scattering (original) (raw)

This article provides a comprehensive overview of the scattering of light. It explains the fundamental distinction between diffuse and stimulated scattering, and details the primary physical mechanisms. Key elastic scattering processes like Rayleigh scattering, which depends strongly on wavelength, and Mie scattering from larger particles are described. Inelastic processes, including Raman and Brillouin scattering, involving energy exchange with the medium, are also covered.

Furthermore, the article discusses the role of scattering as a major source of propagation losses in optical components like fibers and ceramics, and highlights various applications where scattering is essential, such as in microscopy, display screens, and optical diffusers.

(This summary was generated with AI based on the article content and has been reviewed by the article’s author.)

What is Scattering of Light?

In various situations, light can be scattered, i.e., sent into other directions. The term scattering is mostly used for diffuse scattering, where light is sent into a wide range of directions. A classical example is scattering of light at a rough surface, having a microscopically irregular structure.

However, there are also cases with stimulated scattering, where the direction of scattered light is determined by some incident light.

This article describes various physical mechanisms which occur in the context of scattering of light.

Rayleigh Scattering

Rayleigh scattering, named after Lord Rayleigh, is elastic scattering of electromagnetic radiation at particles (or other entities, see below) which are much smaller than the wavelength of the radiation. In the area of optics, that usually implies particle sizes far below 1 μm, i.e., with nanometer dimensions or less.

Elastic scattering means that the wavelength of the scattered light is not changed, apart from a possible Doppler shift due to the movement. That implies that the inner energy of the scattering particles is not changed; there is no electronic excitation or deexcitation involved, for example. The process can be described such that the particles (scattering centers) experience an oscillating polarization due to the light, and that polarization leads to radiation of light in all directions. At the same time, the incident light is attenuated accordingly.

The intensity of scattering scales with the fourth power of the optical frequency, or with the inverse fourth power of the wavelength. This results from the basic properties of dipole radiation.

Further, one can show that the scattering intensity is proportional to ($1 + \cos^2\theta$), where ($\theta$) is the scattering angle (e.g. ($\pi$) for back-scattering towards the source). This means that forward and backward scattering are stronger than scattering to transverse directions. That can be understood by considering that the induced dipoles oscillate in directions perpendicular to the propagation direction of the incident light, and that they cannot radiate along the direction of oscillation.

Rayleigh scattering is the dominant reason for propagation losses in optical glass fibers (particularly single-mode fibers) at shorter wavelengths, e.g. in the visible and ultraviolet spectral range. Even for perfect glass quality, scattering occurs at the density fluctuations in the glass, which are to some extent unavoidable. Therefore, the lowest propagation losses are achievable for wavelengths just below the infrared absorption edge. For silica fibers, this is in the wavelength region around 1.5 μm to 1.6 μm.

Mie Scattering

Mie scattering occurs when the scattering particles have a similar size as the wavelength of light. Here, one has significant variations of optical phase over the scattering contributions of different locations on the particles. Here one finds that — in contrast to Rayleigh scattering — the scattering intensity does not depend that much on the optical wavelength; this is consistent with the white appearance of milk, where Mie scattering occurs at fat droplets in water which are not that small. Another finding is that forward scattering is stronger than backward scattering because the relative phase differences of contributions from different scattering locations on the particles become smaller.

Mie scattering is particularly relevant for meteorological optics, but also in the biomedical area, for example.

Raman and Brillouin Scattering

Raman and Brillouin scattering are examples of inelastic scattering processes, where the inner energy of the scattering particles change. For Raman scattering at gas molecules, for example, the vibration and rotation states of the molecules change; typically, the molecules have a higher energy after the scattering process, which implies a correspondingly lower photon energy of the scattered light (Stokes components → Stokes shift). If the molecules are initially excited, one can also obtain anti-Stokes components with increased optical frequency. Similarly, Raman scattering can occur in solids, involving so-called optical phonons, i.e., phonons with relatively high frequencies (in the terahertz region). Brillouin scattering in solids involves acoustical phonons with much lower frequencies in the gigahertz region.

Both Raman and Brillouin scattering can be stimulated with additional incident light at the corresponding modified optical frequencies and with appropriate propagation directions. For example, stimulated Brillouin scattering in optical fibers is normally possible only in backward direction. The injected lower-frequency backward wave experiences amplification in that process; the same holds for the involved acoustic wave. Even without injecting a counterpropagating light wave, for sufficiently high Brillouin gain one may obtain a strong backward wave which starts with spontaneous Brillouin scattering and then experiences strong nonlinear amplification.

See the articles on Raman scattering and Brillouin scattering for more details.

Optical Propagation Losses Due to Scattering

Scattering is a major cause of propagation losses in optical components.

It has already been mentioned above that scattering limits the propagation losses which are achievable in optical glass fibers.

Scattering is also a fundamental challenge for the use of ceramic media as optical materials. However, there are some ceramic media where the crystallites are so small that scattering losses are no longer substantial. For example, one can fabricate ceramic Nd:YAG with high optical quality.

Light scattering occurs not only within optical materials due to material inhomogeneities, but very often also on optical surfaces. This is because of the high refractive index contrast between optical materials and air. One generally minimizes scattering losses at optical surfaces by polishing them with very high quality.

The strong frequency dependence of Rayleigh scattering also explains why scattering losses are generally a much more severe problem with ultraviolet optics than with infrared optics.

Applications of Scattering

Although scattering losses are a common problem in optics, there are also various applications of light scattering. Some examples (all involving elastic scattering):

Light scattering is often the mechanism which produces image contrast e.g. in microscopy.
Display screens are based on light scattering.
Diffusely scattering surfaces are often used for providing homogeneous illumination. For example, one often uses white ceramics in laser heads of lamp-pumped lasers for obtaining smooth pump intensity distributions.
One can construct optical diffusers based on scattering media. They are used for avoiding unwanted spatial coherence of light.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is scattering of light?

Light scattering is a process where light is deflected from its straight path and sent into various other directions by interacting with small particles or inhomogeneities in a medium. It is often diffuse, meaning the light is sent into a wide range of directions.

What is the difference between elastic and inelastic scattering?

In elastic scattering, such as Rayleigh or Mie scattering, the wavelength of the scattered light is unchanged. In inelastic scattering, like Raman or Brillouin scattering, the wavelength changes because there is an energy exchange with the scattering medium.

What is Rayleigh scattering?

Rayleigh scattering is the elastic scattering of light from particles that are much smaller than the light's wavelength. Its intensity is strongly dependent on wavelength (proportional to the inverse fourth power), causing shorter wavelengths like blue light to scatter much more strongly than red light.

Why is Rayleigh scattering important for optical fibers?

In high-quality optical glass fibers, Rayleigh scattering at microscopic density fluctuations is the dominant source of propagation losses at shorter wavelengths. This effect determines the wavelength region of minimum loss, which for silica fibers is around 1.5 µm to 1.6 µm.

What distinguishes Mie scattering from Rayleigh scattering?

Mie scattering occurs when particles are similar in size to the light's wavelength, whereas Rayleigh scattering involves much smaller particles. Mie scattering is less dependent on wavelength, often resulting in a white appearance (e.g., in milk), and tends to scatter light more in the forward direction.

Is light scattering always a problem in optics?

No, while scattering often causes unwanted propagation losses, it is also essential for many applications. For example, it creates image contrast in microscopy, is the working principle behind display screens, and is used in optical diffusers to homogenize light.

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