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.

Make a penny disappear with water


  1. Read about “Total Internal Reflection”:

    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.


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

    1. 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. 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.

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

  4. 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.

  5. 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. (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.

      From List of common misconceptions

      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.

      EDIT: added wiki link and quote

  6. Cory — well played, good sir, with your sneaky link to SR Delany’s wonderful autobiographical work.  More folks should read it, it’s beautiful.

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