Scattering

Earth's atmosphere contains many molecules and other tiny particles. These particles can scatter sunlight. Scattering means that light is redirected as it passes through a medium. Look at Figure 21. A scattering effect reddens the sun at sunset and sunrise. Most of the particles in the atmosphere are very small. Small particles scatter shorter-wavelength blue light more than light of longer wavelengths. The sunlight encounters more of the molecules and tiny particles that scatter the shorter-wavelength colors. By the time the sunlight reaches your eyes, most of the blue and even some of the green and yellow have been scattered. Most of what remains for your eyes to detect are the longer wavelengths of light, orange and red.

When the sun is high in the sky, its light travels a shorter distance through Earth's atmosphere. It scatters blue light in all directions much more than other colors of light. Scattering explains why the sky appears blue on a sunny day, even though air itself is colorless.

Figure 21 The lower the sun is on the horizon, the more of the atmosphere the light travels through before it reaches Earth's surface. In certain weather conditions, the blue, green, and yellow wavelengths of sunlight are heavily scattered. What's left to enjoy are the beautiful reds and oranges of sunrise and sunset.