Why is the Sky Blue? The Simple Answer

What if I told you that what our eyes show us isn’t entirely accurate? Sounds surprising, doesn’t it? But even though it seems unbelievable, this is true in certain cases. If you’re curious to know more about this interesting fact, keep reading till the end. As a curious person, you’ve probably wondered at some point, why is the sky blue? And when the weather is clear, it’s natural to think that the sky should be blue. But that’s not the whole truth. Despite the rainbow having so many colors, why is it that we only see blue in the sky?

We all know that sunlight consists of VIBGYOR colors (Violet, Indigo, Blue, Green, Yellow, Orange, and Red). But why do we only see the color blue in the sky? The year was 1665, and Sir Isaac Newton was a junior scientist studying at Cambridge University. At the time, his primary interest was in colors and light.

One day, Sir Isaac Newton returned home from the university, shut all the doors and windows, and made his room completely dark. He knew there was a small hole in the window shutter. Through that hole, a small beam of sunlight entered the room. Newton placed a prism in the path of the sunlight.

As the sunlight passed through the prism, he observed that the light took on a new form, which looked very much like a rainbow in the sky. This multi-colored band of light is what we now know as the “spectrum.”

However, humans have been seeing this spectrum of light for centuries, which we recognize as the rainbow. But the sad part is that people in Newton’s time were not ready to accept the rainbow scientifically. The people of that era believed, based on religious teachings, that the rainbow was a sign of God’s promise. They believed that the rainbow meant God would never again bring a flood to the Earth. This belief is mentioned in the Bible.

When Newton’s theory was presented to the world and scientifically proven, people’s misconceptions gradually began to fade. The reason for the sky’s blue color was also hidden in Newton’s discoveries. So, let’s start from the very beginning. We all know that energy or electromagnetic waves from the sun reach Earth in the form of light. The components of this light with the shortest wavelengths are absorbed by the ozone layer, while the components with longer wavelengths enter the Earth’s atmosphere.

Now, let’s learn an important fact about the VIBGYOR colors. There is something called “wavelength,” which increases from violet to red. How? The wavelength of violet light is around 400-440 nm (nanometers), while red light has a wavelength of about 620-720 nm.

As sunlight enters Earth’s atmosphere, it encounters nitrogen and oxygen molecules and naturally begins to scatter.

The shorter wavelengths of light scatter as soon as they enter the atmosphere, and this is why we see the sky as blue. But now the question arises: if violet has the shortest wavelength and scatters the most, why don’t we see the sky as violet? Why do we see it as blue? 

There are two main reasons! The first reason is related to our eyes. Our eyes have a type of photoreceptor in the retina called melanopsin, which is more sensitive to blue light. (Photoreceptors are cells in the retina that respond to light.) When blue light, around 480 nanometers, is intense, our eyes can easily detect it.

The second reason is that, among violet, indigo, and blue light, blue light has the highest intensity. That’s why we perceive the sky as blue.

Now, let’s dive into some history. In the 19th century, there was a renowned scientist named John William Strutt, also known as Lord Rayleigh. He conducted research on various physics topics, such as color vision, electrodynamics, electromagnetism, fluid flow, hydrodynamics, light scattering, sound, wave theory, and gas density.

One of his most notable discoveries was “Light Scattering,” which explains why the sky appears blue. His work has helped us understand the true reason behind the blue color of the sky.

Lord Rayleigh published his research in 1871 in a paper titled “On the Light from the Sky, Its Polarization and Colour.” In this paper, he explained a formula that describes how the intensity of scattered light depends on its wavelength. What we discussed earlier was essentially what he discovered, but in his research, he used Cartesian coordinates.

Cartesian coordinates refer to a system that determines the position of points using perpendicular axes, where the point of intersection is called the “origin.” Simply put, it means he used a map of XYZ coordinates, representing height, length, and width. This coordinate system is widely used in science and mathematics to determine the position of objects or points.