定义：
一种光的散射，散射中心远小于波长。
瑞利散射是一种很常见光学现象，是以英国物理学家瑞利伯爵命名的。它是光的线性散射，散射中心远小于光的波长。
在这种情况下，散射光振幅正比于入射光振幅、波长倒数的四次方和1 + cos2 θ，其中θ是散射角。前向和后向散射（分别为θ = 0和θ = π）通常相等。
散射中心更大时的散射可由Mie散射理论描述（以Gustav Mie命名）。这时散射特性与瑞利散射不同，例如，前向散射的振幅更大，并且与波长的关系也不同。
瑞利散射中心可以是单个原子或分子。也可以描述空气中的瑞利散射，是由微观密度的涨落引起的，而围观密度涨落则是来自于空气中分子的随机分布。
需要注意的是，考虑多个粒子或散射中心的散射时，不能简单的将单个中心散射的功率简单相加，因为它们存在干涉效应：需要将振幅叠加。
因此，在完全纯净和规则的晶体中不存在瑞利散射。并且，空气中的瑞利散射可能只是来自于如上所述的随机密度涨落。
光纤中的散射损耗
在一些非晶材料例如二氧化硅中，由于其微观结构的不规则，总是存在随机的密度涨落。并且在室温时涨落比预想的要强，由于在光纤制备过程中，玻璃软化温度附近的密度涨落被冻结的缘故。
瑞利散射对光纤的传播损耗起了一定的限制作用。不规则的纤芯/包层界面（尤其是折射率差很大时）、杂质的散射和吸收、宏观和围观弯曲都会引起附加损耗。
经过优化的应用于光纤通信中的二氧化硅光纤具有很低的传播损耗，接近于瑞利散射的极限值。当波长远低于常采用的1500 nm时，单单瑞利散射本身都大于光纤在1500nm波长处的总损耗。
而对于很长的波长时，瑞利散射会更弱，但是这时二氧化硅会吸收红外光。
理论上可以采用其它玻璃（例如，氟化物光纤）制备中红外光纤，并且具有更低的损耗，但是实际中二氧化硅光纤已经达到了最好的性能。
光纤中大部分的瑞利散射光都会从光纤的另一侧逸出，只有一小部分的散射光被散射回来并且在光纤纤芯中传播。
因此，光纤装置的回波损耗比较大；光纤装置的总回波损耗常常是由界面处的反射引起的，例如光纤端面，机械焊接点或者光纤连接器。
光纤中由于光强很高，所以存在非线性相互作用，例如拉曼散射和布里渊散射。瑞利散射是一种线性过程，即使在低光强时也很重要。
Definition: scattering of light at scattering centers which are much smaller than the wavelength
More general terms: scattering
Rayleigh scattering is a common scattering optical phenomenon, named after the British physicist Lord Rayleigh. It is linear scattering of light at scattering centers which are much smaller than the wavelength of the light. Under such circumstances, the scattering occurs with intensities which are proportional to the in-coming optical intensity, to the fourth power of the inverse wavelength, and to 1 + cos2 θ, where θ is the scattering angle. Forward and backward scattering (θ = 0 and θ = π, respectively) are equally strong.
Scattering at larger centers can be described by Mie scattering theory (named after Gustav Mie). Here, the characteristics are different; for example, the scattering amplitudes are stronger for forward scattering, and the wavelength dependence is different.
Scattering centers for Rayleigh scattering can be individual atoms or molecules. However, one can also describe Rayleigh scattering in the atmosphere, for example, as resulting from microscopic density fluctuations, which are caused by the random distribution of molecules in the air.
Note that for scattering at multiple particles or scattering centers, one cannot simply add the powers scattered by individual centers, as there are interference effects: amplitudes must be added. As a result, there would be no Rayleigh scattering of light in a perfectly pure and regular crystal. Also, Rayleigh scattering in air is possible only due to the above-mentioned random density fluctuations.
If the scattered light is assumed to be lost, the scattering effectively contributes to propagation losses. For example, in case of a single-mode fiber any scattered light will end in cladding modes and will effectively be lost.
Scattering Losses in Optical Fibers
In amorphous optical materials such as silica glass, there are always random density fluctuations due to the irregular microscopic structure. These are even substantially stronger than they would normally be at room temperature, because during fiber fabrication, the density fluctuations which occurred for the fiber near the glass softening temperature are “frozen in”. Only to a limited extent, the fluctuations can be reduced by an annealing process.
Rayleigh scattering sets a lower limit to the propagation losses in optical fibers. Of course, additional losses can result e.g. from an irregular core/cladding interface (particularly in the refractive index contrast is high), from scattering and absorption by impurities, and from macroscopic and microscopic bending. Silica fibers which have been optimized for long-distance optical fiber communications have very low propagation losses, approaching the limit given by Rayleigh scattering. For wavelengths substantially below the often used 1.5-μm region, Rayleigh scattering alone would be higher than the actual losses of these fibers at 1.5 μm wavelength. At substantially longer wavelengths, Rayleigh scattering would be weaker, but the infrared absorption of silica sets in.
In principle, one could have mid-infrared fibers made of other glasses (e.g., fluoride fibers), which could have even lower losses, but in practice silica fibers have reached the best figures.
Most of the Rayleigh-scattered light in a fiber exits the fiber on the side. Only a small portion of the scattered light is scattered back such that it is again guided in the fiber core. Therefore, the return loss of fiber devices is generally very high; the overall return loss of a fiber setup is more often caused by reflections at interfaces such as fiber ends, mechanical splices or fiber connectors.
Due to the high optical intensities which often occur in optical fibers, nonlinear scattering processes like Raman scattering and Brillouin scattering can also occur. Rayleigh scattering, being a linear process, is equally important at low light intensities.
