Mirages

Light travels though air of different densities at different speeds. The density of air changes with temperature, with cold air being more dense. Near a surface such as a hot road, or an ice field, there can be a strong temperature gradient which produces a density gradient which causes the air to act like a lens near the surface. However these two examples are very different. In the case of the ice field the density gradient will cause surface features a long distance off to appear stretched upwards. While in the case of the hot road the density gradient will be in the other direction which results in the sky being reflected, or bent back up, towards the observer just near the hot surface. The result can be the false appearance of water in the distance, and thus the most commonly known idea of a mirage.

Green Flash Distortion of the sun at sunset by viewing through a strong temperature inversion in the atmosphere.

Lantern Effect Distortion of the sun at sunset by viewing through a strong temperature inversion in the atmosphere.