What is Needed

Four or more Pyrex glass stirring rods, or anything made from Pyrex. Little figurines made by glass blowers generally are made from Pyrex, so they should work well, too.
Two clear plastic or glass containers
One container of Wesson Vegetable Oil
Tap water

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What to Do

Fill one of the jars about half full with tap water. Put two of the glass rods into the water. Look through the jar at the rods. You can see them fairly clearly.

Next, fill the other jar about half full with the Wesson Oil. Now put the Pyrex glass rods into the jar. Look through the jar at the rods. Notice that the rods have disappeared. They are practically invisible in the oil.

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What is Happening?

The activity introduces you to the concept of index of refraction. The index of refraction of a material is a measure of how fast light travels in a material. If light goes from one material to another, and the two materials have different indexes of refraction, the light is bent. When two materials (like glass and air) are in contact and light shines through them, the light bends a little at the surface separating the two materials. The reason that the light bends is because the index of refractions are different for air and glass. When light goes from air into glass, this happens. This is why you can see the glass rods in air.

Now, if the two indexes of refraction are not too much different, the light does not bend too much. When the Pyrex rods were in water, you could see them clearly because the index of refraction of Pyrex and water are slightly different. When the Pyrex rods were put in the Wesson vegetable oil, the story is quite different. The rods are practically invisible in the oil. This is because the Wesson Oil and the Pyrex rods have practically the same index of refraction. The light that is traveling through the jar into your eye is not bending at the surface of the glass. Therefore you cannot see the Pyrex rods.

Purdue University

Department of Physics

Physics on the Road—Hands-On