Invisible Glass

Chemistry, Physics:	Properties of Matter, Refraction
Grade Range:	Elementary School, Middle School, High School
Format:	Hands-on

This demonstration is great for stumping students young and old, and is a good way to introduce refraction. You may want to keep cleaning supplies on hand in case anything is spilled.

Materials

Method A

• Small Test Tubes (Culture Tubes)
• Baby Oil
• Two 600mL Beakers
• Water

Method B

• Two Glass Jars with Lids
• Clear Glass Marbles, Varying Sizes
• Baby Oil or Corn Syrup
• Water
• Paper and Pencil

Safety Precautions

Please read the General Safety section of the Demonstration Safety page before performing this demonstration.

Demonstration

Method A

1. Set out the two glass beakers. Fill the first beaker with 400mL of water, and the second with 400mL of Baby Oil. Fill 1-2 test tubes with water and set them in the first beaker. Fill 1-2 test tubes with baby oil and set them in the second beaker.
2. As students come by the table, have them note the difference between the two beakers. Ask them why you can easily see the test tube(s) in the first, but not in the second. After they think of ideas, explain the demonstration briefly.

Method B

Note: The jars for this method are already prepared, each with 220 marbles in them (200 small, 20 medium). Skip to step two if you are using the prepared jars.

1. Count out the marbles and put the same number of each size in each jar. Pour water into the first jar, being sure to cover the marbles fully, and put a lid on it. Pour baby oil or corn syrup into the second jar, being sure to cover the marbles fully, and put a lid on it. Be sure to write down the number of marbles you put in each jar!
2. Set out both jars, and put out the paper and pencil between the jars. Draw two columns on the paper and label the columns "Water Jar" and "Baby Oil/Corn Syrup Jar".
3. As students come up, have them try to guess the number of marbles in each jar. They will find it much harder to guess the number for the baby oil/corn syrup jar than for the water jar. After students write down their guesses, briefly explain refraction, and let them know that both jars have the same number of marbles!

Why This Works

Material	Index of Refraction
Air	1.00
Water	1.33
Clear Plastic	1.46
Glass	1.47
Corn Syrup	1.48
Plexiglass	1.50

Why are some materials clear? The configuration of the atoms and their charges within a material determine how it interacts with electromagnetic radiation, such as light. When a light wave strikes a transparent material, such as glass, the charges on the atoms start to oscillate. The oscillating charges mimic the pattern of the light wave, and so they transfer the light through the material. The charges are not be able to oscillate as quickly as the incoming light wave, however, which causes the light to slow down as it passes through the material! Although light usually travels in a straight line, it undergoes refraction when it passes through transparent materials. Refraction is when a light wave will take the shortest path through a material, and so it bends, or changes direction, when it enters the material, and bends back to the original direction when it exits the material. How much light bends when it passes through a material depends on the properties of the material, and is called the index of refraction for that material.

In the table to the right you can see a list of indexes of refraction. Glass has an index of refraction of 1.47, which means that light will travel about 32% slower through glass. Water has an index of refraction of only 1.33, so light travels only 25% slower through water. This is why the glass test tubes or marbles are easy to spot in the water; The light travels faster through the water than the glass, allowing your eyes can see the difference between the materials! The index of refraction for corn syrup, however, is 1.48, which is only off from the glass by 1%. This is why the glass test tubes or marbles are much harder to spot in the corn syrup; The index of refraction for either the corn syrup or the glass is nearly the same, so your eyes have trouble telling them apart!

Additional Information

• This demonstration pairs well with the Mirror Mirage and the Polarizers and Corn Syrup.