Spectral Surprise

An activity from Dave Mastie (Ann Arbor, MI)

Type of Lesson: Demonstration/Discussion

Time Needed: 20 minutes

National Standards Addressed

Physical Science, Grades K-4: Objects have many observable properties, including size, weight, shape, color, temperature, and the ability to react with other substances. Those properties can be measured using tools, such as rulers, balances, and thermometers.

Physical Science, Grades K-4: Light travels in a straight line until it strikes an object. Light can be reflected by a mirror, refracted by a lens, or absorbed by the object.

Physical Science, Grades 5-8: Light interacts with matter by transmission (including refraction), absorption, or scattering (including reflection). To see an object, light from the object-emitted by or scattered from it-must be enter the eye.

Physical Science, Grades 5-8: The sun is a major source of energy for changes on the earth’s surface…The sun’s energy arrives as light with a range of wavelengths, consisting of visible light, infrared, and ultraviolet radiation.

Quick Summary of Lesson

This activity builds a model for producing, separating and displaying spectral colors.


overhead projector
holographic diffraction grating
masking tape
4 sheets of paper
box of household paraffin wax (also called gulf wax or canning wax)
aluminum foil
cutting surface
2 rubber bands (1/4 inch thick)


A. Producing the spectrum
1. Lay the 4 sheets of paper on the overhead (flat surface) leaving just 2 inches by 2 inches exposed. This will reduce the amount of light coming from the projector.

2. Tape the pieces of paper in place with masking tape so they won't move.

3. Now tape the holographic diffraction grating on the upper, outward facing lens of the projector (curved lens on arm of projector, but not the lens that points down).

4. Now turn on the projector and you should see a spectrum of colors displayed on the wall or screen or wherever the projector is pointing.

B. Separating/Displaying the Spectrum
(see image that will help in the building of the stack of blocks)
1. Normal paraffin wax should come with 4 blocks in it. Take one of the full blocks of paraffin wax and create an imprint with it on a single sheet of aluminum foil. Use the scissors to cut out this imprinted portion of aluminum foil. Now cut 8 more pieces this size. Place to the side.

2. Now, using the knife cut the 4 wax blocks at the halfway point creating 8 blocks that are approximately 2 1/4 inch x 4 1/2 inch x 1/2 inch thick. You may want to just score the blocks at the halfway point, then just break the blocks using the edge of a desk or table.

3. Fold your aluminum foil pieces in half (dull side in). They should now be the same size of the wax blocks you have cut in half.

4. Place wax blocks and aluminum foil in a stack. Start with an aluminum piece on bottom, then a block, then aluminum foil, then a block, then aluminum foil...until the last wax block is added to the top of stack, topping with an aluminum foil piece. The wax blocks should be stacked so that the "front" of the stack has all smooth faces. It doesn't matter if the back of the stack has some blocks with rough surfaces (from the cutting or breaking of the blocks).

5. Now use the masking tape to secure the block stack together. For extra security, place rubber bands on top of tape. (You want to leave the smooth side facing front without tape or rubber bands on it).

6. Now hold the block stack in front of your projector. You'll want to hold the block stack about 2 feet from the projector. The spectrum of colors should "get caught" in the block stack with each block lighting up a different color! This sure will be a surprise for students and yourself!

Notes to the Teacher

An inexpensive holographic diffraction grating is available through Arbor Scientific (pictured above). Here is the information they have on their web site:
"The brightest holographic diffraction grating ever made! Our replica diffraction grating is the most efficient grating ever offered. A special process was used to achieve transmission capabilities exceeding 99.9%! Use these gratings to demonstrate emission spectra from spectrum tubes. Previously dim lines are now visible! Produce multiple beams from a single laser for great optics demonstrations or experiments. Bright rainbows can be formed to fuel a classroom discussion of diffraction. These holographically produced replica gratings have 750 lines per millimeter and are enclosed in durable 35mm glass slide mount." $3 at Arbor Scientific

Paraffin wax can be found at craft or kitchen stores. The sales people may not know it is called paraffin wax, but they are sure to recognize gulf wax or canning wax (it's all the same stuff!).

The holographic grating is a good alternative to prisms which can be finicky in experiments and demonstrations.

Use this demonstration with younger students having them draw and label the block stack with the colors in it. Use this demonstration to start a discussion with older students with regards to the spectrum, wavelengths, diffraction and how the colors are being trapped in each wax block by the reflecting aluminum foil.

This is a good lifelong teaching tool - once you build the wax block stack, it can be used for many years to come!

Need More Information? Try Using Windows to the Universe

Please use these links for further ideas or more information:
Looking at the World in a Different Light all about light and the electromagnetic spectrum
The Spectrum

Last modified March 15, 2002 by the Windows Team

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