|MadSci Network: Earth Sciences|
This is difficult to envision and to describe, but we'll use some real common sense objects for our model. Picture yourslf standing in front of a tunnel, like you might find under a bridge - a nice even, round, smooth tunnel. Now, if you cast a shadow parallel to the ground, like a shadow from a power line (not the pole, the line - which is parallel to the ground) and let the shadow fall inside the tunnel, you can imagine that the shape of the shadow on the ceiling of the curved tunnel looks as though it is arced, even though the source of the shadow is a straight line. Can you picture that? This is what is happening in the atmosphere. The rainbow is actually a straight line, since light travels in a straight line. Its just that the straight line is falling across a 'fish bowl' like pocket of moist, reflective air, like our tunnel in the model. The rainbow appears, therefore, to be an arc.
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