How does this optical illusion bring invisible distant objects into view?
Commonly seen in the desert or at sea, a mirage is an optical phenomenon associated with light refraction. As light rays pass from a distant object through one material and into another, their refractive index changes, which alters the speed at which the rays travel. This is known as ‘refraction’ and it causes the light rays to bend and change direction. So when light passes from a layer of denser cool air through to a layer of less-dense warm air, refraction occurs. Due to the curvature of the Earth, the bent light rays can bring otherwise obscured objects beyond the horizon into view above it, in line with the viewer.
The ‘vision in the desert’-type mirage is known as an ‘inferior’ mirage. These mirages make objects appear much closer than they are, and light coming from objects on the horizon will appear as images down on the ground. They occur when the ground is so hot that it warms the air just above the surface.
Light rays passing from the overlying cool air through to this warmer air near the surface will bring a distant object into view at a closer range. Where the light would normally go to the ground, it goes up to meet the viewer’s eyes.
Conversely, ‘superior’ mirages can make an image of an object appear unfeasibly high off the ground. This occurs when a layer of cool air sits beneath a layer of warm air (ie, over an area of ice or very cold water, such as the poles). Because the cold air near the ground is denser than the air higher up, light is refracted downwards, bending the light rays towards the viewer’s eyes.