Okay (he says with a redundant sigh). I know how it is that water, prisms and the like have varying indexes of refraction for the coloured constituents of light, thus breaking up "white" light, into all the visible wavelengths. (the pretty colours of the rainbow) However, my question is, why does a rainbow typically appear as an orderly arc in the sky? What is going on? Is this question one of human perception, or the science underlying the phenomena itself? I'm confused. (Theo forges a smile, in order to adequately express his thoughts) > ! (adding an exclamation point for effect )

its due to raindrops being spherical, only one certain angle bounces the spectrum of light back towards you, this angle is the same as there is between both sides of the rainbow as you see it. it is also why they always appear a uniform distance away.

They are actually circular. If the earth was not in the way, the rainbow would appear in a giant ring around your conical zone of vision at the constant angle that allows the light to break up into its constituent colours.

Is there a way of making an experiment that will curve a rainbow coming from a glass prism? Maybe using a fog machine?

I'm thinking of creating one for a classroom environment, so it would be pretty large.

Raindrops being spherical is secondary — 22 degree halos from ice crystals are also circular. The reason for the circle is that for you to see the refracted light, there must be a constant angle between you and the refracting element and the sun. That describes a cone, the projection of which is a circle.

 

Spherical drops and double refraction ensure that the rainbow will be on the opposite side of the sun, rather than the same side.