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Measurements of Sound Speed in the Water by Brillouin Scattering (CAT#: STEM-ST-0112-YJL)

Introduction

Sound is a wave of alternating compression and expansion, so its speed depends on how fast it bounces back from each compression – the less compressible the medium it's travelling through, the faster it bounces back. Water is about 15,000 times less compressible than air, but it is also 800 times denser. The extra density means that the molecules accelerate more slowly for a given force, which slows the compression wave down. So water's high density partly offsets its extreme incompressibility and sound travels at 1,493m/s, about four times faster than through air. The speed of sound in diamond is so high because it is extremely incompressible and yet relatively light.




Principle

From a quantum point of view, Brillouin scattering is an interaction of light photons with acoustic or vibrational quanta (phonons), with magnetic spin waves (magnons), or with other low frequency quasiparticles interacting with light. The interaction consists of an inelastic scattering process in which a phonon or magnon is either created (Stokes process) or annihilated (anti-Stokes process). The energy of the scattered light is slightly changed, that is decreased for a Stokes process and increased for an anti-Stokes process. This shift, known as the Brillouin shift, is equal to the energy of the interacting phonon and magnon and thus Brillouin scattering can be used to measure phonon and magnon energies.

Applications

Brillouin scattering is used to determine acoustic velocities and elastic properties of a number of crystalline solids, glasses, and liquids.

Procedure

1. Sample preparation
2. Measurement by scattering detection instrument
3. Data analysis

Materials

Brillouin scattering measurement system (Brillouin spectrometer)