Why do sunsets appear orange and red rather than other colors, and how does this relate to the way light interacts with the atmosphere and other materials?

-Karthik Gurumurthy

I’ve always been fascinated by sunsets and their beautiful orange hues. The reason behind this colorful phenomenon comes down to basic physics of light and our atmosphere.

When the sun is setting, its light must travel through much more of our atmosphere than when it’s directly overhead. This extended path through the atmosphere makes all the difference in what colors reach our eyes.

What happens is a process called scattering. As sunlight passes through the atmosphere, the shorter wavelengths of light (blues and violets) get scattered away more easily by air molecules. The longer wavelengths (reds and oranges) are more resilient and can penetrate through the thicker layer of atmosphere. So by the time sunset light reaches us, most of the blue and green light has been scattered away, leaving primarily the warm red and orange hues visible.

This selective scattering explains more than just sunsets. It also helps us understand why things like clouds, milk, powdered sugar, and salt appear white. In these materials, the particles responsible for scattering are larger than the wavelength of light, so all colors of light are scattered equally. In the case of milk specifically, much of the scattering comes from lipids (fat). That’s why skim milk, which has had the fat removed, doesn’t scatter as much light and has a different appearance.

Nature’s physics gives us these beautiful displays at the end of each day, reminding us how the interaction between light and matter creates the colorful world we experience.