Melita Wiles
Science & Environment Editor
The atmosphere does so much to make our planet habitable for us. It provides us with oxygen, protects us from dangerous rays and traps heat, thus making the Earth a livable temperature. The atmosphere contributes to other things that we may take for granted as well like a beautiful clear blue sky or the breathtaking colors of sunsets at dusk.
Our world is made up of billions of particles, including dust, gases, water and other particles that linger throughout the atmosphere. Sunlight and the composition of our atmosphere contribute to the colors of the sky, including blue days, reddish-orange sunsets and white clouds.
Sunlight, which is a mixture of all colors, travels in the form of electromagnetic waves and passes through air, which causes electrons and protons in air molecules to wiggle up and down. These electrons and protons emit electromagnetic radiation at the same frequency as the incoming sunlight, but shoot off the electromagnetic radiation in many different pathways. This occurrence is called scattering because the radiation is dispersed in many directions. The faster the particles move, the more scattering occurs, and the blue part of the light causes more scattering.
The explanation behind this phenomenon comes from an idea in physics called the Rayleigh Law of Scattering. This law states that the intensity of light is inversely proportional to the fourth power of the wavelength. This scattering was first observed by Rayleigh in 1871 and thus named after him. Basically, the shorter the wavelength, the more scattering occurs and vice versa. From this law we can conclude that the shortest wavelength of color will scatter the most. Therefore, violet has the shortest wavelength, meaning that we should see a mostly violet sky. However, this is obviously not what we see so, why do we see blue instead of violet? The blue cones in our eyes are less sensitive to violet light, so we see mostly blue light. Therefore, we see the sky as blue, except during cloud coverage. Without this sensitivity, we would see a violet sky.
Sometimes the sky does change color. For example, during sunsets the sky is not entirely blue; it turns a reddish orange. The concept of scattering explains the color of sunsets as well. During sunset, sunlight travels a greater distance to reach Earth, causing it to pass through more particles. The scattering that occurs and creates blue light happens farther out in the atmosphere. In fact, it occurs so far out that we cannot see the blue. Thus, the non-scattered light that extends to us is red and orange, which makes the sunset.
Similarly, this idea of scattering can be applied to other planets as well. To people on Earth, the sky is usually blue, but on Mars, the common, everyday sky color is a reddish-brown or rust. On the other hand, Martian sunsets appear blue to earthlings because the location of the dust particles makes the color blue much more distinguishable. For other celestial bodies or planets and moons in outer space, the color of sunsets varies. The sunsets of Uranus begin blue and gradually transition to turquoise. Titan, one of Saturn’s moons, has a sunset that starts yellow to orange and finally turns brown.
The principle of scattering can be related to looking at light through a prism. When viewing white light through a prism, we see a multitude of colors shooting off the prism. Light that, at first glance, seems white, is actually made up of all the colors of the rainbow. Clouds are white because the sunlight hits different types of particles in areas where an abundance of water droplets are located. There are gas particles also but there are far more water particles (because clouds are made of water droplets). When sunlight hits water particles, they scatter different types of light. This is similar to the prism example, but in reverse, and therefore the color of clouds is white or close to white.
The next time someone asks, “Why is the sky blue?” or “Why are sunsets red?” you can explain that the sunlight interacting with the gaseous, light and dust particles in the atmosphere scatters electromagnetic radiation of mostly the same wavelength, which is blue, or in other words, because of particle scattering.
