Look carefully at this photo of a rainbow over Dinas Head near Fishguard in Wales (above) and you will notice that as it nears land it looks like there is a sharp bend in its arc. What caused this?
• This effect is caused by atmospheric refraction, the same type that makes the sun appear to rise earlier and set later than would be expected from astronomical calculations.
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The density of the atmosphere decreases the higher you get above Earth’s surface, and so it interferes less with the light passing through. This means the speed of light is slightly faster higher in the atmosphere than it is closer to the planet’s surface. In outer space with no air, it is the standard textbook value of almost 300,000 kilometres per second (the speed of light in vacuum).
The increase in light speed with height causes rays to be refracted downwards, and the effect is strongest for rays that are close to horizontal. Thus light rays curve slightly around Earth’s surface. A small curvature is also caused by Earth’s gravitational field.
The curved light rays cause objects below the mathematical horizon to appear above the visible horizon. So the bottom part of the rainbow in the photo includes some from below the mathematical horizon. This slightly longer bit of rainbow has been compressed by the refraction, increasing its apparent curvature.
This is the reverse of the effect that causes desert mirages, in which the hot ground heats air close to it, making it less dense. In this case, the speed of light is faster close to the ground, so light is refracted upwards, giving a mirror effect that looks like water.
Richard Parkins, Cambridge, UK
• I once witnessed an extreme version of this when approaching Sydney airport. Our aircraft was supposed to be landing on the northern runway, but a southerly buster (the Aussie term for a strong summer cold front) was about to pass, and the wind direction flipped from north to south. While changing direction to land on the southerly runway instead, we could actually see the edge of the cold front as it traversed the runways, so large was the temperature difference. It was as if a huge elongated lead crystal dome paperweight was moving northwards. The distortion of the light rays was so big that the boundary between the warm and cold air was clearly visible. It took about a minute for the edge to pass over the runway.
Roger Williams, By email, no address supplied
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