What exactly are you seeing when you observe a "water-on-the-road" mirage?
You are seeing sky. It's almost as if the hot ground were acting as a mirror. Light waves from the sky that pass near and along the ground, are redirected upwards to the eye
a refraction of the light due to the thin air near the road
When observing a "water-on-the-road" mirage, you are perceiving an optical illusion caused by the bending of light as it passes through different layers of air of varying temperature. To understand this phenomenon, we need to consider the concept of refraction.
When light travels from one medium to another with a different density, such as air to water or air at different temperatures, it changes direction. This change in direction is called refraction. This is the key reason why mirages occur.
In the case of a "water-on-the-road" mirage, the road surface is typically very hot, causing the air just above it to heat up as well. As the temperature of the air increases, its density decreases, creating layers of air with different temperatures and densities.
When light from the sky passes through these layers, it gets bent differently depending on the temperature and density of each layer. This bending causes the light rays to curve upward, creating a virtual image of the sky. This virtual image can then be seen by our eyes as a reflection of the sky on the surface of the road, giving the appearance of water.
One way to observe this phenomenon is by driving on a hot day when there is a temperature gradient between the road surface and the air above it. As you approach the mirage, you may notice that the appearance of water on the road seems to shimmer and distort. This is because the layers of air are not uniform, causing the light to bend unevenly and creating a wavy or distorted pattern.
It is important to note that the "water-on-the-road" mirage is just an illusion, and there is no actual water on the road. It is a result of how light interacts with the different layers of air and our brain's interpretation of that visual information.