In a primary rainbow, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.
All about rainbows. What causes a rainbow, why is it curved? What are the rainbow colors, how does a double rainbow work, and what's at rainbow's end?
What is a Rainbow? It takes both the sun and rain to make a rainbow! To put it plainly, rainbows are produced by sunlight entering water droplets, bouncing around each individual bead of water, and changing direction (refracting) to reflect off the back of the droplet to return back toward us.