Gelatin Prism

Concept: Refraction (bending) of light.

Materials Needed:
 

• Knox Gelatin®, 21-grams (3-packets)
• Glycerine, 5-mL
• Razor blade or suitable cutting implement
• Distilled water
• sandwich plastic container or similar
  smooth-bottomed container
• Hot plate
• Light pen or laser pen
• Stirrers (popsicle sticks)
• Beaker, 250-mL
• sandwich-sized zipper-type
  plastic bags
• 100 mL graduated cylinder

Safety Considerations:

There are no safety hazards with materials used in this experiment. The prisms may become moldy after storage for several days or more.  If that occurs, they should be disposed of in the trash.

Disposal:

All materials used in this experiment can be safely disposed of in the trash.

Procedure:

1. Making a gelatin prism:
   1. Measure 100 mL distilled water into a 250 mL beaker.
   2. Add 5 mL glycerin to the distilled water and stir well.
   3. Add 3 packets (about 21 g) of Knox unflavored gelatin to the water mixture and stir well.
   4. Stir slowly so that the gelatin does not get foamy.  Stirring too fast will make the gelatin cloudy.
   5. Warm the mixture on a hot plate while stirring until the gelatin is dissolved, and you have a clear, homogenous solution.
   6. Pour it into a sandwich plastic container so that a prism can be cut later.
   7. Let the gelatin harden for about 30 minutes or more.
   8. Cut a prism in the shape of a right triangle from the solidified gelatin with the razor blade or cutting implement. See diagram below.
   9. Prisms may be stored in a plastic bag for later use.
2. Point the laser or pen light through the base of the triangle.  The light should bend as it passes through the matrix of the gelatin prism.  It may be necessary to darken the room to see the refraction or bending of the light through the prism.

Diagram:

This is the shape that your prism should have.

Explanation:

A beam of light traveling through a liquid, a solid, or through air is invisible unless something is put into the beam's path that can deflect the light.  Refraction is the bending of light rays as they pass from one medium to another.  The speed of light in air and the speed of light in a solid medium (such as the gelatin) is different.  As the light will be going from a less dense material (air) into a more dense one (gelatin), this causes the light to change direction or bend.

Activities:"Doing" science by asking questions.

• What happens when you change the angle of the light beam entering the prism? Can you measure the changes?
  What do the results tell you?
• What would happen if you used a different color of light? Would the light bend more or less?
• What if you used "white" light (try a powerful flash light)? What is white light anyway?
• How does the shape of the prism affect the angle of defraction? What angles and side lengths can you vary to test this idea?
• What would happen if you used colored gelatin? How about yellow, red, and blue?

Author: Wayne Goates: Kansas Polymer Ambassador
           email: [email protected]

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