SCIENCE FILE / An exploration of issues and trends affecting science, medicine and the environment

Q: Why are sunsets often red?

A: The distance light travels in the air before losing half its intensity--the “mean free path”--is about 25 miles. But that value only holds true for the middle of the optical spectrum: yellow light. Red light, which has a longer wavelength, can travel significantly farther before it is absorbed, while blue light, with its shorter wavelength, travels a much shorter distance, according to physicist Robert Ehrlich in “The Cosmological Milkshake.”

At noon, the sun appears yellow, its actual color, because the Earth’s six-mile-thick atmosphere is much shorter than the mean free path, and little light of any wavelength is absorbed. At sunset, however, when light passes through a much greater amount of atmosphere because of the light’s angle, blue--and to a certain extent yellow--light are preferentially absorbed and the sun appears red. Sunsets can also be made more colorful by dust in the atmosphere, as when a large volcano erupts.