Disappearing Glass Lesson Plan
Materials


Bottle of corn oil
 Two old Pyrex test tubes with the labels worn off of them two beakers
 Tongs or test tube holder
Procedure
1.
Take one of the beakers and fill it with corn oil and take the other beaker and fill it with water.
2.
Place the first test tube in the water filled beaker, and show them that they can still clearly see the object.
3.
Place the other Pyrex test tube in the corn oil filled beaker, and the test tube will
“magically” disappear.
4.
Take the test tube back out to show your students that it was just a trick.
5.
Explain your sweet magic skills.
Explanation
Corn oil and Pyrex have nearly the same index of refraction. We’ve all seen someone walk into a glass wall or door because the glass was so clean the person couldn’t tell it was there. The reason we can see glass beakers and test tubes is because they are curved and they bend the light that passes through them at different angles. If you look through the test tube everything on the other side will look really stretched out. The glass stirring rod and the corn oil have a different index of refraction, so the light gets bent going through the corn oil and then gets bent different going through the stirring rod. Since the corn oil and the Pyrex have the same index of refraction, the light only gets bent once since the light goes through the Pyrex at the same angle, we can’t see it.
Reinforcement Activities
Have students discover how the lens in their eye or a camera works. The ray optic demonstration set can be used to show refraction and reflection of light very clearly. It comes with plastic slides to show how the human eye works, explain what near sighted and far sighted is how glasses correct these conditions and well as show how a pinhole camera works.
Have students extend their knowledge of lenses using an optical bench. Students can find focal points of lenses, project images of candles or light bulbs and reinforce drawing ray diagrams by showing real life examples.
References http://www.exploratorium.edu/snacks/disappearing_glass_rods/index.html