Diffraction Gratings


Unfrosted Bulb
1. Either (a) Unfrosted Tubular White Light Bulb (Long Filament Preferred) with Variable Dimmer, or
(b) Temperature Movie

A "live" incandescent filament is best to demonstrate the relationship of temperature and color. The movie option is a quick and clean alternative.

2. Color Analyzers
Color Analyzer
Option A: Make a color analyzer (see diagram at left) for each student with a few spares for good measure. They can be made by drilling four holes (about 3/4" diameter in two pieces of wood or opaque plastic (about 10 x 15 cm). Make a sandwich of those pieces, 2 clear acetate rectangles, 3 color filters (red, green and blue) and a diffraction grating* taped in position so they lined up with  the holes. The whole sandwich is held together with screws or high quality sticky tape. The holes are labelled “A,” “B,” “C,” and “D.” A message (word) written with invisible fluorescent ink or a fluorescent sticker is affixed to it. Making the message with yellow highlighter pen on yellow paper works well. Alternatively, you can leave off the invisible ink message and make a large poster of an invisible message instead.
*For diffraction gratings, we recommend holographic diffraction gratings available from companies such as Frey Scientific, Science First, Rainbow Symphony, and others.
    Option B: Use the movies of filters and spectra provided in the media for this show.

3. Prism, Slide Projector and Slide of a Slit
The bright beam of a slide projector passing through a slit and a stand-mounted prism produces a relatively large spectrum on the dome. This can be used as a supplement or alternative to the activity with the diffraction gratings. The prism itself can be made of glass, plastic or a water-filled glass container. Place the slit slide after Image 12 (Ring Nebula in Full Color). To demonstrate the effect, during the program turn on the projector with the slit slide, and place the prism stand directly in the path of the light beam. The prism should be pre-positioned so that setting it in place produces a good sized beam and a fairly bright rainbow on the dome. 

Presenter's Script:            

Hand out color analyzers.

Using filters is one way of separating light into its individual colors. Another way of separating light into its component colors is using a “diffraction grating.” Window D in your color analyzer is not just a clear window, it is a piece of plastic with thousands of parallel grooves called a diffraction grating. When light goes through the grooves it splits into different parts, so we can see what makes up the original light. Look directly at this light in the center through Window D, holding it very close to your eye.
Gradually turn on the variable white light in the center. Demonstrate how to look through the diffraction grating. Then, check that everyone’s face is directly illuminated by the light from the bulb. Alternatively, use the "spectra" movie. Pause it on the incandescent spectrum to allow time for discussion if needed.
Incandescent Spectrum

What do you see? [A straight rainbow.]
Besides the white light in the center you can see rainbows when you look to the left and right of the bulb. Rotate your color analyzers until the rainbows appear to the sides.
Let’s name all the colors we can see through the diffraction grating starting from the one furthest from the light. [Red, orange, yellow, green, blue and purple or violet; have the whole class say the colors.]
Those are the colors that make up this white light. The diffraction grating allows us to see that white light is a combination of many colors: red, orange, yellow, green, blue and violet. But these are just the colors we can see. There are lots of other colors we cannot see; for instance, there is one before the red, called “infrared,” and there is one after the violet called “ultraviolet.” The areas before the red and after the violet look dark to us even though the invisible colors are there.

Filters are one way to separate light into its individual colors but another way is to use a prism that creates a rainbow. Here I have a source of white light passing through a slit.
Turn on slide projector with slit so a line of white light appears on the dome.
Now I am going to put a prism in the path of that white light.
Move prism to the right place to produce a rainbow on the dome.
Notice that the rainbow doesn’t appear in the same place as the white light. It shifted position because the prism bends the path of the light going through it. But all the colors that are making up the white light are bent different amounts; violet is bent the most, red the least, so all the components of white light are separated. Again we see the rainbow colors: Red, orange, yellow, green, blue and violet. Our eyes are not sensitive to the invisible colors such as infrared and ultraviolet.
Use light pointer to indicate where infrared and ultraviolet would appear if we could see them.