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?
In the case of a rainbow, the sunlight is refracted as it enters the water droplet, which causes the sunlight to disperse, or spread out, into its component colors. This separation of colors through raindrops, similar to what happens in a prism, is what gives a rainbow its distinct bands of color.
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.
To be scientifically correct, a rainbow is not an object but an optical illusion. What reaches our eyes is visible light, which gets processed by the human eye into the vibrant bands of color we associate with a rainbow.