The reason why the sky is blue is that molecules and particles in the air can scatter sunlight, and the blue wave length in the sunlight, with high energy, is easier to scatter. When it rains, the clouds become thicker, and the sunlight reflects more, so the sky is darker at this time. The color of many objects in nature is due to the scattering and reflection of sunlight.
 1、 The reason why the sky is blue
 The sky is blue because of the scattering of sunlight.
 The sky we looked up and saw was colorless. In our eyes, the reason why it appears blue is that molecules and particles in the air scatter sunlight to the surroundings, while the blue in the sunlight is easier to scatter due to its wave length and high energy, resulting in blue sky.
 The sky is gray on rainy days.
 However, the sky is not blue every day. For example, when it rains, the amount of clouds in the sky increases, and the sun will be reflected when it reaches the thick cloud layer, so the sky in rainy days is much darker than that in sunny days.
 The sun has seven colors.
 Through the prism, we can see that the sun has seven colors of visible light. They are red, orange, yellow, green, cyan, blue and purple. These seven colors are also the familiar colors of the rainbow. Many objects in nature are produced by different sunlight, such as the ocean.