cyeokpeng Posted October 26, 2004 Posted October 26, 2004 I reckon we have to use quantum mechanics to explain why glass can transmit visible light through but not objects like ceramics and metals. In addition, why is glass able to block off or reflect totally certain cutoff wavelength of radiation in the infrared range? We know that because this is the mechanism of Greenhouse Effect. In addition, I know that when light propagates and is incident on a material, 3 things can happen. Certain range of wavelength of light is absorbed and other ranges of light is reflected, which is the basis of the theory of colors. Lastly, light can also be transmitted right through the material if the material is transparent to this range of wavelength of light. How can we explain that quantum mechanically? Also, as I have learned, as EM waves of certain wavelength is incident on a cystal lattice, it can get Bragg reflected and there will be peaks at certain angles depending on whether Bragg condition is satisfied and also on the atomic structure factor (ASF directly affects the intensity of the peaks in addition to controlling certain disallowed diffraction planes). Can someone shed some light on this esp. on the atomic structue factor as I still do not understand the mechanism of how the atomic structure factor affects totally disallowed 1st diffraction planes and also on the intensity of the diffraction peaks observed at allowed diffraction planes.
ed84c Posted October 26, 2004 Posted October 26, 2004 just a guess, but are photons in an opaque material adsorbed by the electrons (which go up one energy level) and those in a transparant, the electrons for some reason do not want to adsorb a photon.
1veedo Posted October 26, 2004 Posted October 26, 2004 In QM, everything is, well, quantified. Lets say that you need $1 to get on the train, and the money collector is only collecting exact change. If you have $2 you cant get in, if you have 1.01, you cant get in (of course, 1.01 cant happen in QM). Lets say a photon’s frequency is x. If x is the frequency it needs to be remitted, then it will be. If x is whats needed to be observed and remitted at another wave length, then it will be etc. And, as you’d guess, the same holds true negatively and conversely.
swansont Posted October 27, 2004 Posted October 27, 2004 Note that adsorption and absorption are different phenomena. You meant absorption.
swansont Posted October 27, 2004 Posted October 27, 2004 In QM, everything is, well, quantified. Bound state energies are. Free states are a continuum.
Mokele Posted October 29, 2004 Posted October 29, 2004 Lets say a photon’s frequency is x. If x is the frequency it needs to be remitted, then it will be. If x is whats needed to be observed and remitted at another wave length, then it will be etc. And, as you’d guess, the same holds true negatively and conversely. So, why is it that glass permitts so many different frequencies through, as opposed to just one or two?
Ophiolite Posted October 29, 2004 Posted October 29, 2004 So, why is it that glass permitts so many different frequencies through, as opposed to just one or two?Pure guess: it likely is related to the fact that glass is a supercooled liquid. The amorphous micro-structure must some how facilitate the passage of light. Most transparent 'things' are fluid, most opaque 'things' are solid.Too tired to google for the truth.
Mokele Posted October 29, 2004 Posted October 29, 2004 But quartz crystals are transparent, and they're crystaline, ditto for diamonds. And merucry is liquid, yet not transparent.
Ophiolite Posted October 29, 2004 Posted October 29, 2004 Which is why I said 'most'. Mercury is unusual anyway. Not just quartz crystals are transparent, there are plenty of others too. Interesting point there is the question of purity. Most (there's that word again) opaque or translucent minerals either contain some impurity or are part of continuous reaction series. The transparent ones, like quartz, have a very pure composition and consequently a very uniform crystaline structure. Again, just thinking out loud.
Skye Posted October 29, 2004 Posted October 29, 2004 Glass absorbs in the UV range, so does quartz but at a higher frequency. If we could see UV I imagine it would look opaque.
swansont Posted October 29, 2004 Posted October 29, 2004 Pure guess: it likely is related to the fact that glass is a supercooled liquid. No, it's not
Ophiolite Posted October 30, 2004 Posted October 30, 2004 You vindicated my uneasy feeling when I wrote that, but it's been a couple of decades since I had to worry about binary eutectics, phase transitions and the like. I suspected someone might challenge me on it. Thanks for setting me straight and for the useful link.
Mokele Posted October 30, 2004 Posted October 30, 2004 Ok, I'm a bit confused, and also not a physics major. Are materials transparent to all but a few wavelengths, or opaque to all but a few?
ed84c Posted October 30, 2004 Posted October 30, 2004 yes (in the case of glass and other translucents). Proof can be found from anybody who has tried to get a suntan from sitting near a windows, im sure!
TheProphet Posted November 3, 2004 Posted November 3, 2004 Note that adsorption and ab[/b']sorption are different phenomena. You meant absorption. adsorption? neverheard of that please more info Swansont!
swansont Posted November 3, 2004 Posted November 3, 2004 ad[/b']sorption? neverheard of that please more info Swansont! Very basically it's molecules sticking to a surface. The attraction is fairly weak and you don't form a chemical bond, in the case of physisorption (IIRC it's van der Waal's forces) You can usually undo it by heating the solid. A lot of filtration uses this method, like charcoal filtering of water. If you do form a bond, it's known as chemisorption. And if we Google on the right terms...Here is more.
Mokele Posted November 5, 2004 Posted November 5, 2004 Ok, since noone answered by question in an understandable way, I'll repeat it: Are materials transparent to all but a few wavelengths, or opaque to all but a few?
cyeokpeng Posted November 12, 2004 Author Posted November 12, 2004 Yes Let's take for example glass. Glass is transparent to the visible light spectrum and shorter wavelength EM radiation like the UV light. However, glass is opaque (or block/totally reflect) infrared radiation and radio waves (Not sure about radio waves. Correct me if I m wrong) which are your EM spectrum at longer wavelengths. It also depends on the material. Most materials are opaque to a lot of frequencies of EM radiation, otherwise you can see through your tables, walls and most of your common materials.
YT2095 Posted November 12, 2004 Posted November 12, 2004 plastics like Polyprop or PVC will allow UV to pass through, that`s why poly tunnels are better than greenhouses for many plant growing applications. my 2 cents
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