# HOWTO make a penny disappear by changing the speed of light

A physics teacher created this video showing how to make a penny "disappear" by placing a Pyrex beaker over it and filling it up with water, asking why and how this illusion worked. On IO9, Esther Inglis-Arkell explains how the effect is achieved -- it's all down to the strange motion of light in water.

So when the penny is at the bottom of the beaker, there's only one sharp turn that the light from the penny has to make, from the pyrex dish to the air. The water-to-pyrex transition is comparatively mild, with little bending. The penny is distorted, but it's visible. When the wet penny is beneath the dish, but under another layer of water, the light also only has one sharp turn — back into the air at the end of its journey. Before that it only travels through water and pyrex, which have similar indices of refraction, and so it isn't bent much.

When the penny 'disappears,' though, it is taking two sharp turns, the massive turns between the pyrex and the air both at the bottom of the beaker and at the side. And, because of the way light bends, both turns are in the same direction, away from the eye of the viewer. Imagine the beaker full of water like an immense piece of rectangular carpeting on the concrete floor, and the light like a person on roller skates. (No, seriously, this will help.) The viewer is on the right side of this carpet. In order for them to see the penny, just under the bottom of the carpet, the light has to get from the penny to them.

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1. PhosPhorious says:

Fucking pennies!  How do they work?

1. digi_owl says:

well there is a bunch of elderly guys in suits sitting in a room…

2. I would have said this is about reflection.

3. wrwetzel says:

It is the same effect that makes some of the surface of a swimming pool look like a mirror when viewed from under water. If the water is completely still you can see up and out through a disk directly overhead. The rest of the surface looks like a mirror, reflecting the image of the walls of the pool.

Bill

4. Gemma says:

Cory, in your first sentence, change “wet” to “dry”. He shows the illusion fails if the penny is wetted.

5. Ehlyah says:

And this person is qualified to teach physics? This is basic stuff.

1. travtastic says:

Golly, aren’t you special!

1. Ehlyah says:

Yeah, so special I took physics through high school. Wow.

1. travtastic says:

Cool!

2. i was actually impressed that he was asking the question DESPITE being a physics teacher. i think a culture of asking questions when you don’t understand is better than one where everyone is afraid of looking dumb, and so no one asks questions, and so everyone stays ignorant.

he can now explain all this to his students. he is trying to improve rather than just teaching them what he knows. geez.

1. Ehlyah says:

It is indeed great that he dared to ask. I am glad he did.
That doesn’t take away that it’s disappointing he didn’t already know.

6. C.J. Hayes says:

I would’ve used a different analogy.  “Imagine the penny is there, but you can’t see it! Light is involved!”

7. A physics TEACHER not understanding this is like an English teacher being bamboozled by The Cat in The Hat.

1. why dont YOU interperet the true meaning of the cat in the hat?

1. I’m not qualified as an English teacher. Obviously.

1. alright, you have a point there.

8. The Ouroborus says:

This guy sounds incredibly bored.

9. Brian Koss says:

If he is a science teacher, he should have an explanation for this phenomena.  It’s simple refraction, people.  Nothing of value here, move along.

10. ryuchi says:

Well, it’s like putting the pennie on the other side of a mirror–you can’t see it. Then u put it infront of the mirror–u see it. Then you put water–you RUIN the condition to produce a mirror-effect: two types of dencities–Air & Glass/water/

Glass and water are similar (in fact glass is liquid that “spills” slow in time) so when u make the pennie wet and stick it to the “mirror surface” you provide the same densitiy from both sides of the mirror (water) which ruins the mirror effect (needs two far different densities: air & water/glass) blabla

my two pennies anyways

1. flosofl says:

(in fact glass is liquid that “spills” slow in time)

I wish people would stop spreading this. Glass is NOT a liquid. It is an amorphous solid.

Glass is not a high-viscosity liquid at room temperature: it is an amorphous solid, although it does have some chemical properties normally associated with liquids. Panes of stained glass windows often have thicker glass at the bottom than at the top, and this has been cited as an example of the slow flow of glass over centuries. However, this unevenness is due to the window manufacturing processes used in earlier eras, which produced glass panes that were unevenly thick at the time of their installation. Normally the thick end of glass would be installed at the bottom of the frame, but it is also common to find old windows where the thicker end has been installed to the sides or the top

It’s almost as bad as the “We only use 10% of our brain” canard.