Mirages

When light passes from one medium to another, its speed changes. This causes the light to bend, or 'refract'. This can be seen by placing a pencil in a glass of water, which seems to bend where it enters the water.

Refraction also happens when light passes through air of a different density, such as when the temperature changes. The diverted light reflected from the object creates an image in another position. This result's in an optical illusion known as a mirage.

Inferior mirage
The inferior mirage occurs when the temperature of the air decreases with height, causing distant objects to be displaced downwards; a patch of sky in the desert could be reflected on to the sand, so to appear as a pool of water.

Most people are familiar with the inferior mirage and have seen 'pools of water' on the road on a hot day. In this case the refraction is caused by a large temperature difference near the road. The light bends so much that an image of the sky is seen on the road.

Superior mirage
A superior image is formed by a warmer layer of air above some colder air, called a 'temperature inversion'. When the temperature difference is large enough, the warmer air refracts the light and amazing images can be seen.

An image is diverted upwards, so that something on ground level over the horizon, which would normally be out of sight due to the curvature of the Earth, is reflected in the sky above its actual position. These images are often upside-down. A common superior mirage is a ship that looks like it is sailing in the sky.

A superior mirage is less common than an inferior mirage, as it also includes a setting or rising sun. The sun that we see setting or rising is a mirage of the actual sun, which is really below the horizon.

A superior mirage could explain some apparent UFO sightings. Refracted light from the headlights of cars, gives the impression that there are lights in the sky. Due to the unpredictability of mirages, these lights can seem to move quickly through the sky and suddenly vanish.

Mirages do not only occur in the desert but also in cold areas like ice caps. Sailors in the Arctic waters often see superior mirages of far off ships sailing towards them, inverted in the sky. Now that's some trick!

In April 1977, people in Grand Haven, USA saw lights across Lake Michigan. The nearest town was Milwaukee, 75 miles away. A temperature inversion across the lake allowed this faraway city to be briefly visible. With an average April temperature of around 13°C, this mirage at least would not have been due to heat stroke!