Ryan McIntosh, Kings High School, asks :-

Evan Blanch, of Alexandra, asked:-

What causes mirages?

Carol Miles, an optical physicist with Selim Systems, Christchurch, responded.

When the sun warms the dark surface of a road, the air just above the surface is heated 4 to 6 degrees Celsius hotter than the air well above the road. Light travels a little faster in the warmer air near the road surface than it does in the cooler air above.

We see, say a cyclist down the road, because sunlight is scattered in all directions from the cyclist and some reaches our eyes. We usually think of light as travelling in a straight line which is what normally happens to a ray of light travelling directly to our eye. However when the air just above the road is hot, another light path is possible from the cyclist to our eye and this path is curved. A light ray scattered by the cyclist downwards in our general direction travels faster and faster, curving as it does until it is parallel with the road and then it curves upwards again and finally reaches our eyes. To us it seems that the light has come from below the road surface. We see an image of the horizon, sky and cyclist as though reflected by a horizontal mirror. The image of the blue sky looks like a pool of water, the classic mirage seen in deserts.

You can do a simple experiment on any hot day. Stand and look down a smooth road or supermarket car park and you will see a weak mirage a block or two away. Now squat closer to the road surface and you will see the mirage stronger and closer. (For safety's sake do this where there is no traffic.)