ABCs of DEW Software chapter 8:
Why Is the Sky Blue?
The Sun produces white light, a combination of all of the colors of the rainbow. When we view sunlight through a prism, we bend the light so that it is separated into all of the colors. When sunlight shines through our atmosphere the same process occurs; it is bent and scattered by particles. Our atmosphere, because it is largely made up of nitrogen and oxygen, is most efficient at scattering blue wavelengths of light. What effect does this scattering have what we see in the atmosphere?
If you look back at what you discovered from experimenting with color, you saw that if we remove blue from our white light, the color that we are left seeing is yellow. So looking directly at the Sun, which we do not recommend, the Sun would theoretically have a yellowish hue. But the main reason the sky appears blue to us is not that the blue light is removed—it's that the blue light is scattered by the predominant molecules in air: nitrogen and oxygen.
Why Are Sunsets Red?
We all know that the sky is not always blue. At sunset and sunrise, the sky includes many more colors from throughout the spectrum.
Other colors of light are scattered by our atmosphere, but not as efficiently as the blue. When the Sun is high in the sky, its light has a shorter distance to travel through the atmosphere and the blue light is scattered. When the sun is low in the sky, the path is much longer, the atmosphere through which the sunlight has to travel is much thicker and even more of the blue light is scattered. The remaining reds, oranges and other longer wavelength colors are left to reflect off anything in the atmosphere especially clouds. It is particular arrangements and mixtures of cloud types that make the most vibrant sunsets is any observer who is lucky enough to see or takes the time to enjoy a memorable sunset.
Modern technologies let us monitor plant health by analyzing the proportions of light reflected from leaves. This helps us to gauge the quality of the environment. In some ways, plants are key indicators of environmental quality: if they die, then other organisms will likely follow. By measuring the health of plants, we are measuring the environmental conditions that affect all nearby organisms, including humans.