Why does a rainbow appear after a rainy day?

This is because after a thunderstorm or shower, there are many small water droplets suspended in the air. These small water droplets produce refraction and internal reflection under the sunlight. Thus forming a rainbow.

Among them, the reflection between 40 and 42 degrees is the strongest, causing the rainbow we see. When this kind of reflection occurs, sunlight enters the water droplet, is refracted once, then reflects on the back of the water droplet, and finally refracts again when leaving the water droplet. In total, it is refracted once and twice.

Introduction to the rainbow:

In fact, as long as there are water droplets in the air and the sunlight is shining behind the observer at a low angle, an observable rainbow phenomenon may occur. It often appears in the afternoon, when the sky just turns sunny after a rain. At this time, the air is less dusty and full of small water droplets, and one side of the sky is darker because there are still rain clouds.

The rainbow will be easier to see when there are no clouds above or behind the observer and sunlight can be seen. Another place where rainbows are often seen is near waterfalls. On a clear day, you can create an artificial rainbow by sprinkling water or mist in the air with your back to the sun.