Demonstrating refractive index
[I]f the refractive index of two materials is the same, as is the case for sunflower oil and Pyrex, then light doesn’t bend at all, and you end up with the nice effect demonstrated [in the video]
swans on tea
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It is about matching refractive index and eliminating interface impedence discontinuities. They are separable. Antireflection-coated lenses illustrate the latter. Phase contrast microscopes exploit both to visualize structure in transparent colorless samples without doping the sample.
An interesting experiment would place the “invisible” test tube between crossed linear polarizers. Not stir or stir the matching medium. All the fun is in the footnotes – and the spectral interrogation. Ultrasonic pinging still sees the test tube.