Index of Refraction and the Disappearing Beaker
This demonstration lets you set up a cloaking shield that makes a glass object disappear.
What You Need
- Pyrex beaker
- other glass objects such as Pyrex stirring rods and magnifying glasses
- transparent container large enough to hold the beaker
- cooking oil
- Place the beaker in the larger container.
- Fill the container with the oil.
- Immerse the beaker in the oil and slowly pour the oil into the beaker.
- Observe what happens when other glass objects are placed in the oil (Figure 88-1).
As the oil level rises above the beaker, the glass can no longer be seen. If any markings are on the side of the beaker, they will still be noticeable, as seen in Figure 88-2.
If you use other liquids, similar results may be obtained, but you may have to do some fine tuning. Other types of glass and other oils (including the "lite" version of cooking oils) may be less perfect, leaving some ghost images that are less noticeable the farther away your "audience" is. A mixture of a heavy and light mineral oil (in a ratio of about 2:1 to start) should match Pyrex and be adjustable for other types of glass. Karo syrup is a close match to Pyrex and can be diluted with water to match other types of glass.
The magnifying glass will not enlarge images when submerged.
The precise index of refraction for these materials may vary slightly with temperature. Also, imperfections in the glass may make it difficult to make the very last trace disappear on some samples.
Why It Works
Objects are visible to the extent that they are able to reflect light. If an object immersed in a liquid has an index of refraction that is different than the object, some of the light is refracted through the object and some is reflected back to the observer. However, if the object has exactly the same index of refraction as the immersed object, the light will neither reflect nor refract at the interface between the object and the liquid it is immersed in. In that case, the object will appear to be invisible.
Lenses, such as magnifying glasses, work by virtue of their index of refraction being different than the index of the medium around it. If the index of refraction surrounding the lens is increased from 1.0, which is the index of refraction of air, to an index very close to glass, the light rays will not be bent through a focal point and magnification will not occur.
Both Wesson cooking oil and Pyrex glass have a nearly identical index of refraction of about n = 1.474, making them particularly well matched for this demonstration.
Other Things to Try
A similar experiment along these lines is first to pour a water-alcohol mixture to a beaker, and then cover it with cooking oil. If a small enough amount of alcohol is in the mixture (less than twice as much oil as water), the oil will float and it will also not mix with the alcohol-water layer. If you add a few drops of food coloring to the water-alcohol mixture, the effect is easier to see. Viewing from the top, the water-alcohol layer is invisible. This is the result of total internal reflection at the interface between the layers.
Transparent objects are visible because of reflection from their surface. Transparent objectswill partially reflect and partially refract light if there is a difference between their index of refraction and that of their surroundings. If the index of the material and their surroundings is the same, the object will appear to be invisible.
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