Why Can a Hot Road Look Covered in Water?

On a hot, clear day, a road ahead can show a trembling bright patch that resembles shallow water. As the car moves forward, the “water” keeps retreating. The road is not leaking, and the effect is more than shiny bitumen: it is an inferior mirage.

Sunlight heats the road, making the air immediately above it hotter, less dense and slightly lower in refractive index than the cooler air higher up. When light from the sky travels at a shallow angle through these continuously changing layers, its path bends gradually upwards into the driver’s eyes. Vision normally traces arriving light backwards in a straight line, so the light is judged to have come from below the road surface. A virtual image of the bright sky or horizon therefore occupies the apparent position of reflected water.

Moving air continually rearranges the temperature gradient, so the image shimmers and breaks up; the lower parts of distant objects may also stretch or invert. When the observer reaches the apparent location, the angle and air layers have changed, so that image disappears and another forms farther ahead. The key point is that the light is real, but its curved path leads the eye to assign it the wrong position.

Source: https://www.physicsclassroom.com/class/refrn/lesson-4/mirages