| Author | Message | ||
     Donald J Dahm (djdahm) Advanced Member Username: djdahm Post Number: 22 Registered: 2-2007 |
Howard: Thank-you for the amplification. For absorption to occur, there must indeed be an absorption mechanism. It is probably wise not to make general statements about absorption or scatter as a funtion of wavelength until one have more fully described the sytems involved. Don | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 210 Registered: 9-2001 |
Don - I have to make one small correction to what you said, in particular your statement: "As wavelength increases, the scattering power goes down, and the absorption power varies with wavelength depending on the composition, but on the average, will tend to increase with wavelength." The part of that statement dealing with absorption is not generally true, although it may be true in particular parts of the spectrum, and for which you give one example. Another example of that behavior is in the NIR and IR regions: in general, IR absorptions are stronger than NIR absorptions, and in the NIR region the absorptions tend to get weaker as you get closer to the visible portion of the spectrum. However, in comparing the behavior of NIR to visible light, for example, in general visible light is much more strongly absorbed than NIR light is. The reasons for this lie in the physics controlling the absorption processes in the two spectral regions. Visible light (and UV, for that matter) interact with the outer electrons in atoms and with the bonding electrons in molecules. NIR interacts with the nuclei in molecules. The difference in the physics makes for substantial differences in the absorption properties, so in general, you have to consider each spectral region separately, to know how it behaves, there's no single rule that covers the entire E-M spectrum. In your own example of X-rays, for instance, there is still different physics that is operative, and the behavior of X-rays is still different from any of the longer-wavelength regions. The fact that X-rays have higher penetrating power is almost coincidental; you could not tell that a priori by comparing the mechanisms. Incidentally, before Karl Norris did his groundbreaking work, that ushered in the era of modern NIR analysis, every chemist "knew" that all useful absorbances were in the visible and UV regions, and that there was "no" absorbance in the NIR. This is due to the fact that instruments that were typically used for analysis in the UV-Vis parts of the spectrum were not sensitive enough to detect the orders-of-magnitude smaller absorbances that is characteristic of the NIR. BTW - Your summary of the behavior of scattering vs absorption is the best one I've seen - EVER \o/ /_\ | ||
| Donald J Dahm (djdahm) Advanced Member Username: djdahm Post Number: 21 Registered: 2-2007 |
Depth of penetration is a rather complex issue. As you say, it does depend on both the absorption and scattering. Since we are dealing with a complex issue, we try to understand it by learning about what happens in special cases, and apply those special cases to the situation at hand. First off, light (a wave) travels in a straight line until it encounters �matter�, frequently called an �object� but which I will call a scattering center. When light encounters the scattering center, we may picture some of it as being absorbed and some of it scattered in all directions. 1)�A special case we understand pretty well is that of light travelling in a medium where the scattering centers are very small compared to the wavelength, and the objects are placed randomly, but many wavelengths apart. The transmitted light intensity at any depth falls off exponentially as a function of absorption and scatter (the power of which we may describe by the use of �coefficients�). As wavelength increases, the scattering power goes down, and the absorption power varies with wavelength depending on the composition, but on the average, will tend to increase with wavelength. [For example, we know x-rays are more penetrating than visible light.] The above case describes visible light traveling through the atmosphere, where absorption by individual molecules of air are small and the scatter of blue light is greater than that of red light. So the sky is blue. This case is so classic, that it dominates our thinking about spectroscopy. I would like to update your head by saying (overstating) that this is the only time that the depth of penetration is wavelength dependent due to scatter. 2)�For dense systems, where the �objects� are placed closely together compared to the wavelength, the directly transmitted light falls off primarily due to absorption. Here, a random placement of scattering centers tends to cause cancellation of scattered waves except in the forward direction. (Non-random placement causes diffraction effects. These effects are most obvious when the spacing is of a size of the order of the wavelength.) At edges and corners, there is not complete cancellation and the wave appears to �bend�. This case describes transmission of light in liquid water. At depths of 100 feet the red light is absorbed to a greater extent than the blue, so my underwater photos look eerily blue. 3)�The case where the depth of penetration is limited by scatter is where absorption is small, and the objects are separated by distances large compared to the wavelength. The everyday example is that of a water cloud and visible light. A cloud lets little direct light through (so you can�t see your shadow), and the amount of scattered light that gets through depends on the number of water droplets encountered. However, notice that the light scattered is white, not blue. The larger size of the water droplets (compare to air molecules in 1� above) means that there is little wavelength dependence for the scatter. I notice in the other thread that your interest is scattering in tissue. Here there are many scattering centers. In regions of high absorption the depth of penetration tends to be limited by absorption (the effect of which is increased by the scatter). In regions of low absorption, the depth of penetration tends to be limited by the scatter. | ||
| Jerry Jin (jcg2000) New member Username: jcg2000 Post Number: 2 Registered: 1-2009 |
And again,I am confused by the penetration depth of electromagetic radiations. In long-wavelength radiation such as radio and sound, the "light" or wave can easily bend around the medium particle and travels more distance. That's why we can hear around corner. It takes more time for the energy of long wavelength to get dissipated in the transmission medium. So, longer wavelength travels further. In short-wavelength region such as NIR whose wavelength is smaller than the size of medium particle, it is a totally different story. It is the shorter wavelength that travels further,with regard to the same medium. It makes sense since short wavelength has more energy. Am I right? I know the penetration depth of any radiation should be a compromise between transmission, absorption and scattering. Is there a systematic explanation of dependence of penetration depth on wavelength? Is that relationship also medium dependent? |