Coin in a cup
Can you push a coin through a cup of water and not spill a drop? With this trick of the light it's more than the truth that gets a bending.
20 replies
This is one of the easiest, yet most impressive magic tricks you'll ever learn. Your audience will gasp when you magically push a coin through the side of a plastic cup full of water without spilling a drop.
1. Half-fill a plastic cup with water. Now carefully drop in a 10-cent coin so it stands upright against the inside of the cup.
2. Rotate the cup and you'll notice that, when viewed at just the right angle, the coin completely disappears from view. Show the cup to your audience this way and they'll believe there is nothing but water inside.
3. With your audience convinced there is nothing funny going on inside your cup, press a second 10-cent coin against the outside of the cup.
4. Cover this coin with your fingers. Now comes the tricky bit. Tilt the cup forward slightly so that the coin inside rolls forwards.
5. Remove your hand, sneakily taking the coin you pressed against the outside of the cup. While your audience marvels at how the coin got into the cup, pop the other one in your pocket. Hey presto, you're a magician!
What's going on?
Water is a liquid so it assumes the same shape as the inside of your cup, which is very close to a cylinder. So, a cup full of water is basically a cylindrical lens, which focuses light to a straight line. Spherical lenses, like the one in your magnifying glass, focus light to a point.
The real trick here is that light entering the water at the outer edges of your cup is bent so much that it doesn't actually reach your eyes. The angles are so great that, instead of passing through the water, those light rays are reflected back where they came from.
This phenomenon is called 'total internal reflection'. You can see it in action underwater by looking up at the surface. If you're wearing goggles, you'll see a reflection of the bottom of the pool.
Those total internal reflections at the edges of your cup mean you can sneakily 'hide' a coin there. The coin's image bounces away from your audience's eyes so they'll have no clue it's lurking in the water right before their eyes.
Tilt the cup forwards so the coin rolls into view behind a veil of four strategically placed fingers and the illusion is complete.
But be warned! Practice in front of a mirror so you know exactly how to hold and tilt the cup without being obvious or allowing the coin to roll into view before you're ready.
Nice trick, thanks for sharing 
i had read it in physics, nice trick
Nice trick
hmmm, i'l do it
Great... I appreciate d use of physics here. I'm a physicist in d making.
I like that
Nice trick
