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Can you explain why a penny under a glass beaker of water disappears? If you add some water to top of the penny and return it to under the beaker it is visible.

Please see link for complete demonstration and question.
http://www.youtube.com/watch?v=uz99SnUl2lo

beaker

Picture courtesy of Atoma


beaker

Figure courtesy of RJ Hall

To help this physics teacher "get it," a physics phenomenon known as the index of refraction must be explained. You may have heard the word refraction before. It sounds a lot like reflection and has to do with light; however, it is very different. Refraction is the bending of light. Have you ever tried to grab something at the bottom of a fish tank only to look at your arm from the side of the tank and have it scare you because it looks like it's broken?

If you've never tried that, you can experiment in a much less scary way. Take a straw and put it in a clear glass. Look at the straw from the side of the glass. You should see the straw appear to be in two separate parts, breaking where the water level starts. If it doesn't appear to be broken it should at least appear to be completely bent. When you pull the straw out, you can see that the straw didn't bend. This is an optical illusion created by refraction, or the bending of light.

When light travels from one density to another it bends. In the case of the straw in the water, light went from the lesser dense air to the more dense water. When light moves between these two densities it bends, which make objects, appear to bend when the object is passing between both areas of density. How much the light bends is known as the index of refraction.

Physicists have tested the index of refraction for many materials. You can see a list at http://physics.info/refraction/. If you examine this list you will find that the index of refraction of air is about 1.00 and the index of refraction of water at room temperature is 1.33, giving you a difference of .33. This means that light bends enough to make the penny appear to be invisible at certain angles.

Now, don't forget about the Pyrex beaker that the person in the video uses to hold the water. The index of refraction for Pyrex is 1.47, which is only a .14 difference from water, which is enough to make the penny appear distorted if it's under the beaker, whether or not the beaker is filled with water or not. However, it's not enough to bend the light around the penny. So, when the penny is wet in the beaker, the index of refraction goes from the air, through the beaker, through the water, through the beaker and back into water to the penny, which should bring the index of refraction almost back to its original angle (Figure 1).

The same thing happens with a dry penny under an empty beaker. The light travels through the air, through the glass, back through the air, through the base of the beaker, and then through the air before it hits the penny; therefore, making it possible for you to see the penny, by correcting any refraction that was caused by the glass (not pictured). If you change the angle from which you view the penny it changes the angle of refraction as well.

When the light travels from the air, through the beaker, into the water, back through the base of the beaker, and through the air to the penny (Figure 2), the index of refraction is too great for the viewer to be able to see the penny.

However, if the penny under the beaker is wet, the viewer will be able to once again see the penny. The light travels through the air, through the beaker, through the water, back through the beaker and through water on top of the penny to the penny (Figure 3), where the water on top of the penny minimizes the change in angle of the light, making it possible for the viewer to see the penny.

The following diagrams show the difference.

Penny in the beaker

Figure 1: Penny in the beaker (can see the penny)

Wet penny under the beaker

Figure 3: Wet penny under the beaker (can see the penny)

Dry penny under the beaker

Figure 2: Dry penny under the beaker (can't see the penny)

 

More information:

http://en.wikipedia.org/wiki/Refractive_index
http://dev.physicslab.org/Document.aspx?doctype=2&filename=GeometricOptics_IndexRefractionGlass.xml
http://mintaka.sdsu.edu/GF/explain/optics/refr.html

Submitted by: Christina Wood, Lincolnshire IL