Is the Sky Blue?

By Jaron Bobay

Many people ask, “Why is the sky blue?” and the answers are usually uncertain. In actuality though, it has been known for 150 years. John William Strutt, better known as Lord Rayleigh, was a 19th-century physicist from Essex. In 1871, two papers he wrote were published, explaining the phenomenon of Rayleigh Scattering, the answer to our infamous question. So, what exactly is Rayleigh Scattering?

In the world of physics, photons are particle-like electromagnetic fields that carry energy as light. When these light photons hit gas particles in our atmosphere, the wavelength of the waves these photons carry shifts from the initial position and scatter the light in every direction around. The shifts seen in the sky cause the light to become an ultraviolet color due to the interactions the light has with oxygen and nitrogen atoms of the atmosphere. For our eyes, the ultraviolet wavelength is not within the visible spectrum of light, but the closest wavelengths we can see to the ultraviolet light are shades of blue. From this, the light in the upper atmosphere shifts to scatter the remotely blue wavelengths and create a blue sky during the day.

Rayleigh scattering does not only cause this colorful effect in the sky, but also leads to the red, orange, and pink colors observed during the sunsets and sunrises each day. The same light scattering in the sky that makes it appear blue during the day makes the blue light continue to scatter on each of the particles the light must travel through. The blue light scatters out as the light travels farther, where the longer wavelength light goes through the air to meet our eyes.

Now, this effect is not just a natural occurrence. In the last twenty years, programs like NASA (National Aeronautics and Space Association) in the United States have produced studies by using Rayleigh Scattering to observe laser interactions.

So, the next time someone asks the infamous question, you can think of John William Strutt, 3rd Baron Rayleigh and give them an honest, fact-based answer to shine some light on the reason behind blue skies.