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tranthuongbn



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Join date : 2010-12-21

PostSubject: Diffuse reflection   Sat Jan 15, 2011 9:42 pm

Diffuse reflection from a solid surface generally is not due, as one may think, to the roughness of its surface: a flat surface is indeed required to give specular reflection, but it does not cancel diffuse reflection. We can levigate and polish at will a piece of a white marble, but it continues to be white, and will never become a mirror: simply it will give a small specular reflection, while the remaining light continues to be diffusely reflected.
The most general mechanism by which a surface gives diffuse reflection does not involve exactly the surface: most of the light is contributed by internal scattering centers, beneath the surface[2] [3], as illustrated in Figure 1 at right. If we imagine that the figure represents snow, and that the polygons are its (transparent) ice crystallites, we have that an impinging ray is partially reflected (a few percent) by the first particle, enters in it, is again reflected by the interface with the second particle, enters in it, impinges on the third, and so on, generating a series of "primary" scattered rays in random directions, which, in turn, through the same mechanism, generate a large number of "secondary" scattered rays, which generate "tertiary" rays.... [4]All these rays walk through the snow crystallytes, which do not absorb light, until they arrive at the surface and exit in random directions[5]. The result is that we get back in all directions all the light we sent, so that we can say that snow is white, in spite of the fact that it is made of transparent objects (ice crystals).
For simplicity, here we have spoken of "reflections", but more generally the interface between the small particles that constitute many materials is irregular on a scale comparable with light wavelength, so diffuse light is generated at each interface, rather than a single reflected ray, but the story can be told the same way.

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