Light scattering

When I let light be incident on a material, the light scattering (ひかりさんらん) means a phenomenon to release light in all directions at the same time to take this in.

Table of contents

Principle of the light scattering

Explanation by the classical theory

The light scattering depends on the second wave being released by the vibration of the electric dipole evoked by incident radiation as well as a reflection of the light. For example, when light is incident on an atom, the vibration of the electric dipole is evoked, and then the second wave is released, but when I observe the second wave in the future in an arbitrary direction if many atoms are distributed sparsely and at random, as for the strength, it is to the extreme sum of second wave from each atom, and generally there is not this to 0. This is light scattering.

In contrast, the second wave from each atom interferes it each other, and strength becomes 0 any place other than the specific direction when an atom is dense, and the density is the same. The second wave which does not disappear by a result of the interference becomes the reflection wave, and I interfere it with an incidence wave again, and there is refracted wave. The light scattering is caused by a material not being uniform generally in this way, and the material surface is not same in this, and the diffused reflection to open is included in the direction where borrowed light is various there, but I ignore the surface effect here and think about the light scattering to have inside of the material.

Explanation with the quantum theory

By handling with the quantum theory, the light scattering is transition of 2 photon processes happening by light and the interaction of the material.

The basic formula of the regular light scattering is called dispersion formula (K-H dispersion formula) of クラマース Heisenberg. In addition, the K-H dispersion formula is expressed in the polarizability tensor of molecules approximately by insulation approximation and Placzek approximation.

The mean of the polarizability contributes to elastic scattering of the light, and Rayleigh scattering means this. On the other hand, a fluctuation of the polarizability contributes to inelastic scattering of the light, and Raman scattering means this.

Various light scattering

A Raman scattering process and Rayleigh scattering

Dispersion with fine particles

• Rayleigh scattering (elastic scattering with the particle which is smaller than a wavelength of the light)
• Mie scattering (dispersion with the particle which is bigger than a wavelength of the light)

Dispersion by the electron

• Thomson scattering (elastic scattering of the long wavelength light by the electron)
• Compton dispersion (inelastic scattering of the short wavelength light by the electron)

Dispersion by the phonon

• ブリュアン dispersion (inelastic scattering by a sound wave and the phonon)
• Raman scattering (inelastic scattering by the optics phonon)

Phonon band and phonon state density is found by an experiment

Example of the light scattering phenomenon

• It has a big place where light of the sun performs atmospheric air molecules and Rayleigh scattering that the sky is blue
• It depends on multiple scattering + Mie scattering that a cloud is white.
• A fat ball (around 1-100 micrometers in diameter) causing a micelle (around 20-150 nanometers) of protein casein causing Rayleigh scattering to milk and Mie scattering exists. The fresh cream with much fat content looks white by the Mie scattering of the fat ball. The no fat milk is bluish by Rayleigh scattering, but looks white by multiple scattering.
• The Tyndall phenomenon is dynamic scattering caused by the colloids. It was studied theoretically by Reilly and me. I can find colloidal molecular weight or size by checking light scattered in.

Microwave dispersion meter

I measure a target state by receiving the microwave that it is targeted for observation and irradiates a microwave into a pulse form and reflected.

References

• Fumiaki Shibata "theory of the light scattering" (アグネ publication "solid physics" Vol.20 1985)

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