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Old Oct 8th 2011, 06:26 AM   #1
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Angular distribution of diffusely scattered light

Hey,

I recently did a pretty simple experiment to model the angular distribution of diffusely scatter light. I had four different surfaces, two were obviously smooth and two were obviously rough. I use an apparatus shown in the below picture where I used horizontally polarised light.



The surface was placed in position of the "triangle" (it wasn't actually a triangle the apparatus is actually from a previous section of the experiment where I used a prism) such that the angle of incidence was 70deg. I then rotated a detector around the surface between a range of angles to measure the intensity reflected light. I plotted the detectors angle against the intensity. The graph is below.



What I found was the two smooth surfaces (S1,S3) gave results that were expected, the peaks were centred when the angle of incidence = angle of reflection (the plateau of each peak is because the detector plate was large). However with the rough surfaces the peaks were not centred at the expected angle, they shifted up by around 4degrees. The only explanation I can think of is that the rough surfaces resemble a disc or a "dirty" diffraction grating.

Does anyone have any ideas why the peaks have shifted? We ran the experiment three times and received the same result with the rough surfaces
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Old Oct 8th 2011, 01:02 PM   #2
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Might be the 'direction' of the small structures of the surface... Have you tried 'flipping the prism vertically'?

Like in the diagram, the top vertex will then be in the place of the bottom one and vice versa, while the initial base of the prism now becomes the top.

Except that if I understood well, you would just have to flip the surface mounted on the prism.

Well, just my 2 cents to explain why this happens, otherwise I'm stumped
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Old Oct 8th 2011, 06:17 PM   #3
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Thanks for your reply,

Yea I was thinking about flipping the surface yesterday so I might see if I can give that a shot tomorrow.

I've been trying to picture what would happen, at first I thought it might shift the peak to a lower angle but I'm really sure.

I should of also tried with vertically polarised light, maybe ill try that tomorrow as well.

Some one in another forum suggested it can be explained by "bidirectional reflectance distribution function"

Bidirectional reflectance distribution function - Wikipedia, the free encyclopedia

Which I think would mean that those particular surfaces provide those results due to the properties of the surface. But I'm not sure if that's the correct thing to say
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Old Oct 9th 2011, 03:38 AM   #4
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Hmm interesting. That would support the idea that flipping the surface would indeed change the angle and this time would shift in the other direction by the same magnitude.
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