The Real Reason Why The Sky Is Blue

Have you ever looked up at the sky on a clear day and wondered "why is the sky blue?" While there's some complex physics behind it, the answer is actually more simple than you might think. As you probably already know, light from the Sun appears white, but is actually made of all the colors in the rainbow (via NASA Space Place). Prisms allow us to see these colors separated, because each color has its own wavelength. Blue and violet have shorter wavelengths, while red and orange have longer wavelengths.

Because blue and violet light have shorter wavelengths, they can get caught up more easily in the gas and particles in the atmosphere. When this happens, those wavelengths become scattered throughout the sky. The rest of the lightwaves — green, yellow, orange, and red — all go straight through the atmosphere and into our eyes. Because these colors are still all mixed together, the Sun still appears white, even though it is missing the violet and blue that are scattered in the sky. Though both blue and violet are caught up in the atmosphere, the sky appears blue because the human eye is more sensitive to blue than violet (via NOAA).

The physics phenomenon responsible for the scattering of blue and violet wavelengths in the sky is known as Rayleigh scattering. This concept was discovered by Lord Rayleigh in 1871, and was named by later physicists in his honor.

When it comes to the sky, Rayleigh scattering essentially states that because the blue light is scattered, an observer will see only that scattered light in all directions, except directly toward the Sun (via Britannica). Since the observer can only see the scattered light, and that light is blue, the sky looks blue. The atmosphere itself is not actually blue, but the scattered light makes it appear that way. This same phenomenon is also the reason the ocean looks blue.

Rayleigh's work with light has also helped physicists predict variations of the intensity of scattered light, including the discovery that there is symmetry in the patterns of scattered light. Rayleigh laws also help physicists calculate the polarization of scattered light.

Not only is Rayleigh scattering responsible for the blue sky during the day, it is also responsible for the beautiful orange hues of sunsets and sunrises. Shorter wavelengths are the first to get caught up in the atmosphere, but longer wavelengths can be scattered in the atmosphere if there are more particles and gas for the light to go through.

As the Sun gets lower in the sky, that light has to pass through more of the atmosphere in order to get to your eyes. This causes wavelengths like yellow and orange to be scattered across the sky, creating a beautiful sunset-colored sky. The same thing happens during sunrise. During a sunset or sunrise, the color of the Sun will appear orange or red, because almost all wavelength of light are scattered through the sky. This leaves only orange and red wavelengths that aren't scattered, which go directly to your eyes from the Sun (via NASA Space Place).