According to the Merriam-Webster dictionary, the definition of a mirage is the following:
- something (such as a pool of water in the middle of a dessert) that is seen and appears to be real but that is not actually there
- something that you hope for or want but is not possible or real
The reason a mirage is seen is because the light from the sun is bent. If all the air is at the same temperature -hot or cold- the photons (light particles) travel in a straight line into our eyes. If there is hot air, it travels through it to get faster to its destination. It can travel faster in hot air because it is less dense than cold air.
In a highway or dessert, the air is kind of normal in temperature and the air just above the pavement (or sand) is hot because of the sun. Because of that, the light travels through the hot air in the ground although it bends its way, but it wants to go faster. The light that gets to our eyes usually comes in a straight line from the sun. Our brains know that, so it thinks that water is reflecting the sun’s light. That´s why we see mirages.