Rayleigh effect

rayleigh scattering

Throughout history, human beings have felt a deep admiration for the sky, not only during contemplative nights that provoke existential reflections, but also during daylight hours, when it presents a vibrant spectrum of colors. At some point in our lives, we have all wondered why the sky appears blue or why it turns orange and red tones during sunset. This question was initially resolved by Lord Rayleigh, also known as John William Strutt, a mathematician who made this discovery in the late 19th century.

In this article we are going to explain to you the Rayleigh effect, its characteristics and why the sky is blue.

Rayleigh effect

rayleigh effect explained

The Sun emits a wide range of electromagnetic radiation, including visible light, commonly known as white light. Interestingly, white light is actually a combination of all the colors of the rainbow, with violet being the shortest wavelength and red being the longest. As Sunlight travels through the atmosphere, interacting with various substances such as gases, solid particles and water molecules. When these particles are smaller than a tenth of a micrometer, they cause the scattering of white light in all directions, with greater emphasis on blue light.

This preference for blue light can be explained by the dispersion coefficient, which is calculated by the formula 1/λ4, where λ represents the wavelength. Since violet and blue light have the shortest wavelengths in the visible spectrum, they produce the highest ratio when substituted into the formula, which leads to a higher probability of dispersion. This phenomenon is commonly known as Rayleigh scattering.

As a result, the scattered rays intersect with the gas particles that function as a reflective surface, causing them to bend once again and amplify their strength.

Why is the sky blue?

rayleigh effect

Considering the information mentioned above, one might expect the sky to appear purple instead of blue due to its shorter wavelength. However, this is not the case because the human eye is not very sensitive to the color violet. Besides, Visible light actually contains a higher proportion of blue wavelength radiation than violet.

In cases where particles exceed the wavelength in size, differential scattering does not occur. Instead, all components of white light are equally dispersed. This phenomenon explains the white appearance of the clouds, since the water droplets that make them up exceed a tenth of a micrometer in diameter. However, when these water droplets become densely compacted, light cannot pass through them, resulting in a grayish appearance associated with extensive cloud cover.

However, it must be recognized that the sky does not maintain a constant blue hue. As a result, the phenomenon of Rayleigh scattering does not fully explain the presence of various shades of red during sunrise and sunset. However, there is an explanation for this fact.

As the Sun sets and enters the twilight phase, its position on the horizon causes the light to travel a greater distance to reach us, no longer being perpendicular. This change in angle results in a lower incidence, causing the blue light to disperse before reaching our eyes. Instead, Longer wavelengths dominate, manifesting as reddish tones. It is important to note that Rayleigh scattering continues to occur, but at a different location within the atmosphere where the Sun is at its zenith.

History

lord rayleigh

Throughout history, the sky has captivated our attention both during the day and at night. It has served as a canvas for our imagination to wander. Naturally, curiosity and scientific research have not been exempt from this fascination. As with other everyday phenomena, such as the changing color of leaves or the origin of rain, researchers have tried to discover the mysteries of the sky. Rather than diminishing its mystical appeal, its discoveries have only deepened our understanding and admiration.

During his infrared experiments in 1869, Rayleigh stumbled upon an unexpected finding: the light scattered by tiny particles had a subtle blue tint. This led him to speculate that a similar scattering of sunlight was responsible for the blue color of the sky. However, he could not fully explain why blue light was preferred or why the color of the sky was so intense, ruling out atmospheric dust as the only explanation.

The innovative work of Lord Rayleigh on the color and polarization of light from the sky was published in 1871. Their objective was to measure the Tyndall effect in water droplets by quantifying the presence of small particles and the refractive indices. Building on James Clerk Maxwell's earlier proof of the electromagnetic nature of light, Rayleigh showed in 1881 that his equations were derived from electromagnetism. Expanding on his findings in 1899, he extended the application to individual molecules, replacing terms related to particle volumes and refractive indices with terms of molecular polarizability.

Dispersion in porous materials

Porous materials have the ability to exhibit Rayleigh-type scattering, which follows a λ-4 scattering pattern. This phenomenon is particularly evident in nanoporous materials, where there is a significant contrast in refractive index between the pores and the solid portions of sintered alumina. As a result, the Light scattering becomes incredibly intense, causing it to change direction approximately every five micrometers.

This remarkable dispersion behavior is attributed to the unique nanoporous structure achieved through the sintering process, which involves the use of monodispersive alumina powder to create a narrow distribution of pore sizes, typically around 70 nm.

I hope that with this information you can learn more about the Rayleigh effect and its characteristics.


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