Physics · Ch 6 — Optics
Mirage and Looming
Mirage and Looming
The refractive index of air increases as its density increases. In hot places, air near the ground is hotter (and hence less dense, lower refractive index) than air higher up, so in effect the refractive index of the air decreases towards the ground. Light from a tall distant object such as a tree, travelling down through these progressively lower-index layers, is bent continuously away from the normal, and eventually undergoes total internal reflection near the ground once its angle of incidence there exceeds the local critical angle. This produces an inverted image that looks exactly like a reflection off a pool of water or a wet road surface -- the phenomenon known as mirage. In cold places, the opposite gradient occurs (air near the ground is colder, hence denser and higher-index, than ai …
What this figure shows. In panel (a), over hot ground the air's refractive index decreases closer to the surface (hot air is less dense); light from a distant tall object like a tree bends progressively away from the normal through successive air layers and undergoes total internal reflection near the ground once the incidence angle exceeds the local critical angle, producing an inverted image that looks like a reflection in a pool of water -- a mirage. In panel (b), the reverse situation over a cold surface like a glacier gives the opposite refractive-index gradient, producing an inverted image …