Andy Dyson, of Keri Keri School, asks :-

How does the physics demonstration work whereby a test tube can be made invisible when immersed in a liquid?

John Campbell, a physicist at the University of Canterbury, responded.

We have a similar demonstration at Canterbury. As four glass tubes are lowered into the liquid two become invisible. The trick is to have the liquid's refractive index match that of the glass exactly.

We see surfaces because some light is reflected at the surface. e.g. for window glass in air about 4% of incident light is reflected straight back in the visible region. We can calculate this from the theory of how electromagnet waves reflect off of a dielectric. This theory shows that no light is reflected if the speed of light in the liquid matches exactly that of the speed of light in the dielectric.

We use crown and flint glass tubes and index match to one. The best fluid is peanut oil. However, to make a tube truly invisible we must control some other properties.

In dielectrics blue light travels slightly slower than red light so the index matching can only be done at one frequency. Usually sodium light (e.g. a sodium vapour yellow street lamp which is quite cheap) is used.

The speed of light in liquids is fixed for the liquid but fine tuning can be done by altering the temperature of the liquid. Then glass can become truly invisible.

Forensic scientists match glass fragments from hit and run accidents to identify the batch of glass the car headlight was made from. The small sliver is immersed in the appropriate oil on a microscope stage and the temperature of the oil adjusted until the sliver vanishes.

There is another way to make something invisible, but it is not really reversible. During the war Niels Bohr in Denmark had auctioned his gold Nobel Medal for Finnish Relief. But he held for safekeeping those of two refugees, Max von Laue and James Franck. When the invading forces overran Denmark the medals were dissolved in aqua regia, a mixture of acids that dissolves gold, and stored safely in open view in a bottle on the chemistry laboratory shelf. After the war the Nobel Institute recovered the gold and made each a new medal.

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