# Making Pyrex Invisible

09 October 2005

## pyrexoil2.jpg

### Ingredients

 A couple of glass bowls - the smaller one should be ovenproof - the Pyrex(TM) brand definitely works others may not) Vegetable oil - enough to fill the larger container a few cm deep

### Instructions

Poar the vegetable oil into the large container

Put the small pyrex bowl into the oil

Look for the bowl under  the oil.

### Result

The part of the pyrex bowl that is completely under the oil seems to disappear, you can't see it any more!

### Explanation

The first question to ask is why can we see the transparent bowl in the first place - it is see through so how can we see it?

Although glass is transparent, the light travels at a different speed in the glass than in the air. This means that if light hits the glass at an angle some of it will be reflected and some will get bent around a corner (refracted). When you see a glass object you don't see the glass itself, but distortions and reflections of other things in the room.

However light goes at the same speed in vegetable oil as in the pyrex. This means that the light isn't reflected or refracted when it passes from the oil to the pyrex, so you have no way of seeing the bowl.

 Light goes at different speeds in air to pyrex so it is distorted. In oil the light travels at the same speed as pyrex, so it carries straight on.

## Why does changing the speed of light cause distortions?

Imagine a formation of soldiers marching fast on tarmac, then veering off into some sand which they can't march nearly as fast on.

Because the soldiers on the sand are marching more slowly, they are closer together. The only way they can maintain their formation stay next to their neigbour is to change direction.

When the soldiers leave the sand the first ones to get off it will speed up so the process hapens in reverse.

Light is a wave, and wavefronts (the line long the top of a wave), behave in much the same way as our soldiers. So if light enters a substance it travels more slowly in, it will change direction.