Why Is the Sky Blue?

On a clear sunny day, you can look up into the sky and see the beautiful blue color ... did you ever wonder why is the sky blue? "Sky blue" has become synonymous with that famous color of the atmosphere and is the name for paint colors, crayons and numerous other colors that try to capture the color of the sky... but where does that color come from? There are a few reasons that you see the color you see when you look up at the sky.

To understand why is the sky blue, it is important to look at some basic information that can help you understand how your eyes see color.

Sunlight is made up of different colors of light. Newton made this discovery when he passed a beam of light through a prism. Red, orange, yellow, green, blue and violet can all be split out of a beam of bright light. Each color of light has a specific wavelength and frequency associated with it. For example, red light has a long wavelength and smaller frequency, while blue light has a shorter wavelength and a higher frequency.

The light that reaches your eyes travels through the atmosphere. The atmosphere is primarily composed of molecules of oxygen and nitrogen. When light reaches your eyes, it interacts with the rods and cones in the retina. The rods distinguish white and black or shades of gray. The cones are responsible for color vision. 

Most people begin to learn the principles of color in elementary school or earlier, often by using finger paints. When painting, mixing one color with another one will create a new color. Red and violet create purple, yellow and blue make green and red and yellow make orange. This type of color recognition is based on reflective color perception. If you take solutions of different colors and mix them together, you will perceive very different results. The color you perceive is based on complementary color principles. The reason you perceive a color in a solution is that the solution is selectively absorbing more light of a particular wavelength. A solution that appears red is absorbing a greater quantity of light in the wavelength region that is red’s complementary color, green. Red and green are complimentary colors, as are blue and orange or violet and yellow.

All of these factors contribute to understanding why is the sky blue.

Why Is the Sky Blue?

Light from the sun shines on the Earth and passes through the atmosphere before you see it with your eyes.

As the light passes through the atmosphere, the spectrum of light experiences Rayleigh scattering. This occurs when light passes through a medium that has small particles in it. The scattering effect is stronger for shorter wavelength light, which means that longer wavelength light can pass through in greater quantities. Shorter wavelength light is towards the blue end of the spectrum.

In the eye of the beholder, the cones respond best to red, green and blue light. If the light reaching the eye is disproportionately toward the blue end of the spectrum, then the color perceived by the eye is made even bluer by this response from the cones.

All of these effects work together, and the result is that the white light from the sun is filtered by the atmosphere to remove a larger portion of the blue wavelengths. As the blue light is scattered to a greater extent, your eyes respond to the light waves by registering larger amounts of blue light. This is why you see the sky as being blue.

These same principles can also be used to explain why the sunset is red. The light from the sun travels to your eye by passing through a greater volume of atmosphere where many more scattering interactions occur. The increased scattering thus reduces the amount of blue reaching your eye. The light in the yellow, orange and red wavelengths have fewer scattering interactions, allowing more of the red end of the spectrum to reach your eye. This means the sunset will look red.

Using the principle of physics, common everyday observations of the physical world can be explained. Applying the knowledge that you learn is extremely useful in being able to make sense of the world around you.