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

keywords: disappear invisible glass water transparent magic trick

(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

Material IOR Comments
Silicone Oil 1.520 Seems easier to buy pyrex rods on ebay than to get this although it sounds safe, nontoxic (I think?) and I assume you can mix it with wesson.
Standard Glass 1.512 Used in drinking glasses, windows. Most common glass. aka soda-lime. ior is much too high to use vegetable oils
Vegetable Oil 1.48 to 1.47 I only like Wesson because it has less color
Pyrex Glass 1.47 to 1.474 aka borosilicate. Ideal for making disappear.
Mineral Oil 1.461 to 1.462 aka baby oil
Quartz Glass 1.4585 Uncommon. Used in cases where you need UV transparency or very high temperatures.
Water 1.33
more IOR values here and here.

A reader from Canada claims Becel oil works well also: Another reader suggests this source for cheap pyrex glass rods (although ebay has them also).

Another reader writes:
In our lab we occasionaly use N-Methyl-Pyrrolidone (IUPAC: N-Methyl-2-pyrrolidone , wikipedia link , short:NMP). It matches the IOR of pyrex glass almost perfectly without mixing with anything else. Problem: it's kinda toxic! If anyone has it in their lab though (and knows how to treat it safely) then you easily get that cool disappearing glass rod effect! I realize that not many people may have access to this chemical but It might be useful information for someone teaching students in a university lab.

FINALLY - IF YOU LIKE THIS PAGE - please send me an email. I wouldn't put this up if I didn't think I'd get any emails. I like to hear about successes and failures and would be happy to add more information to this table (such as a list of other easily found liquids that work well and perhaps details about plastic rods?) Also correct my spelling errors etc. When I start getting too many emails - I will remove this part of the web page. Also if you have a website and you link to this page that would also be nice as it will raise my page in the page orders of various search engines. I don't advertise or make any money - wouldn't you rather find my page in your search than some pyrex supplier?