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Physics/Refractive Index

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Question
Sir,

A science demonstration...a clear, transparent glass cup was placed in a clear, transparent glass container..roughly 5-6 times the size of the cup. Cooking oil was added to the brim. The glass cup could not be seen, it just disappeared.

The explanation was that the container was made of Boro-Silicate, which has the same refractive index as the cooking oil, hence the disappearing cup.

Sir, why does same refractive index make the cup "invisible"..the producers of the show did not explain.

Thank You.

Answer
I used to do this in my science show, and I'm actually preparing to do it again soon! The reason that you can see glass, even though light passes through it, is mostly that at the glass-air interfaces the light bends (Snell's law) due to the waves passing into materials with different indexes of refraction. That means that the wave fronts change speed, causing the light wave to change direction.  If you have very clear glass, like a clean window (where the direction change changes back as it goes through), you can't perceive it because there's no image distortion. If you have a curved object, like a glass, you see the distorted path of the light. If you put another glass inside, you can perceive the light angle changes. But if you fill all that air space with oil, the indices of refraction are the same and light doesn't bend at the interfaces. Therefore you can't see the inner glass, only the outer glass where you have an air-glass interface that bends light rays.

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Dr. Stephen O. Nelson

Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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