Re: Why glass disappears when submerged in baby oil?

First, ask yourself this question: if glass is transparent, then how can you see it?

The answer is that glass reflects a little of the light that strikes it, and, more significantly, it refracts, or bends, light. When we look at a piece of glass, most of the light we see comes from objects somewhere not-quite-directly behind it, but slightly to one side. Because that doesn't look exactly like the objects which are directly behind it, we notice the difference and perceive the glass.

The reason light bends as it moves through glass is because one particular property of the glass -- its index of refraction -- differs from that of the air. The index of refraction can be described in several ways; it measures the speed with which light moves through a medium, and it also indicates the amount by which light rays will be bent if they enter it at an angle. Ordinary air has an index of refraction of about 1.0, while glass has an index of refraction which ranges between about 1.4 and 1.5, depending on the composition of the glass. Because these two materials have different indices of refraction, light bends as it moves from air to glass and back to air again. We easily notice the effect of the bent light, and say to ourselves, "Hey, there's a piece of glass there."

Now, baby oil (and other liquid oils) has an index of refraction which is about 1.45 -- in the middle of the range of typical glasses. Because the index of refraction of oil is so close to that of glass, when light passes from oil to glass or glass to oil, it hardly bends at all; instead, it passes straight through. We have to look very closely to see any wierd effects that give away the presence of the glass. Putting one material inside a liquid of similar index of refraction is called "matching indices", and is a common trick in several branches of optics.

You can read a longer description of this topic at this web site run by the Exploratorium: http://www.exploratorium.edu/ snacks/disappearing_glass_rods.html

Can you find any other liquids which have an index of refraction similar to glass?