Why are hot roads reflective when looked at from a flat angle

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Why are hot roads reflective when looked at from a flat angle

In: Physics

Its basically a mirage. Light is actually infinitely interesting.
If you remember refraction from school, you should also recall a phenomenon called total internal reflection (TIR). TIR occurs only when light travels from a denser to a rarer medium and hits the boundary between the two media at a the “critical angle” or any angle above that value, so its a bit difficult to observe in everyday life (since we usually perceive light in air, which has quite a low density).
When the air near the tarmac heats up, the molecules move about more rapidly than the relatively cooler air above it, making the air above denser than the air below. This satisfies the condition for TIR.
An important thing to note is that the road is only reflective a certain distance ahead of you. So when you drive ahead, the reflective surface also moves with you. This is where the critical angle comes into play. Only light hitting the rare air closer to the road is totally internally reflected to your eyes.

TLDR; read up on how mirages are formed.