User: Can you succinctly explain why rainbows happen?
Assistant: Sure, the reason rainbows form is due to scattering of sunlight by water droplets in the atmosphere.  The rainbow colors are due to the specific angle at which light is refracted by the droplets, resulting in different colors being scattered in different directions.  The order of colors in the rainbow (red, orange, yellow, green, blue, indigo, violet) is due to the structure of the electromagnetic spectrum, where red light has the lowest frequency and purple/violet light has the highest frequency.  The apparent arc shape of the rainbow is due to the fact that the whole spectrum of colors is scattered in all directions, but we only see a narrow arc due to the fact that our eyes are located at a particular angle relative to the direction of the Sun.  Hope this was helpful!  Best wishes,  -- The Assistant