of many colours. Other coloured lights are made up of a mixture of a subset of these colours. See the first photo to see how light colours mix to make new light colours.
CDs/DVDs:
Tip the disc towards the room lights, the sun, or shine a flashlight on them. When light hits the groves in the CD, they spread the light out into its component colours, which line up into a rainbow. See the second photo.
Cut glass or prisms:
A piece of cut glass or a prism in a box can be arranged so that a flashlight shining onto a cut edge of the glass separates the colours of the white light into the colours of the rainbow. See the third photo.
A hanging ornament (sometimes called a "rainbow catcher") of cut glass separates sunlight into its rainbow of colours.
Scratched plastic/diffraction gratings/rainbow glasses:
The scratched lines in the plastic of these materials separate light into its component colours.
White light separates into the spectrum of the rainbow. With objects or light sources that only emit some colours e.g. holiday lights or various coloured objects, they only emit a portion of the whole colour spectrum, so when their light is separated, a reduced range of colours are seen.
(Astronomy connection lesson plan on star spectra.)
Using marker pens, the scratched plastic can be used to find out the primary colours of light (cyan (a light blue), magenta (pinky red) and yellow:
Give students purple or dark blue, green and red markers. Ask them to make a blob of colour on white paper, then look at the blob to see what colours "bleed" out of the sides. They should find that cyan, magenta and yellow appear around many colours. This is because the scratched plastic is splitting up the light coming from the marker pen and splitting it into its component colours - these are the colours that make up light (cyan, magenta and yellow). Just like paint there are also secondary colours, which might be seen where primary colours overlap.
Oil:
A thin layer of oil (e.g. on the road from a car) can also make a rainbow of colours when light from the sun hits it.
With older students, discussion can include how the colours are separated:
The edge of a glass prism, or cut glass, bends each of the wavelengths (colours) of light slightly differently, so that they are separated out. The bending of light is called refraction.
The colours in a CD or scratched plastic, in bubbles or an oil slick are formed by interference. When light is reflected from the grooves of the CD or the top and bottom surfaces of a bubble or oil, the light waves interact with each other. Interference of the waves enhances some wavelengths and cancels others in different places, resulting in a rainbow.
