Ask a Physicist Answers

Why is the sky blue? CL, Hong Kong

blue skyOur atmosphere creates the blue sky. If there were no air, the sky would always be jet-black and all you would see when you looked up is the stars. Even during the day, the closest star—our sun—would appear as a brilliant blue-white circle surrounded by darkness.

But air exists and it’s this air that gives the sky its blue glow. The glow is created when sunlight tries to pass through the air particles on its way to the earth’s surface. Some of the sunlight, particularly bluish light, is redirected by those air particles and reaches your eyes from angles at which no stars are present. When you look at the sky, you see this redirected blue light coming at you from all directions and note that the sky appears blue.

To understand why the air redirects primarily blue light, we have to look at the physics of light interacting with matter. Sunlight consists of countless tiny electromagnetic waves, each with an approximate frequency and wavelength, and each with a small amount—a quantum—of energy. Because of the discreteness of their energies, these basic electromagnetic waves have many particle-like properties and are known as photons of light.

The energy of a photon of sunlight determines its color, with higher-energy photons appearing at the blue end of the light spectrum and lower-energy photons appearing at the red end. The bluish photons also have higher frequencies and shorter wavelengths than the reddish photons. These differences in wavelength are responsible for the blueness of the sky.

Electromagnetic waves consist only of electric and magnetic fields, and they interact with matter by exerting forces on the charged particles in that matter. Through these interactions, matter can absorb a light wave and may then reemit it in a new direction, a process known as Rayleigh Scattering. That is just what the air particles do. In effect, the air particles act as antennas for light, absorbing light heading in one direction and reemitting it in another.

But an antenna’s efficiency for absorbing or emitting electromagnetic waves depends on its length. An ideal antenna is about a quarter as long as the wavelength of the light it handles. Although the wavelengths of visible light are quite short, they aren’t nearly as tiny as the particles in air—individual gas molecules, clumps of water molecules, microscopic dust grains, and even local density fluctuations. As a result, these particles make poor antennas for sunlight and most sunlight passes through the atmosphere unaffected. But some sunlight is redirected during its flight, particular that light which has the shortest wavelength and thus uses the air particles most effectively as antennas. This shortest wavelength light is at the blue end of the spectrum. Since bluish light Rayleigh scatters more effectively from air particles than reddish light, most of the redirected sunlight is bluish. That’s why the sky has a blue glow during the day.

Answered by Louis A. Bloomfield of the University of Virginia