Everybody knows the color of the sky in a sunny day and how beautiful sunsets are. Have you ever asked yourself: Why?
What are the lights we see?
To answer this question, we have to understand what are the lights that humans see. As mentioned in my previous post Different Lights in Life, visible light is actually a combination of many different lights with wavelengths ranging from 400-700 nanometers. These lights exhibit different properties because of their wavelengths.
One great example is when lights travel through a prism, they will be deflected by the prism at different angles, because of their wavelengths.
This spectrum of colors produced from the prism are ordered based on their wavelengths. The table below shows these color and their wavelengths, although the spectrum is continuous with no clear boundaries between these colors.
| Color | Violet | Blue | Cyan | Green | Yellow | Orange | Red |
| Wavelengths (nm) | 380–450 | 450–485 | 485–500 | 500–565 | 565–590 | 590–625 | 625–740 |
Light Scattering
There are lots of small particles in Earth’s atmosphere, such as dust and droplets of water vapour. When light travels through them, it bounces off them and gets scattered in lots of different directions.
Depending on the light wavelengths compared with the particle size, this scattering effect changes. Scientists call this Rayleigh Scattering and the graph below shows the percentage of light get scattered depending on wavelengths.
We can see that the shorter the wavelength is, the stronger the scattering is. Blue light is more strongly scattered than other longer-wavelength lights like yellow and green. Hence the result that when we look up the sky is blue.
But what about violet? It has the shortest wavelength and should be scattered even more, right? Well, there is less violet in sunlight than blue. Also our eyes are less sensitive to violet. Otherwise we’ll see a violet sky on a sunny day.
Red sunsets
At sunset, the sunlight must travel a long path through the atmosphere to reach us. Most shorter-wavelength lights (blue) have been scattered and removed. Only the longer-wavelength lights (red, orange) are left to be seen. The result is that the sunlight takes on a red or orange cast.