If the earth were flat, would a rainbow be straight?

A rainbow is circular because it is made up of every point that is about 42° away from the point opposite the sun (this is because of the angle light reflects off raindrops at). That produces a circle. read more

No. The form of the rainbow is generated by optics, not by the form of the planet. In a fly you can even see a full-circle rainbow. The article in Wikipedia is excellent: Rainbow - Wikipedia read more

The curve of a rainbow isn't related to the shape of the Earth's surface. If that were the case, even on a Round Earth, with such slight curvature, a rainbow would be almost a straight line still. From an aeroplane, sometimes the entire circle of a rainbow is visible. read more