To use a diffraction grating, hold it right up to your eye, and look through
it at a light source. Five degrees to either side of the source you
will see a spectrum, and fainter, broader spectra farther out (these are
"echoes" of the spectrum). Small bright lights are better sources to look
at, because otherwise the different parts of the object give spectra that
overlap, which confuses or even obliterates the effect; a dimly lit room
or a black backdrop against which to see the spectrum is also useful.
A diffraction grating is made of rather ordinary plastic, with a pattern
of fine grooves printed on it. The plastic will melt if you touch
it to a light bulb (but why would you get that close?). The grooves
will not have as much effect if they get filled with grease and dirt; encourage
students to hold the diffraction grating by its edges. Diffraction gratings
are not very expensive, and are neat, fun, and educational -- so don't
be too protective of them.
To help keep gratings clean and not lost, mount each one on an index card.
First cut a square hole slightly bigger than the viewing part of the grating,
and tape the cardboard frame to the card.
About 6% of the population is color-blind (that's about one student per
class!). Most commonly, color blind people cannot distinguish red from
green. These people will still see a spectrum, but the coloring of it will
seem to be different; they will see the pattern made by different sources.
