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Look up at the sky in the evening and you’re likely to see the blue sky of the afternoon being changed into a reddish-orange one across the sky. Why does the colour of the sky change at different times of the day? The answer has to do with how the sun’s light and the earth’s atmosphere（大气层）have an effect on each other.

Light travels in waves（波状运动）. Every colour we see is decided by the length of those waves. Red wavelengths are the longest, and as you go through the rainbow, the waves get shorter and shorter until you reach the blues and violets, which are the smallest.

As the sun’s light travels towards us, at first all it meets is space without any particles（微粒）, such as dust and water droplets. But on earth, there are thousands of particles in the air which are hardly seen. The wavelengths that make it to the earth’s atmosphere hit these particles, which sends light bouncing（反弹） in many different directions.

But not all wavelengths bounce around to the same degree. Blue wavelengths, because of their smaller size, are scattered（散射） more easily and in many directions. Red wavelengths, which are larger, are scattered the least in the air.

During the day, when the light has the shortest distance to travel through the atmosphere, what we end up seeing are the blue waves that have been scattered all across the sky. At the day’s end, though, the light has to travel through much more of the atmosphere, and those short, blue light waves have a hard time getting past all particles. They mostly get scattered away before they ever reach us. The result is that red sky we see at sunset, since longer red wavelengths are still able to pass through the atmosphere.

So when you see a red sky at dusk, just remember that it’s actually the last remaining colour of the whole rainbow.