MB Science - 10 - Vanishing Coin

'Try this' challenge 10: Vanishing Coin

You will need:

Quick start:

A glass with straight sides
Small jar or dish with a dip in the bottom (a concave part) – a ramekin is ideal
A coin
Water
Jug for pouring
Lid for your glass (can be anything to cover the top)
Tea towel/ drying cloth

Genius:

Different glasses to experiment with
Torch

MIGHT/WILL GET WET DO IT SOMEWHERE THAT WILL BE EASY TO CLEAN UP

What to do:

To measure how much water you need, fill your glass all the way up with water then pour carefully into your jug.
Dry off your glass inside and out. Now you’re set up.
Get your family into position and introduce your trick.
Place the small jar upside down and the coin on top.
Place the glass on top of that. There should be a gap of air between your coin and your glass but your family should be able to see the coin. Make a big deal about how you can make it disappear.
Hold the lid open - but make sure your family can’t see through the top of the glass.
Pour the water into your glass all in one go and drop the lid. Your coin should have vanished from view!
Once your family have recovered from the shock, lift the lid.

Genius challenges:

Find the tallest straight-edged glass in the house. How many reflections of the coin can you see?
Remove one of the air layers by filling the dip in the jar with water or run a wet finger behind the reflection of the coin . What happens?
Replace the coin with a torch. Switch it on and off. Where does the light come out?

email your photos/ videos / explanation to trythisathome@mountbatten.hants.sch.uk to get your reward!

What is going on?

Making the coin disappear might seem like magic, but like many optical illusions it only works if you're looking at it from a specific angle.

When you look at a coin normally, you can see it because light bounces off it in all directions and some of that light travels in a straight line to your eye.

In the first part of the trick, with the coin underneath the empty glass, things are a little bit more complicated. Light bouncing from the coin now has to travel through the air, the bottom of the glass, the air again, the side of the glass and – finally – though the air to your eyes.

Each time the light goes from one material to another it changes direction but most of it still gets through and so your family can still see the coin.

But only most of the light reaches your family, a small amount will be reflected off the side of the glass instead, bouncing back inside the glass and not reaching your eyes.

Whether the light travels through a material or gets reflected depends on the angle the light is travelling at and the materials it is travelling between. That is why adding water to the glass is the final stage to our trick. The light from the coin at the bottom now has to travel through:

The air
The bottom of the glass
The water
The side of the glass
The air again to your family

But the water changes the angle enough that instead of leaving the glass all of the light is reflected. When 100% of the light reflects in this way it's called total internal reflection.

With the glass filled all the way to the top, none of the light from the coin at the bottom can escape through the sides. It’s trapped. It can never reach your eyes and so the coin seems to vanish.

Of course this only works if you’re looking through the side of the glass. We need a lid for the top so no one can see in. This is also why you need a straight edged container; the size doesn’t matter, but it can’t have any curvy bits or the light can "leak" out.

Knowing the physics behind what’s going on will help you set everything up, and soon you‘ll have all your family wanting to know the secret of the vanishing coin. Lifting the lid gives a clue – there's a very clear reflection of the coin and if they look closely the writing is back to front. Our five layers combine to make the perfect mirror.