- To show that the boundary between materials which have the same Refractive Index is not detectable by the eye.
Immerse the ordinary glass rod (sodium glass) in the bottle, and note that it is easily seen through the side walls of the bottle.
Now immerse the Pyrex rod in the oil. This glass rod is not visible in the oil, it "disappears."
This is because the oil and the Pyrex have identical indices of refraction.