OK. So, you're walking alone through the desert as a result of an accident and you are trying to get back to civilization. Suddenly you observe a water-like feature a few hundred meters in front of you and you don't know if what you see is real. You have three possibilities. One, you are dillusional as a result of being dehydrated, two, the image you see in front of you is actually real and three, you are experiencing a mirage.

We have seen it so many times in cartoons and movies, that it almost became some kind of a myth, but the physical phenomena is as real as it gets. It's all due to a property of transparent materials called refraction, which involves slightly changing the angle of the light as it passes through. You probably remember playing with a table spoon inside a transparent glass full of water and observing a certain image distortion. The same effect is responsible for the mirage.

This light bending phenomenon usually occurs naturally and extremely often in the atmosphere due to slight variations in the density of air, as a result of temperature differences between certain layers of the atmosphere, causing the light to travel in other directions than in a straight line as usual. The refraction effect is triggered by the fact that light does not travel at constant speed through all the mediums. For example, in void light has the maximum velocity in the universe although when it hits the Earth's atmosphere its speed decreases slightly, causing a change in trajectory as well.

Most mirages that occur naturally, and we see them relatively often on sunny days, usually appear on man-made road surfaces as the Sun heats them up, which determines the surrounding air to heat up as well. Hot air has less density than cold air thus triggering an amplified refraction effect that occurs between the two different temperature air layers.

For example, as you watch an image in the distance the refraction effect produces an apparent image of the same object, only inverted, due to the fact that light reflected from the image refracts oddly in the air fooling the brain into believing that light coming from the apparent image actually travels in a straight line.

A similar effect can be observed when a superior image is forming, due to the fact that a relatively warmer layer of air is covering a colder layer, and is usually associated with mirages that appear at the horizon when the Sun sets, giving a spectacular illusion.