[Duncan Moore] As an example of a gradient index material I have here a piece a plastic. And you notice as I
shine a laser beam straight through it, it goes straight through as you would expect. But as I move the laser beam up, it curves down and so what we’ve got is we’ve got a change in the composition from the top to the bottom. So everybody’s observed this phenomenon. When the light goes down here at an angle notice it comes out about the same angle
going uphill. This is exactly what happens when the road appears to be
wet on a hot summer’s day. The road is black, it’s absorbing the heat, it’s heating up the air it’s changing the density of the air as
a function distance above the road surface and what’s happening is
light that should have gone in a straight line, which would have gone right straight into the road bends around and it comes back up to your eye. So as the observer on my left and on your right, you think
that the light has come from below the road’s surface. So what you’re actually seeing is an
image of the sky in the road. Now, in the case of the road, it takes about a half a kilometer for that phenomenon to occur,
because the change is very, very small in the air. In this
case I have 10,000 times more change in the composition then is the case in the air and so I can do something about four
inches or 10 centimeters that it takes about half a kilometer in the
air