Why Is the Sky Blue?
வானம் ஏன் நீல நிறமாகத் தெரிகிறது?
Sunlight looks white, yet the daytime sky is blue. The answer lies in how air scatters different colours of light.
Look up on a clear day and the sky is blue. Watch the Sun near the horizon at sunset and it can look orange or red. It is the same sunlight with different colours, so what is going on?
Sunlight is a mixture
Sunlight looks white, but it is really a mixture of all the colours of the rainbow. You can see this when sunlight passes through raindrops or a glass prism and spreads out into red, orange, yellow, green, blue and violet. Each colour of light has a different wavelength. Red light has the longest wavelength, and violet the shortest.
Air scatters light
When sunlight enters Earth’s atmosphere, it meets gas molecules, mainly nitrogen and oxygen. These molecules scatter light, which means they send it off in all directions. But they do not scatter every colour equally. Short wavelengths, such as blue, are scattered much more strongly than long wavelengths, such as red.
So when you look at a part of the sky away from the Sun, you are seeing blue light that has been scattered towards your eyes from all directions.
Why not violet?
Violet light has an even shorter wavelength than blue, so why isn’t the sky violet? The Sun gives out less violet light than blue, some of the violet is absorbed high in the atmosphere, and our eyes are much less sensitive to violet. The mixture that reaches our eyes looks blue.
Red sunsets
At sunrise and sunset, the Sun’s light has to travel through much more air to reach you. Along that long path, most of the blue light is scattered away from the direct beam, leaving the oranges and reds to reach your eyes.
Why clouds are white
Cloud droplets are much larger than air molecules, and they scatter all colours of light in roughly equal amounts. That is why clouds look white or grey.
Key points
- Sunlight is a mixture of colours, each with its own wavelength.
- Air molecules scatter blue light much more than red light.
- Sunsets look red because the light travels through more air and the blue is scattered away.
- Clouds look white because their droplets scatter all colours about equally.
