Hand out prisms, CDs, other cut glass objects. Ask students to tip the objects towards one or more light source(s), including the sun, and make a spectrum - the colours in the light spread into a rainbow, or some colours within it.
Hand out scratched plastic to see spectra emanating from objects or lights.
With objects or light sources that only emit some colours e.g. holiday lights or various coloured objects, a fraction of the colour spectrum is seen.
Discuss where the colours come from (with youngest grades and older grades if necessary: while repeating the experience with the whole class looking at each light source in turn):
White light is a mixture of many colours. When white light hits the groves in the CD, the edges of cut glass or the surfaces of bubbles or oil, the light is separated into its component colours, which line up into a rainbow.
Coloured lights, or coloured objects are only emitting 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.)
With older students, discussion can include the method of making the colours:
The edge of a glass prism, or the cut glass of a gem or drinking 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.