Rainbows and the paths of sunlight

Rainbows are optical patterns produced when sunlight interacts with water droplets in the air. Their familiar bands of colour appear when light enters a droplet, changes direction, reflects inside, and emerges again. Different colours follow slightly different paths. The result is not an object hanging at one fixed place, but a pattern seen from a particular position relative to the light and the droplets.

The Sun is usually behind an observer looking at a rainbow. Water droplets in the opposite part of the sky send suitable light toward the observer’s eyes. Moving to another position changes which droplets contribute to the visible pattern. This is why a rainbow cannot be approached in the same way as a distant building or a hill. Its apparent location depends on the geometry of observation.

The colours in a rainbow form a continuous range rather than a set of sharply separated strips. Names such as red, green, and blue provide useful descriptions, but the transitions are gradual. The size of the droplets and the lighting conditions can affect the appearance of the bands. A rainbow may look bright and well defined on one occasion and pale or incomplete on another, even when rain is present in both cases.

More than one rainbow can sometimes be seen. In a secondary bow, light has undergone an additional internal reflection, and the order of the colours is reversed. This outer pattern is generally less bright than the main bow. Other subtle features may also appear under suitable conditions. Their visibility depends on a combination of droplet properties, the brightness of the background, and the position of the observer.

Rainbows do not require a large rainstorm. Similar patterns can appear in spray from a waterfall or in a fine mist when the lighting geometry is suitable. These smaller examples make the relationship between droplets, sunlight, and viewpoint easier to explore. Watching how a rainbow changes as clouds pass or the Sun becomes lower reveals how an ordinary interaction of light and water can produce a striking and temporary scene.

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