Rayleigh scattering
Rayleigh scattering is a phenomenon that describes how light is scattered by small molecules in the atmosphere, specifically particles that are about one-tenth the size of the wavelength of the light. This scattering effect is primarily responsible for the blue color of the sky, as shorter wavelengths of light, such as blue and violet, are scattered more efficiently than longer wavelengths like red. Despite violet light being scattered even more, the human eye is more sensitive to blue light, which is why the sky appears blue rather than violet.
The term Rayleigh scattering is named after British physicist Lord Rayleigh, who established a mathematical formula in the 19th century to describe the scattering process. As sunlight travels through the atmosphere, it interacts with nitrogen and oxygen molecules, causing shorter blue wavelengths to diffuse and create the blue sky we observe. During sunrise and sunset, when the Sun is lower in the sky, its light encounters a greater thickness of the atmosphere, scattering the shorter wavelengths and allowing the longer red and orange wavelengths to dominate, resulting in the warm hues often seen during those times.
Factors such as air pollution, humidity, and seasonal changes can also influence the color and intensity of sunsets, with particles in the atmosphere enhancing the scattering effect and altering the colors displayed. Overall, Rayleigh scattering plays a crucial role in our perception of sky color and the beauty of natural light phenomena.
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Rayleigh scattering
Rayleigh scattering refers to the dispersal, or scattering, of light off tiny air molecules about a tenth the size of the wavelength of the light. The effect is what gives the sky its blue color, and it is responsible for the deep reds and oranges observed at sunset. Rayleigh scattering was first observed in the nineteenth century and was named after a British physicist who calculated its mathematical properties.
Background
Light is a form of energy also called electromagnetic radiation. Light is formed when a subatomic particle called an electron is accelerated to a higher energy state. When the electron returns to its original state, it releases energy in the form of an oscillating electric and magnetic field. This field travels as a particle of light energy called a photon. Photons move at incredibly fast speeds—about 186,000 miles per second (299,300 kilometers per second)—a figure also known as the speed of light.
A photon of light moves in the form of waves, with rising and falling peaks and valleys, similar to the waves on a body of water. The measure of distance between each wave peak is called the wavelength. Scientists measure wavelengths in meters; the longest wavelengths stretching over kilometers and the shortest measured in trillionths of a meter. Light waves with shorter wavelengths contain more energy, while those with longer wavelengths have less energy.
The full range of light energy is known as the electromagnetic spectrum. Most of the electromagnetic spectrum consists of light that is invisible to the human eye. At one end are longer wavelength radio waves and microwaves; at the other end are shorter wavelength X-rays and gamma rays. The light capable of being seen by humans is known as visible light and falls within a narrow band of the electromagnetic spectrum. The visible light spectrum is separated by wavelengths into red, orange, yellow, green, blue, and violet. Red light has the longest wavelengths at about 620 to 750 nanometers (nm)—a nanometer is a billionth of a meter—and violet has the shortest at about 380 to 450 nm.
Overview
Seventeenth-century British physicist Sir Isaac Newton was the first to observe that white light can be scattered into the colors of the visible light spectrum by passing it through a glass prism. In 1859, Irish physicist John Tyndall discovered that when light passed through a clear fluid containing small particles, the light seen from the side appeared blue, while the light that passed through the fluid appeared red. The Tyndall effect, as it was called, demonstrated that shorter wavelength blue light was scattered more strongly than the longer wavelength red light.
In 1871, British physicist John William Strutt, also known as Lord Rayleigh, devised a mathematical formula showing that the percentage of light scattered was inversely proportional to the fourth power of the wavelength. This meant that blue light encountering a particle at least one-tenth the size of its wavelength would scatter far more efficiently than a red light. For example, blue light with a wavelength of 400 nm would be 9.4 times more likely to be scattered than red light with a wavelength of 700 nm. The phenomenon was later named in Lord Rayleigh's honor.
Earth's atmosphere is predominantly made up of the gases nitrogen and oxygen. The molecules of these gases are very effective at scattering the shorter wavelength light at the violet and blue end of the visible spectrum. Scattering occurs when an atom first absorbs a light wave and then retransmits that wave in several directions. As light from the Sun enters the atmosphere, it encounters nitrogen and oxygen molecules. The molecules scatter more of the violet and blue light waves, diffusing them throughout the atmosphere and making the sky appear blue.
Humans perceive color based on the light wavelengths transmitted from an object to the eye. For example, a green plant contains chlorophyll molecules that absorb most visible light wavelengths except for green, which is reflected. For that reason, plants appear green to the human eye. While violet light is more easily scattered by the atmosphere, the eye is more sensitive to blue light waves, which is why the sky appears blue rather than violet-tinted. Rayleigh scattering is more pronounced when the Sun is at an angle, which is why the sky appears to be a deeper shade of blue when looking away from the Sun.
Red, orange, and yellow light waves are more likely to pass through the atmosphere without scattering. As a result, the midday Sun appears yellow because more of those light waves are able to reach the human eye. As the Sun nears the horizon at sunset and sunrise, its light must pass through considerably more of Earth's atmosphere than during the day. Light with violet and blue wavelengths have about forty times more air molecules to travel through. These wavelengths can be absorbed and redirected multiple times, scattering away in all directions.
The red and orange wavelengths that are left over make their way through the atmosphere, giving the Sun and the surrounding sky a red and orange glow. This effect is more pronounced if the atmosphere contains a greater number of small particles. For example, pollution or the smoke from forest fires can often give the sky a deeper reddish tint. Hot and humid weather is more likely to trap smoke, pollution, or water droplets in the atmosphere, creating summer haze and redder sunsets. In the fall and winter, cooler air with fewer foreign particles allows for more light to pass through the atmosphere, resulting in more colorful pink, yellow, orange, and red sunsets.
Bibliography
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