When this kind of reflection is caused, the sunlight enters the water drop, refracts once, then reflects on the back of the water drop, refracts again when leaving the water drop, and always refracts twice after one reflection. Because water has dispersive effect on light, the refractive index of light with different wavelengths is different. The refractive index of red light is smaller than that of blue light, and the deflection angle of blue light is larger than that of red light.
Because light is reflected in water droplets, the spectrum seen by the observer is reversed, with red light at the top and other colors at the bottom. So the heights of rainbows and neon lights are different, and the hierarchical order of colors is just the opposite. Rainbow means that light is refracted twice and reflected once, while neon means that light is refracted twice and reflected twice.
Extended data:
When a rainbow appears on an object on the water, two mirror arcs with complementary optical paths may appear on the water and underwater respectively. Their names are slightly different. If the water surface is calm, the reflected rainbow will appear as a mirror image below the horizon of the water surface. Sunlight is deflected by raindrops before reaching the observer, and then reflected by water. Even in small puddles, reflected rainbows, at least in part, are often visible.
When sunlight is reflected by the water surface before it reaches the raindrops, it may produce a reflected rainbow. If the water surface is large enough, the whole water surface is calm and close to the rain curtain, the reflected rainbow may appear on the horizon. It meets the normal rainbow at the horizon, and its arc will be higher in the sky because its center is above the horizon, while the center of the normal rainbow is below the horizon. Due to the above conditions, reflected rainbows are rare.
If the reflex arc is reflected again, the neon reflex arc and other reflex arcs will appear at the same time, and it is also possible that six (or eight) color bands will appear at the same time.
References:
Baidu encyclopedia rainbow