Rayleigh Scattering

Introduction

Rayleigh scattering, named after the British physicist Lord Rayleigh, is a type of scattering that occurs when the size of the particles or the wavelength of the light makes the scattering medium or obstacles irrelevant. It is most commonly used to explain why the sky appears blue.

The Principle of Rayleigh Scattering

Rayleigh scattering is a specific type of scattering that occurs when the size of the particles () in the medium is much smaller than the wavelength of the light () being scattered. The condition is typically defined as .

The scattered light can be observed in all directions, and the intensity of the scattered light () varies with the fourth power of the frequency or, equivalently, inversely with the fourth power of the wavelength. This is expressed by the Rayleigh scattering formula:

Rayleigh Scattering and the Blue Sky

Rayleigh scattering is responsible for the blue color of the daytime sky. Light from the Sun must pass through Earth’s atmosphere before it reaches our eyes. As it passes through the atmosphere, it is scattered in all directions by the oxygen and nitrogen molecules in the air.

Because Rayleigh scattering favors short wavelengths, blue and violet light are scattered in all directions across the sky. However, our eyes are more sensitive to blue light and much less sensitive to violet light. Furthermore, some of the violet light gets absorbed in the atmosphere, then gets re-emitted in other directions (a process called fluorescence), which further shifts the light towards blue. This results in a blue sky.

Rayleigh Scattering in Other Phenomena

Rayleigh scattering is also responsible for other optical phenomena, such as the reddish color of sunrises and sunsets. When the Sun is near the horizon, sunlight has to pass through more atmosphere than when the Sun is high in the sky, so most of the short-wavelength blue and green light is scattered out of the line of sight, and the longer-wavelength red and yellow light is transmitted directly to the observer.

Conclusion

Rayleigh scattering plays a fundamental role in various natural phenomena we observe, such as the color of the sky. Understanding the principle behind Rayleigh scattering not only allows us to appreciate these natural occurrences but also applies to numerous applications in science and engineering, such as in optical fibers and lidar technologies.

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