How to make glass disappear in a liquid - index of refraction

(Click on pictures for larger versions - then try clicking on the larger version for even larger)

Wesson Oil Baby Oil Mixture

I wanted to make glass disappear in a liquid as a demonstration to a fifth grade class. I found 2 great ways to do it.

The first step is to get a glass rod made from pyrex. This was the hardest part. I got one from the high school chemistry department. Using hollow glass objects like test tubes and pipets and eye-droppers isn't as impressive. The double wall nature of these objects makes it more than twice as hard to make the object disappear. Plus having a glass rod results in a simple test to see if your liquid's index of refraction is too high or too low (explained below in step 4). Pyrex has a low IOR compared to other glasses. Fortunately it is a very common glass. Most glass blown decorations are also pyrex so you could try one of those if you have one.

Step 2 is to pick two liquids that can be mixed together (not oil and water!). One with an index of refraction higher than glass and one lower. I was very successful with: Wesson Oil and Baby Oil but I was also able to use Karo Corn Syrup and water. Baby Oil's IOR is slightly lower than glass. Wesson Oil is slightly higher. Karo Corn Syrup is slightly higher and water is much much lower. Of all the liquids - Wesson Oil was the closest. The oils were much easier to work with. You only have to mix for 10-30 seconds before testing. Karo syrup requires 1 to 3 minutes each time you add a drop of water. Also it gets bubbles in it easily and they take 30-60 minutes to reach the surface and pop. Also I imagine the Syrup will get thicker if you don't keep it covered. However it worked quite well despite all this.

Step 3 is to find a suitable transparent container and put a horizontal line on it or behind it for testing purposes. I was lucky to have some small beakers which already had red horizontal lines on them.

Step 4 - Testing! Put your liquid in your container, insert glass rod. Hold rod vertically and tilt it to the left or the right. If the horizontal line tilts in the same direction, then the index of refraction of the liquid is too low. Water is a good example of this. Here is a picture with water - note that the red line is angled roughly twice that of the rod but in the same direction. If the line tilts in the opposite direction your IOR is too high. As you get close, the tilt is very subtle and you have to look at the edges, tilt the rod more, or hold the rod farther from the horizontal line (towards the front of the container).

Above picture shows water. Notice that as the upper red horizontal line passes behind the glass rod, the red line tilts dramatically in a kind of S shape.

Step 5 - Mixing - obviously steps 4 and 5 are repeated alot to get it perfect. My final mixtures were about 4 parts wesson to one part baby oil and about 19 parts syrup to 1 part water. If you add too much water (easy to do) and your container is getting too full, put this mixture aside and use it instead of water next time. The mixing goes much faster because the difference in IOR between the two liquids isn't as dramatic.

TEMPERATURE! - I've read that temperature will affect IOR so make sure you do your testing with the liquids at room temperature or at the temperature which they will be used. This goes for your glass also.

Above picture shows a mixture of Karo Corn Syrup and Water

Above picture shows wesson oil only. If you don't want to mix or test - use Wesson only - it's pretty close. Note that as the red line passes behind the glass rod, the red line tilts slightly counter clockwise even though the rod was tilted clockwise. This indicates the index of refraction is higher than the glass rod. If you can't see the effect easily in this picture, click on the picture for a larger view. The effect is only obvious at the edges.

In order of IOR