# 6A40.30b-Disappearing Beaker

Description: This demonstration shows that an object with the same index of refraction as a liquid will look transparent when immersed in it.

Index of refraction is measure of the bending of a ray of light when passing from one medium into another, simply how fast light passes through an object. If the index of refraction is different, the light bends, and if the index of refraction is the same then the light goes right through in a straight line making the pyrex on the inside of the oil look invisible, but normal glass will be visible because the light refracts in a non linear path.
Wesson brand vegetable oil index of refraction=1.474
Pyrex glass index of refraction= 1.474
Normal glass index of refraction= 1.512
Water index of refraction=1.33

Equipment:

• Lots of Wesson vegetable oil
• A large Pyrex beaker with as few markings as possible
• A small Pyrex beaker with as few markings as possible
• Paper towels
• Tongs
• Two extra Pyrex beakers filled with water (Optional)

Setup Procedure:

1. Spread a few paper towels down where this demo will be used.
2. Fill the large beaker up with Wesson vegetable oil, taking into account that the small beaker will also be going in later (so don’t fill it up all the way).
3. When the professor is finished, pour the vegetable oil back into the main Wesson container.
4. You will need to clean the Pyrex beakers out with soap and water.

Demonstration Procedure:

1. Show students the large Pyrex beaker. Explain that it has Wesson vegetable oil in it.
2. Show them the small Pyrex beaker. Fill it up with the Wesson oil.
3. Lower the small beaker into the large beaker, and note that it “disappears” using the tongs.
4. This is because the index of refraction of pyrex and wesson vegetable oil is about the same.
5. Since the light does not bend, it’s as if there is nothing there, which makes it seem like there is only oil.
6. A video camera can be used for lage classes. Point the camera at the side, since from the top the lid of the small beaker can be seen easier.
7. The extra beakers can be used as a comparison to this demo, by showing that with different indexes of refraction, the beakers can be seen.

28-M2