Acousto-optics

The Debye- Sears effect allows a very accurate determination of the speed of sound in media based on wave optics, and is named after the physicists Peter Debye and Sears Francis.

Measuring the speed of sound using the Debye -Sears effect

The medium to be tested (usually a piezoelectric crystal), for example, made ​​to oscillate with a ultrasonic oscillator. Through the body, and the fluid in a cuvette, parallel light is transmitted. The sound waves cause differences in density and this in turn to a change in refractive index at the appropriate point in the medium, which thus acts as a phase grating.

Measurement by the diffraction image

After passing through the medium, the bundled parallel light again by means of a lens, can be mapped diffraction phenomena. By the measurement of the diffraction image, a determination of the speed of sound in the analyzed medium is possible. The diffraction image is easier to measure when the light is first passed through a filter.

Measurement with lens:

Measurement without the lens:

= Phase velocity of ultrasonic

= Focal length

= Order of diffraction

= Wavelength of light

= Distance of the N th diffraction order from the 0 - th

= Frequency of the ultrasound

= Distance between ultrasound and screen

Measuring the diffraction phenomenon

The diffraction image can be projected on a screen in order to make it visible. If, in the focus of the lens behind the medium equal to a camera can photograph the diffraction image directly and then via a photometer

Measure the distances on the negative.

Measurement over the wave grating image

If the sound waves reflected in a suitable liquid via a reflector ( metal plate ), a standing wave is formed. The resulting diffraction pattern here is directly observable through a microscope.

If, with respect to the wave grating and the diffraction pattern method, so much to be said for the wave grating image method. Here there is a possible on 15-20 maxima distances to average the diffraction pattern to get hardly up to third order. This minimizes the error enormous, as this is the spot where lies the experimenter the largest margin. Therefore, the tolerance is three to five times in the diffraction pattern method.

Quartz winds

Quartz winds in the Flüssigkeitsküvette occur when the vibrating piezoelectric crystal in stretching the liquid strongly pushes away from himself and then contracted faster than the fluid can flow back. The result is laterally inflowing fluid particles, which then go through the same cycle. The result is a flow in the cuvette, which brings a density gradient and hence unwanted diffraction effects. This can be proved by projecting the silent state after the cuvette onto a screen. If you switch now the ultrasonic transducer at so proves a possible shadow cast by refraction effects occurring quartz winds.