Fresnel fringes

Reliable characterization techniques are now available to describe all kinds of carbonaceous materials, from aromatic molecules to graphite. They are reviewed in Sections 1.2 and 1.3. 

2 BASIC STRUCTURAL UNITS (BSUs) AS ELEMENTAL BRICKS OF DISORDERED CARBONS 

Carbonaceous matter (i.e., a disordered carbon) is not transformed into pnre carbon until heat treatment at 2000°C. Pure carbons are never found in nature, with the exception of natural graphite. Nevertheless, natural biomass-derived carbonaceous matter is omnipresent as coals, kerogens, soot, and derivatives such as petroleum, bitumen, or asphalt. It is now more than 60 years that the presence of heteroatoms (H,0,S,N) covalently bound to the carbon skeleton has been swept under the rug, at best implicitly they are known to be responsible for many troubles in the interpretation of carbonization and graphitization data. Because heteroatoms are ignored or neglected, the various calculations do not fit well with experiments. It was thus necessary to add adjustable parameters or include additional phases to obtain agreement between theory and experiments. Additional 

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O. S. Heavens. Optical Properties of Thin Solid Films. Buttcrworths, 1955. Chapter 4 presents a detailed mathematical description of the Fresnel fringing phenomenon for the transmission of light through thin films. 

However, when HRTEM was employed on the SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual intergranular films was not detected even at the triple junctions. Hence, Kleebe concluded that SEM imaging and Fresnel fringe TEM imaging alone do not enable a safe conclusion to be drawn about interface wetting in ceramic polycrystals. 

Figure 2.6. Fresnel fringes at the edge of a hole in a carbon him, observed in the electron microscope (a) imderfocus, and (b) overfocus.

The Fresnel fringe provides a satisfactory criterion for focusing if a suitable edge is available on the specimen. An alternative method of finding exact focus is mentioned in Section 6.3.7. 

Note also the bright Fresnel fringe at the edge of the hole in the film. 

That phenomenon is, in some respects, analogous to the classical phenomenon of Young and Fresnel fringes produced by two closely parallel slits. You observe on the screen the impact of photons which have passed through two undistinguishable paths (slit 1 or slit 2). If you wish to know the path of the photons, you can obscure one of the two slits but in these new conditions you do not observe fringes. 

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