Optical Density Determination

Optical Density Determination 

1/24/2005

Question -   Why does light pass through some pure substances, but not others (eg. diamond vs graphite ... both are Carbon)? On a molecular level, what exactly determines optical density (it is not the same as mass density)? Why does light slow down in optically dense media? ----------------- Theresa- There are at least two kinds of "optical density", maybe more like three: 1) absorbance (it is clear, but it is to some degree dark,  like smoky glass.   Light is diminishing as it travels through.) 2) refractive index    (it is clear, but light goes slower through   it.   So it changes direction at surfaces. for large index, some of the light bounces off each   surface.  Metallic reflection is an extreme case of this.) 3) scattering density  ( it is clear but messy with refractive index   surfaces, so it becomes cluttered or frosty or milky or white)  Any given substance has some amount of each of these three "densities".  Only refractive index has any connection with mass density. That being: heavy substances are made of high-atomic-number elements,   which have many electrons, which cause higher refractive index. High concentrations of bound electrons (bound in one place, but   elastically movable by a short distance) are the "water" that slows down   the flight of light.  Viscously-movable electrons absorb light (1) at all wavelengths.  This is  graphite black, and it enforces the opacity of metals. You can see through metals if they are less than 0.1 micrometer  thick.  One-way mirrors are this. But if thicker, the part of light which is not reflected at the front   surface will be completely absorbed.  Metallic opacity.  Light is also absorbed by bound electrons using the energy to climb out of  their trapped state, or at least climb to a higher trapped state. But it has to be the right amount of energy, so it is a more  color-selective absorption.   It creates most of the non-neutral colors of  objects.  Chemical purity helps a clear substance be clearer, but it cannot help an  inherently absorbing substance like graphite become clear. Perfect single crystals of graphite are a lighter silvery color than  typical poly-crystalline graphite. Certain impurities in graphite actually donate more mobile electrons,  which sometimes make it lighter still.  Chemically pure glass is silicon dioxide. Light can go for tens to  thousands of meters in this, depending on color, if it is pure. Give it unnecessary surfaces by grinding it up, and you have sand: more  white than clear. Chemically pure aluminum oxide can be white if it is many small  crystallites, or clear if it is one large crystal (colorless sapphire). Pure water is not a crystal, it is random inside, but it can be clear in a  uniform mass, or milky if dispersed as fog. Ice can be either clear or cloudy or fractured with flaws.  Diamond vs Graphite: The carbon atom  can make 4 chemical bonds (shared-electron-links) to  its neighbors. In diamond, each atom links to 4 different neighbors, and every electron  is bound (confined) within its own link. So there are no mobile electrons, and diamond is a dielectric not a  conductor, and  it is clear, not absorbing. In graphite, each atom links to only 3 neighbors, making a flat sheet, and  each has one link left over. All these leftover links are shared in common by the whole sheet. The electrons of this pool of links are mobile, so graphite conducts  electricity and absorbs light. There are not many elements which have a choice of whether or not to be  conductive.  Those are the classical optical properties. Iridescence and dichroism are a different story.  Jim Swenson ===================================================== If a substance has an unoccupied electronic state whose energy difference from initial state is the same as the energy of the incident radiation (light), and given certain other restriction, then the substance will absorb the incident radiation. The electronic structure of the substance determines whether or not such unoccupied but accessible electronic states exist; however, the details of determining such states is rather involved. The bonding in diamond and graphite is a good example. Both are carbon, but in diamond the carbon atoms are bonded to one another by single bonds and these electrons do not respond to visible light. The electronic structure of graphite on the other hand is stacks of sheets of carbon in which the electrons are highly delocalized in such a way that essentially all visible light is absorbed. As a result graphite is black.      The measure of the ratio of the transmitted power, Ptrans. (energy / sec) to the incident power, Pincid.:  Ptrans. / Pincid. = Tr is called the transmittance (or in the older literature the transmission). This ratio has a range: 0 < tr =" 10^-1" tr =" 10^-5" n =" 1">