How Does Light Scattering Work
Light scattering is both a physical process and a research technique in physics based on the same principle. When light encounters particles as it travels through a medium, the light is scattered in all directions. The intensity of the scattered light depends on the size of the particles and the wavelength of the light. Rayleigh theory, a theoretical description of light scattering involving particles much smaller than the wavelength of the light, helps us understand why the sky appears blue. Does this Spark an idea?
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Light Basics
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Like all electromagnetic radiation, light consists of electric fields that vary with time; these induce time-varying magnetic fields, which in turn induce electric fields and so on. This wave-like disturbance propagates through space. The electric and magnetic fields oscillate in planes at right angles to each other, and the orientation of these planes is called the polarization. As for any other wave, the distance from peak to peak or trough to trough is called the wavelength, while the number of waves that pass a given point in a second is called the frequency.
Rayleigh Scattering
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When light encounters a molecule much smaller than its wavelength, the electric fields temporarily polarize the molecule, redistributing the electrons in the molecule such that one end has a weak positive charge and the other has a weak negative charge. This separation of charges is called a dipole moment. As the oscillating electric fields interact with the molecule, the molecule's dipole moment oscillates, and the molecule re-radiates the light in all directions. The intensity of the scattered light depends on the wavelength of the light and the polarizability of the molecule--how easily it can be polarized, in other words. Polarizability depends in turn on the size of the molecule.
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Mie Scattering
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Rayleigh theory doesn't work well for molecules or particles with diameters greater than 1/10 of the light's wavelength. For larger diameters, another theoretical description called Mie theory applies instead. Unlike Rayleigh scattering, Mie scattering is wavelength-independent, so all wavelengths of light are scattered equally. Rayleigh scattering applies in Earth's atmosphere; since light at shorter wavelengths scatters more than light at longer wavelengths, blue light is scattered more, which is why the sky appears blue. Mie scattering, on the other hand, describes the process that takes place in clouds of water droplets like fog. Since all wavelengths of light are scattered equally by these larger particles, the light in clouds or fog appears white.
Applications
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Scientists sometimes use these principles to measure molecular mass. These types of experiments typically involve laser light since the light emitted by the laser all has the same wavelength. When the light encounters molecules in the solution, it is scattered and its intensity can be measured by a detector. The intensity is proportional to the size of the molecules in the solution, so scientists can use the intensity data to calculate the molecular weight.
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References
- Photo Credit sky image by Darren Nickerson from Fotolia.com
