They know what a prism does - tell them that we're going to give them a tool that does the same sort of thing, breaks up the light into components. Distribute spectroscopes and work with pronunciation of the word spectroscopes. Have them look first at the plain light. Make sure everyone can see the rainbow, is looking through the correct end, etc. Try taking tube off and using just the grating if they have trouble.
Light pollution is a problem - used screens to block adjacent tubes.
Experiment, part 2 Tell them to pick a gas tube and look at it. They don't - distribute data sheets and explain what to fill in where. They had a hard time with this. Show how to rotate grating if they have problems finding rainbow. Point out that undiffracted light has color.