Rainbow is a meteorological phenomenon, which is caused by reflection, refraction and scattering of light in water droplets, resulting in spectrum in the sky. It takes the form of a colored arc. Rainbows caused by sunlight always appear in the sky area directly opposite the sun.

A rainbow can be a complete circle. However, ordinary observers can only see the arc formed by the illumination droplets above the ground, and its center is located in the straight line from the sun to the observer's eyes.

In the main rainbow, the outer part of the arc is red and the inner part is purple. Rainbows are caused by light being refracted as it enters the droplet, then reflected on the back of the droplet and refracted again as it leaves the droplet.

In the double rainbow, the second arc can be seen on the outside of the main arc, and its color order is opposite, and the inside of the arc is red.