Coherent backscattering is a phenomenon where photons scattered at angles very near to 180 degrees self-interfere constructively leading to an observed intensity cone in the exact backscattering direction. The intensity cone has been related to various transfer coefficients of the scattering media, and the mean free path which light takes within the media. The goals for the following experiment was to determine the optimal alignment for a Coherent Backscattering Experiment, and to try and to try and observe the backscattered intensity cones in samples of milk. Experiments were done to determine the optimal distances between optical components, distances between the milk samples and detection mechanism, and optimal camera settings for the CCD Camera used for detection.
Here is a diagram of the setup that I am using to
observe CBS, and an actual photograph of the setup. The sample holder
is cut off at the top, and the beam dump is not visible, but the main
components are all there (click on the pictures for larger versions).
Samples of whole and skim milk were observed using the setup described
above. The results are displayed in the figure below. The results
were obtained by exporting the images produced by the CCD camera into
Scion Imaging Software, and averaging vertical and horizontal
cross-sections of the cone.
The blue curve and red curve represent cones observed from 40 mL samples of commercially available whole milk and skim milk, respectively. The fluctuations in the data could be attributed to background light and/or varying pixel sensitivities in the CCD Camera. Also, the artifacts could be attributed to reflections and scattered light from the beamsplitter and lenses. According to theory, the widths of the cone are inversely proportional to the mean free path that photons take within the scattering samples. The shape of the cones have also been related to different optical transfer coefficients.
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