October 2000
Almost everyone admires the lovely blue sky displayed on a crisp, cool autumn day. As early as 1500, Leonardo da Vinci tried to explain the sky’s color. His notebooks show he monitored sunlight passing through wood smoke, suggesting he understood the basic phenomenon of light scattering. Although the complete physical explanation for the blue sky is complex because of length limitations we will focus on the primary mechanism.
In the 1870’s the English scientist, Lord Rayleigh, weighed in with part of the explanation. But to understand his rationale, we need to know two things about how we perceive light. First, all colors of visible sunlight from the short-wave violet to blue, green, yellow, orange, and the long -wave red, are emitted by the Sun, yet not in equal amounts. The predominant color in the visible spectrum is blue. Second, our eyes detect green light better than the other colors and they perceive blue light pretty well. Mixed together, all the colors of visible light appear white to us.
Imagine a cork at rest on the surface of a tranquil pond. A stone is then dropped into the pond, sending out waves that cause the cork move up and down. The cork receives wave energy and then oscillates up and down, and in a similar manner light waves can interact with air molecules. However, the air particles do not keep the energy from light waves that fall upon them, but quickly re-radiate that light energy in random directions. Hence the initial light that was coming from a specific direction is now scattered (re-radiated) in all directions.
This scattering depends upon particle or molecule size and the wavelength of the incident light. Scattering is proportional to one divided by wavelength to the fourth power. The long waves of sunlight (red) are less effectively scattered than the shorter ones (blue) by the small air particles in our atmosphere. Since red light has a wavelength (700nm) about 1.7 times greater than blue light (400nm) Rayleigh's findings show that blue light has about 9 times greater chance of scattering than red light. This scattered blue light goes out in all directions through the atmosphere and comes to us from throughout the sky during the day.
However, when the sun is near the horizon sunlight must pass through a thicker amount of atmosphere than when it is overhead. As the light travels the longer distance through the atmosphere most of the blue light gets scattered out and the light that remains has proportionally more orange, leading to the beautiful sunrises and sunsets we are often blessed to see. These scattering effects are not however constant. Lord Rayleigh’s explanation ignores the effect of water vapor, dust particles, ozone, chemical pollutants, and eye response. All of these mechanisms can enhance or diminish the beauty or color of a sunrise or sunset.
In space or on the Moon there is no atmosphere to scatter light. The light from the sun travels a straight line without scattering and all the colors stay together. Looking toward the sun we thus see a brilliant white light while looking away we would see only the darkness of empty space. Since there is virtually nothing in space to scatter or re-radiate the light to our eye, we see no part of the light and the sky appears to be black.
As an avid enthusiast and expert in atmospheric optics and the physics of light, I bring a wealth of knowledge to shed light on the intriguing phenomenon of the blue sky, a subject that has captivated minds for centuries. My understanding extends beyond the mere appreciation of the azure expanse above; it delves into the intricacies of light scattering, an essential component in unraveling the mystery of the sky's color.
In the realm of scientific exploration, the journey to decipher the blue sky's enigma began long ago. In October 2000, the article references the endeavors of the renowned polymath Leonardo da Vinci, who, as early as 1500, engaged in experiments involving sunlight passing through wood smoke. This insightful exploration demonstrated da Vinci's grasp of the fundamental principles of light scattering, laying the groundwork for subsequent discoveries.
Fast forward to the 1870s, where the English scientist Lord Rayleigh made significant contributions to the explanation of the blue sky. To appreciate Rayleigh's insights, it's imperative to comprehend two key aspects of human perception of light. Firstly, sunlight emits a spectrum of colors, with blue being the predominant hue. Secondly, our eyes exhibit varying sensitivities to different colors, detecting green light more effectively than others, and perceiving blue light quite well.
Now, let's delve into the core mechanism that gives rise to the captivating blue sky. Picture a tranquil pond with a cork at rest on its surface. When a stone disrupts the calm, waves propagate, causing the cork to oscillate. Analogously, light waves interact with air molecules, inducing a phenomenon known as scattering. However, unlike the cork retaining wave energy, air particles swiftly re-radiate light energy in random directions. The extent of this scattering depends on particle or molecule size and the wavelength of the incident light.
Here, Lord Rayleigh's groundbreaking findings come into play. His work established that the longer wavelengths of sunlight, such as red light, are less effectively scattered than shorter wavelengths, like blue light. Rayleigh's scattering equation, which is proportional to one divided by the wavelength to the fourth power, reveals that blue light has approximately nine times greater chances of scattering than red light.
As sunlight traverses the Earth's atmosphere, the scattered blue light permeates the sky in all directions, creating the mesmerizing blue canopy we observe during the day. However, the dynamics change when the sun is near the horizon during sunrise or sunset. The longer atmospheric path causes increased scattering of blue light, allowing orange hues to dominate the remaining light, resulting in the breathtaking colors of dawn and dusk.
Yet, this narrative wouldn't be complete without acknowledging the complexities introduced by factors such as water vapor, dust particles, ozone, and chemical pollutants. These elements can either enhance or diminish the beauty and color of a sunrise or sunset, influencing the scattering effects.
Moreover, the article touches upon the absence of atmospheric scattering in space or on the Moon. In such celestial realms, light travels unimpeded in a straight line, maintaining its pristine composition. Consequently, when gazing toward the sun, we perceive brilliant white light, while looking away reveals the blackness of empty space, devoid of the scattering effects that adorn our terrestrial skies.
In essence, the captivating tapestry of the blue sky is woven through a symphony of scientific principles, from the meticulous observations of da Vinci to the groundbreaking revelations of Lord Rayleigh. As we marvel at the celestial canvas above, we glimpse the intricate dance of light and atmosphere that orchestrates the ever-changing masterpiece of our sky.
