Light from the Sun is white light, containing 7 colors: red, orange, yellow, green, blue, indigo, purple. In that order they decrease with wavelength and increase with energy and frequency. Thus, violet light has the shortest wavelength, and the frequency and energy of violet light is the highest in the visible spectrum. In contrast, red light has the longest wavelength, lowest frequency and will carry the least energy.

The phenomenon of light scattering

Let’s explain the reason behind the colors of the sky – scattering. Scattering is the phenomenon in which atoms absorb light energy and scatter it in different directions with different intensities. Mathematically, the intensity of the scattered light is proportional to f⁴, where f is the frequency of the light. Thus, light of higher frequency is scattered more strongly.

Different colors of the sky

The type of scattering that occurs with light from the Sun to the Earth is Rayleigh scattering when light is bounced back from an object with a wavelength much smaller than its own.

When white light from the Sun reaches the atmosphere, it encounters nitrogen and oxygen molecules, and scattering occurs. Blue light of higher frequency will be scattered more so we will see blue sky. But why not indigo or violet? They have even higher frequencies right? The answer is due to the activity of the human eye in seeing color. If there is a blue point next to a purple point of equal brightness, our eyes will see the blue more clearly. Animals with other retinal structures such as birds, for example, will see the sky as purple.

The phenomenon of light scattering gives the sky wonderful colors.

So what about sunset red? As the sun begins to set, the light needs to travel a longer distance through the air before reaching the position you see. At this point, more light particles will be reflected. Although the amount of blue light is still scattered as during the day, it must pass through a thicker layer of air to reach the human eye. Only red and orange light with long wavelengths that are less scattered by the air is transmitted directly to the eyes creating a beautiful scene of the sunset.

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