The term "mirage" can refer to a few different concepts depending on the context: 1. **Optical Illusion**: In the most common sense, a mirage is an optical phenomenon that occurs when layers of air at different temperatures create a distortion in the way light travels. This often results in the appearance of water or a reflection of the sky on a hot road or desert surface. The light refracts due to temperature gradients and creates an image of something that isn't actually there.
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A mirage is an apparent displaced image caused by refraction through a spatially varying refractive index. Above a hot surface, low-index air near the ground can bend a light ray back upwards before it reaches the surface. An observer tracing the arriving ray backwards along its local tangent can then infer an apparent image below the turning region. This explains the appearance of a distant reflective surface without requiring reflection at the road itself.