Asked by jayy
n a hot day, the air is much warmer than the air above it. Warm air is less dense than cool air, so
light waves travel faster through i
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On a hot day, the air near the ground is much warmer (and therefore less dense) than the air above it. Warm air is less dense than cool air, so light waves travel faster through it (v = c/n, and n is smaller for less-dense air).
Because the refractive index of air depends on its density (approximately n − 1 ∝ ρ, and for an ideal gas ρ ∝ P/T), n decreases as temperature increases. A vertical gradient in n makes light rays bend continuously (not abruptly) according to Snell’s law: rays curve toward regions of higher n (slower speed) and away from regions of lower n (faster speed).
On a hot road this produces an “inferior mirage”: light from the sky that would otherwise hit the ground enters the hot, low-n layer and is bent upward toward your eye. Your brain traces that bent ray back in a straight line and therefore places the apparent source of the light below the horizon — you see what looks like a patch of sky or a pool of water on the road. Turbulence and small-scale temperature variations in the boundary layer cause the shimmering, wavy distortions. (If the temperature profile is reversed—cool air near the ground, warmer air aloft—you can get a superior mirage instead, where distant objects appear lifted or inverted.)
Because the refractive index of air depends on its density (approximately n − 1 ∝ ρ, and for an ideal gas ρ ∝ P/T), n decreases as temperature increases. A vertical gradient in n makes light rays bend continuously (not abruptly) according to Snell’s law: rays curve toward regions of higher n (slower speed) and away from regions of lower n (faster speed).
On a hot road this produces an “inferior mirage”: light from the sky that would otherwise hit the ground enters the hot, low-n layer and is bent upward toward your eye. Your brain traces that bent ray back in a straight line and therefore places the apparent source of the light below the horizon — you see what looks like a patch of sky or a pool of water on the road. Turbulence and small-scale temperature variations in the boundary layer cause the shimmering, wavy distortions. (If the temperature profile is reversed—cool air near the ground, warmer air aloft—you can get a superior mirage instead, where distant objects appear lifted or inverted.)
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