Mirages

Refraction by Air

Although light travels in straight paths through a constant medium, because the atmosphere is not uniform, light refracts to travel in curved paths. Air is generally denser near the earth's surface; light rays bend toward denser air. One consequence is that when an observer sees the sun just touching the horizon, the geometrical position of the sun is below the horizon (Figure 1a). The apparent direction of an object is determined by the angle that light rays enter the eye, rather than by the object's geometrical position.

If the observer watches the sun carefully as it sets, he or she could see two other refraction phenomena: First, the sun appears flattened (color insert, Figure 27d). Its upper part looks approximately hemicircular, but its lower half is compressed vertically. This is a consequence of the greater refraction of light from the part of the sun lower in the atmosphere; it sends light into the eye at relatively constant angles. Second, when the sun had almost completely disappeared below the horizon, the remaining sliver would appear green, then blue: the green flash (color insert, Figure 27e) and the blue flash. This is because shorter wavelengths, which appear green and blue, are refracted more by the atmosphere; the longer wavelengths, which appear red and yellow, are refracted less, so are blocked from the eye by the curve of the earth.

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