Why Is the Sky Blue?
As I trudge through the knee-deep snow in the mid-December Vermont winters with the wind burning my face and the cold piercing through my clothes, my mind often wanders to warm, sunny days on the beach, breathing the heavy saltwater air and feeling the sand between my toes. Even if you don’t have the same vitamin D-deficient lifestyle, a dip in the saltwater is always enjoyable. Jumping through and over waves is always my favorite part of the day. In some sense, we overlook waves — most people will tell you that they are around “because of the moon,” but very people actually know what that means. Nevertheless, they’re cool. They’re not the only waves around though; sound waves, light waves, radio waves, microwaves, and others, are all vastly more abundant than the waves that you ride on a sunny day at the beach. However, they all essentially operate in the same way.
Waves are defined as disturbances that travel through some medium transporting energy from one location to another, without transporting matter. Think back to the ocean: when you travel out past the crashing waves and get to the calmer region, you’ll still feel smaller waves passing through you. You can feel them lift you up and put you down, but you don’t travel towards or away from the beach. Although you experience vertical motion, a kind of disturbance, your matter does not experience horizontal motion in the direction of the wave. That’s why you can float on your back, close your eyes, and open them a few minutes later in essentially the same place — you are the medium that energy is transported through.
Light waves are much smaller than water waves. You can’t see them individually, but you can clearly see what a bunch of them together looks like. Your eyes are finely tuned to pick up these signals, which respond to the waves by starting a process that results in cascades of signals being sent to the brain. It’s this precision in integrating light waves into a meaningful image that allows us to perceive the world around us without thinking of the underlying cognitive processes.
When we look at the sky, we often see blue, and the reason why lies in the fundamental physical properties of the different colors of light. The light that illuminates everything around us, which comes when the sun rises and leaves when the sun sets, is termed “white light.” White light consists of a mixture of all the wavelengths of the electromagnetic spectrum, meaning all the different colors of light that the sun emits. When you see a rainbow, you are seeing every color of light that combines to form “white light.” The different colors of the rainbow come from them having different wavelengths, and white light represents the amalgamation of all the different wavelengths into one bundle.
In reality, the ‘white’ of white light is very different from the white color of printer paper or snow. A more technical term for white light would be polychromatic light, which is light that consists of more than one color, but for simplicity’s sake, it has been described as white. This white light is emitted from the sun and travels the 93 million miles to the Earth (which takes about 8 minutes), where it hits the atmosphere.
The atmosphere is a thick blanket of gases that both contains the air we breathe and protects us from other types of radiation emitted from the sun and rocks that would hit us. It is mainly made out of nitrogen (78%), and oxygen (21%), along with smaller amounts of argon (0.93%), and carbon dioxide (0.04%). These gases, although invisible to us, are made of real particles of matter that create a physical barrier against the light emitted from the sun. Because of this, just as the atmosphere blocks the path of stray rocks that come in front of the Earth’s path, light cannot pass through it without being interrupted.
When sunlight reaches Earth’s atmosphere, it is scattered in all directions by the atmospheric gas particles. However, the amount of scattering that takes place is different depending on the wavelength of light. The wavelength of visible light ranges from around 380 nanometers to 720 nanometers (there are 1 billion nanometers in a meter), where blue light lies on the shorter, higher frequency side of the spectrum and red light lies on the longer, lower frequency side. This is important because the wavelength of light affects its interaction with the atmosphere and how we perceive the light. Simply put, the high energy, higher frequency blue light will bounce around more in the atmosphere and will scatter more down towards Earth. In comparison, a lower energy, lower frequency light such as red, will have a lower chance of bouncing down to Earth. You might think: wait, if the red light will bounce less, wouldn’t it just pass through the atmosphere right to us? That’s a good thought. However, that might only make sense if the sun was directly over us all the time. Think about it. When is the sun actually right above us? In the middle of the day, for a few minutes at most. If you look up at the sun then, you may see a reddish tint around it, which occurs because the red light drifts through the atmosphere easier and lands on us. However, most of the time, the sun is at a slight angle to us, and the light that reaches us has to pass through the atmosphere somewhat sideways. The higher wavelength light will pass through it and travel out to space, and the lower wavelength light will scatter greatly with the gas particles and have a better chance of bouncing down to Earth.
So that blue sky is a little more complicated than it might seem. When learning about it, the most interesting thing I had to wrap my mind around was that the blue sky, which seems like such a constant, isn’t really actually blue at all — the color of the sky is a product of the Earth’s atmospheric content. If the makeup of the atmosphere was thinner or thicker, the color of the sky would change to the color of the longer-wavelength light or shorter-wavelength light. However, we will always see it as the solid, brilliant blue color, so the next time you get on a beach, you can breathe the heavy saltwater air in, feel the sand between your toes, and be sure that you’ll look up and see a bright, blue sky.
