By Owais AliReviewed by Lexie CornerUpdated on Aug 9 2024This article offers an overview of scattering, including its fundamental principles, types, and applications across various fields.
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What is Scattering?
Scattering is a fundamental physical process that occurs when particles, waves, or radiation interact with matter, causing them to deviate from their original path. This deviation leads to changes in the radiation's direction, energy, frequency, or phase and may result in diffuse reflections instead of specular (mirror-like) reflections.
The scattering pattern varies with particle size and shape, with small particles producing a nearly uniform scattering pattern and larger particles creating more complex patterns with distinct peaks and troughs.
Scattering is observed in phenomena such as the blue appearance of the sky, acoustic diffusion in concert halls, and particle interactions in quantum physics. Understanding scattering is essential for interpreting a wide range of physical phenomena and advancing applications in optics, acoustics, and material science.1
Single vs. Multiple Scattering
Single Scattering
Single scattering occurs when radiation interacts with a single scattering center, such as an atom or a molecule. These events are often random due to the unpredictable position of the scattering center relative to the radiation path. For instance, when a photon of sunlight collides with a single molecule, it is scattered in various directions, contributing to the blue appearance of the sky.
Multiple Scattering
Multiple scattering happens when radiation is scattered many times by grouped scattering centers. This process is generally more deterministic than single scattering, as the randomness tends to be averaged out by many scattering events. It is analogous to diffusion, where the scattered radiation produces a more uniform distribution. For instance, when light travels through thick fog, it undergoes multiple scattering by the numerous water droplets, creating a diffuse, hazy appearance.
Not all single scattering is purely random, nor is all multiple scattering fully deterministic. For example, a laser beam directed at a microscopic particle can result in precise single scattering. Conversely, multiple scattering, especially with coherent radiation, can exhibit random patterns like speckles.2
Types of Scattering
Rayleigh Scattering
Rayleigh scattering describes the interaction of light with particles much smaller than the wavelength of light. This phenomenon is responsible for various natural effects, such as the blue appearance of the sky and the red hues at sunset.
Rayleigh scattering applies when the dimensionless size parameter, α=2πr/λ, is much less than 1, where λ is the wavelength of light and r is the particle radius.
The scattering cross-section of a particle, which measures its effective scattering area, increases with the fourth power of the light frequency, explaining why shorter wavelengths, like blue light, scatter more than longer wavelengths.
Mie Scattering
Mie scattering extends Rayleigh scattering theory to account for larger particles where the quasi-static approximation is no longer valid. It provides a more accurate description of scattering by solving the full boundary value problem.
Unlike Rayleigh scattering, where the scattering cross-section increases with frequency, Mie scattering shows that the cross-section approaches a constant value of πa2 at high frequencies, which explains why the sky is not purple.
Mie scattering occurs when the wavelength of electromagnetic radiation is comparable to the size of the particles in the air, affecting photons in the near-ultraviolet to mid-infrared regions. It is prevalent in the lower atmosphere during overcast conditions, where larger particles like pollen, dust, and pollution are more common.3
Tyndall Scattering
The Tyndall effect or scattering occurs when light scatters off colloidal particles, enabling the differentiation between solutions, colloids, and suspensions. This effect is evident when light from car headlights scatters in fog, resulting in a bluish tint due to the more effective scattering of shorter wavelengths.
Raman Scattering
Raman scattering involves the inelastic scattering of light, where the scattered photons have different energies from the incident photons. This process provides insights into molecular vibrations or rotational energy levels.
When light interacts with molecules, it can either gain or lose energy, resulting in a shift in the frequency of the scattered photons. This shift is called Raman scattering and is observed as spectral lines known as Stokes lines (shifted to lower energy) or anti-Stokes lines (shifted to higher energy).
The energy difference between the scattered photons (Es) and the incident photons (Ei) is given by:
Δ=Evi−Evs
; where Δ represents the vibrational transition energy difference.
Raman scattering is useful for studying molecules without a net dipole moment, offering insights into their molecular structure and moment of inertia.4
Thomson Scattering
Thomson scattering measures the radiation emitted by excited electrons and atoms after interaction with an electromagnetic wave. The method is characterized by the Debye length (λD) and the differential wave scattering vector (σ):
σ=1/kλD
; where k is the wave number.
Depending on the value of σ, the scattering can be incoherent (σ<0.1), coherent (0.1≤σ≤1.5), or incoherent again (σ>1.5). Thomson scattering also requires calibration via Rayleigh scattering, using a laser to induce a radiation spectrum.3,4
Applications of Scattering Techniques
Nanoparticle Characterization
Rayleigh scattering is commonly used to characterize nanoparticles ranging from 1 to 100 nm, often produced through wet chemical methods such as sol-gel deposition. In a solution containing these nanoparticles, Rayleigh scattering generates patterns that reveal the different electron configurations within the particles, enabling researchers to differentiate between various nanoparticle types in a mixture.
This method can also be applied to monitor emissions from industrial exhausts, enabling environmental agencies to track atmospheric pollutants.4
Density and Temperature Measurements in Plasmas
Thomson scattering, particularly its incoherent form, is widely used for density and temperature measurements in electron cyclotron resonance (ECR) plasmas created by aligning microwave frequencies with electron rotation frequencies in a gas. It provides noninvasive, in situ measurements of key plasma parameters like electron temperature and density, which are crucial for nuclear research and plasma processing in manufacturing.
Thomson scattering is also applied to study the Earth's ionosphere and measure species density, temperature changes, and high-frequency wave activity, particularly in phenomena like auroras. This technique has recently gained prominence in assessing plasma parameters in manufacturing processes, offering valuable data for improving industrial systems.4
Construction and Soil Analysis
In the construction industry, dynamic light scattering (DLS) is employed for quality control to ensure that materials adhere to industry standards. For instance, DLS is used to monitor cement hydration, optimize concrete hardening processes, and fine-tune mixing ratios. Additionally, DLS assists planners in analyzing the soil or sediment at construction sites, informing decisions about the appropriate materials and procedures for laying building foundations.5
Cancer Diagnosis and Exosome Analysis
Raman scattering plays a crucial role in cancer diagnosis and treatment by enhancing the detection and differentiation of cancerous tissues. It has been used extensively to distinguish between precancerous, cancerous, and normal tissues in ex vivo samples, stage tumors, and predict surgical margins.
Surface-enhanced Raman scattering (SERS) further aids in diagnosing lung cancer by analyzing exosomes released from cells. This technique allows for identifying exosomes from cancerous versus normal cells based on their Raman spectra, enabling precise classification and improved diagnostic accuracy.6
Flow Visualization in High-Speed Gas Flows
Mie scattering has been instrumental in the biological sciences for studying the cytoplasm and the orientation of cells in suspensions. However, its primary application lies in flow visualization in high-speed gas flows, particularly in engine development and combustibles research. This technique involves introducing particulate tracers into the gas, allowing researchers to observe and analyze flow structures.
A more advanced approach, planar Mie scattering (PMS), uses a two-dimensional array to collect data from illuminated planes within a flow field, creating a concentration map of two-dimensional flows. This method is commonly used in flow velocimetry, providing detailed insights into the behavior of various gases.4,7
Conclusion
The study of scattering has greatly advanced our understanding of light-matter interactions, from explaining natural phenomena like the color of the sky to enabling technologies in medical imaging and materials science.
As technology progresses, our ability to manipulate and utilize scattering phenomena will open doors to new technologies and a deeper understanding of the world around us, promising exciting developments in the future.
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References and Further Reading
- Bohren, CF., Huffman, DR. (2008). Absorption and scattering of light by small particles. [Online] John Wiley & Sons. Available at: https://staff.cs.manchester.ac.uk/~fumie/internal/scattering.pdf
- Newton, RG. (2013). Scattering theory of waves and particles. [Online] Springer Science & Business Media. Available at: https://books.google.com/books?id=KWTyCAAAQBAJ&printsec=frontcover&dq=Scattering+Theory+of+Waves+and+Particles&hl=en
- Purdue University. (2024). Rayleigh Scattering, Mie Scattering. [Online] Purdue University. Available at: https://engineering.purdue.edu/wcchew/ece604f19/Lecture%20Notes/Lect34.pdf
- Wade, DM., Drake, DJ. (2019). A brief review of modern uses of scattering techniques. Georgia Journal of Science. https://digitalcommons.gaacademy.org/gjs/vol77/iss2/7/
- Meritics Ltd. (2020). The Applications of Dynamic Light Scattering. [Online]. Metrics Ltd. https://www.meritics.com/the-applications-of-dynamic-light-scattering/
- Qi, Y., et al. (2024). Applications of Raman spectroscopy in clinical medicine. Food Frontiers. doi.org/10.1002/fft2.335
- Handapangoda, CC., Premaratne, M., Pathirana, PN. (2011). Plane wave scattering by a spherical dielectric particle in motion: A relativistic extension of the Mie theory. Progress In Electromagnetics Research. doi.org/10.2528/PIER10102901
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