Materials
- Bottle of corn oil
- Two old Pyrex test tubes with the labels worn off of them
- Pyrex beaker
- Glass stirring rod that is NOT made out of Pyrex
- Two thick mailing envelopes
- Something to break glass with like a 500 g mass or rubber mallet
- Tongs or test tube holder
Procedure
There are several variations of this “magic” trick depending on what concept the students are or have learned:
- Before the students enter the classroom take one of Pyrex test tubes and fill it with corn oil and place in a beaker, also filled with corn oil and the test tube will “magically” disappear
- When the students enter the room you can tell them that scientist have discovered this magic liquid that will reverse entropy, so instead of increasing the disorder of thing, it increases the order
- In front of the students take the other Pyrex test tube, put it in the mailing envelope, then put that envelope in the other envelope. (You should wear safety goggles and make sure you are not near the students when you do this next part) Smash the Pyrex test tube
- Take the Pyrex shards and place it in the container of corn oil that already has the whole test tube in it
- Say your favorite incantation while stirring the test tube with the glass stirring rod. (This is actually to help you locate the test tube and make it easier to pull it out.)
- Then pull the whole test tube out of the beaker it will look like the container magically put itself back together
- Of course don’t let the students actually believe that you can do magic, you’ll want to explain or have them help you explain what really happened
Another Variation
- Fill the test tube with corn oil and have a beaker of corn oil ready to dip the test tube in
- Tell the students that you have an acid that is so strong it will dissolve glass on contact
- Put the test tube in the test tube holders and place the test tube in the corn oil, the part in the corn oil will appear to disappear
- Your more observant students will notice that the beaker holding the corn oil is glass as well and will ask why the beaker doesn’t dissolve, now is a good time to admit you were fooling with them
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.
