Scattering of Electromagnetic Waves
When an electromagnetic wave meets an object, it induces oscillating charges that re-radiate, scattering the wave in new directions.
Definition
Electromagnetic scattering is the process by which an incident wave induces oscillating charges and currents in an object that re-radiate the wave into other directions, characterized by cross sections and angular distributions that depend on the object's size relative to the wavelength and its electromagnetic properties.
Scope
This topic covers the scattering of electromagnetic waves by particles and obstacles: the scattering and absorption cross sections, Rayleigh scattering by particles much smaller than the wavelength, Mie scattering by particles comparable to the wavelength, the optical theorem, and the dependence of scattering on size, shape, and refractive index. It links incident waves to the radiation re-emitted by induced currents.
Core questions
- How does an object re-radiate an incident electromagnetic wave?
- Why does Rayleigh scattering favour short wavelengths?
- How does scattering change when the particle size approaches the wavelength?
Key concepts
- scattering cross section
- absorption cross section
- Rayleigh scattering
- Mie scattering
- optical theorem
- induced dipole
- differential cross section
Key theories
- Rayleigh scattering
- For particles much smaller than the wavelength, the induced dipole re-radiates with an intensity proportional to the inverse fourth power of wavelength, explaining the blue colour of the sky.
- Mie scattering
- For particles comparable to the wavelength, the full solution of Maxwell's equations for a sphere gives the Mie series, with complex angular patterns and resonances depending on size and refractive index.
Clinical relevance
Scattering theory explains the colour and polarization of the sky, atmospheric remote sensing and lidar, radar cross sections, light scattering used to size particles and study aerosols and colloids, and optical diagnostics in biological tissue.
History
Rayleigh explained the blue sky through scattering by small particles in the 1870s. Lorenz and, independently, Mie in 1908 solved scattering by a sphere of arbitrary size, providing the framework now central to aerosol, colloid, and atmospheric optics.
Key figures
- John William Strutt (Lord Rayleigh)
- Gustav Mie
- Ludvig Lorenz
Related topics
Seminal works
- mie1908
- bohren1983
Frequently asked questions
- Why is the sky blue?
- Air molecules scatter sunlight by Rayleigh scattering, which is much stronger for short (blue) wavelengths than long (red) ones, so the scattered light reaching us from across the sky is predominantly blue.
- What is the difference between Rayleigh and Mie scattering?
- Rayleigh scattering applies to particles much smaller than the wavelength and depends strongly on wavelength, while Mie scattering applies to particles comparable to the wavelength and produces more complex, less wavelength-sensitive patterns, as with water droplets in clouds.