Why is the sky blue when you see it on a sunny day?

The reason why the sky is blue is that the molecules and particles in the air scatter sunlight, and the blue wave in sunlight is short and energetic, so it is easier to be scattered.

When it rains, the clouds become thicker. At this time, the sun reflects more, so the sky is darker at this time. The colors of many objects in nature come from the scattering and reflection of solar light.

However, the sky is not blue every day. For example, when it rains, there are more clouds in the sky, and sunlight will be reflected when it reaches thick clouds, so the rainy sky looks much darker than sunny days.

Color principle

The clear sky is blue, not because the atmosphere itself is blue, nor because it contains blue substances, but because atmospheric molecules and tiny particles suspended in the atmosphere scatter sunlight. Due to the inhomogeneity of the medium. The phenomenon that light deviates from the original propagation direction and scatters sideways is called the scattering of light by medium.

The scattering of fine particles follows Rayleigh's law: the intensity of scattered light is proportional to the fourth power of frequency. When sunlight passes through the atmosphere, green, blue and violet light with higher frequency are most easily scattered; However, the light scattering of red, orange and yellow with lower frequency is weak, and the sky appears blue due to this comprehensive effect.