Disappearing Glass Rods

Pour some vegetable oil into the beaker.

Immerse a glass object in the oil. Notice that the object becomes more difficult to see. Only a ghostly image of the object remains. (Note: If you do this as a demonstration, keep your audience at a distance to make it harder for them to see the ghost object.)

Experiment with a variety of glass objects, such as clear marbles, lenses, and odd glassware. Some will disappear in the oil more completely than others.

You can make an eyedropper vanish before your eyes by immersing it and then sucking oil up into the dropper.

If you have a magnifying glass, immerse it in the oil. Notice that it does not magnify images when it is submerged.

You see a glass object because it both reflects and refracts light. When light traveling through air encounters a glass surface at an angle, some of the light reflects. The rest of the light keeps going, but it bends or refracts as it moves from the air to the glass.

When light passes from air into glass, it slows down. It’s this change in speed that causes the light to reflect and refract as it moves from one clear material (air) to another (glass). Every material has an index of refraction that is linked to the speed of light in the material. The higher a material’s index of refraction, the slower light travels in that material.

The smaller the difference in speed between two clear materials, the less reflection will occur at the boundary and the less refraction will occur for the transmitted light. If a transparent object is surrounded by another material that has the same index of refraction, then the speed of light will not change as it enters the object. No reflection and no refraction will take place, and the object will be invisible.

Wesson vegetable oil has nearly the same index of refraction (n) as Pyrex glass (n = 1.474). Different types of glass have different indices of refraction. In Wesson oil, Pyrex disappears, but other types of glass, such as crown glass or flint glass, remain visible. Fortunately for us, a great deal of laboratory glassware and home kitchen glassware is made from Pyrex glass.

For most Pyrex glass, the index-matching with Wesson oil is not perfect. That’s because Pyrex glass has internal strains that make its index of refraction vary at different places in the object. Even if you can match the index of refraction for one part of a Pyrex stirring rod, for example, the match will not be perfect for other parts of the rod. That’s why a ghostly image of the rod remains even with the best index matching.

The index of refraction of the oil (and of the glass, too) is a function of temperature. This demonstration will work better on some days than others.

Index of refraction is sometimes called optical density, but optical density is not the same as mass density. Two materials can have different mass densities even when they have the same index of refraction. Although Pyrex glass and Wesson oil have similar indices of refraction, Pyrex sinks in Wesson oil because it has a higher mass density than the oil. Wesson oil has a higher index of refraction than water (n = 1.33), but a lower mass density, so it floats on water. The index of refraction depends not only on density but also on the chemical composition of a material.

You can make Pyrex glass disappear by immersing it in glycerin or mineral oil. However, mineral oil comes in different weights, and each variety has a different index of refraction. To match the index of refraction of Pyrex glass, you’ll need a mixture of mineral oils of different weights. To create the proper mixture, place a Pyrex glass object into a large glass beaker and pour in enough heavy mineral oil to submerge it partially. Slowly add light mineral oil and stir. Watch the glass object as you pour. Most Pyrex glass will disappear when the mixture is two parts heavy mineral oil to one part light mineral oil. Notice the swirling refraction patterns as you mix the oils.

Karo syrup has an index of refraction close to that of glass. Karo can be diluted with water to match some types of glass. Other light-colored corn syrups may also work—you may want to experiment and find out.