Band Gap and Conduction Bands

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Offline McQueen

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Band Gap and Conduction Bands
« on: 09/10/2005 08:30:30 »
This thread is actually an extension of the topic dealt with in “Patency of UV light through glass.” I hope that the Naked Scientist Forum will forgive this slight lapse in protocol , as I feel that the topic is important enough to merit a more in depth discussion . Briefly the question is why does ordinary glass permit the visible spectrum to pass through without impediment while blocking  the UV component. The answer as suggested by David  Sparkman has to do with conductance bands or the ability of the material in question to absorb and emit electrons within a certain range of energies. The energy range for visible light is from 1.8 to 3.1 eV. Materials with band gap energies in this range will absorb those corresponding colors (energies) higher and lower than the band –gap and transmit   the others. They will thus  appear transparent and colorless. For example, the band gap energy of cadmium sulfide photocells is about 2.4 eV and so it absorbs the higher energy (blue and violet) components of visible light. It has a yellow-orange color as a result of the transmitted portions of the spectrum. This type of light-induced conductivity is called photoconductivity. The reason why light travels more slowly in a transparent material is because the atoms are absorbing and reemitting the photons, and this process slows down the photons. The slowing down of light as it travels through a medium results in refraction.  If the photons were absorbed without being reemitted,   then the material would not be transparent.  In order to understand this it is necessary to define , transmittance , absorption and reflection.  Transmittance or photoconduction , as has already been described , this is a rapid sequence of absorptions and emissions of photons that travel at the speed of light in the medium (glass )  through which they are passing. Absortion  occurs when the electrons are held tightly, and they pass the vibrations along to the nuclei of the atoms. This makes the atoms speed up, collide with other atoms in the material, and then give up as heat the energy they acquired from the vibrations. In the case of Ultraviolet light the UV photons are absorbed by tightly bound electrons which might cause other valence atoms to also gain energy resulting in the generation of heat , with infra – red absorption the whole atom might vibrate !  Reflection When light is absorbed and reemitted from the surface at the same wavelength, it is called reflection. Metals, for example, are highly reflective, and those with a silvery appearance reflect the whole range of visible light. The energy levels of electrons are quantized, i.e., each electron transition between levels requires a certain specific amount of energy. The absorption of energy results in the shifting of electrons from the ground state to a higher, excited state. The electrons then fall back to the ground state, accompanied by the reemission of electromagnetic radiation. The energized electrons  vibrate between the two levels and  send the energy back out of the object as  light  with the same frequency as the incoming light. This raises several very interesting questions. Firstly , how do we know that photons pass through glass by absorption and re-emission , why can’t they just pass through glass without being affected ? The answer is that apart from being transparent , glass is also reflective , meaning that absorption and emission can take place. Secondly , if it is photoconductivity  that is responsible for the transmission of  light through glass , why can’t metals , which have a wide open band-gap also transmit light. The answer is that the electrons in glass are tightly bound , while metals have numerous , free electrons and loosely bound valence electrons , thus visible light of energies of 3.1 to 1.8 e V is effectively blocked because free electrons and loosely bound valence electrons cannot absorb the forces of recoil entailed by absorption and emission. Thus in metals most of the light is reflected from the  surface , penetration is not possible. The question of light being reflected at the same frequencies as incident light , has raised the question of whether this process has to do with the intensity of light , which was discussed in the thread  ”On the Propagation of Light (In two parts )”. having to do with the number of photons absorbed in rapid succession by an electron at the receiving end , the high frequency of the emitting photon resulting in a favoured direction of emission. Lastly , the phenomenon of photoconductivity raises the possibility that   electricity is also conducted by photons. Thus if the photon is of low enough energy ( about 1.4 eV , just on the extreme lower border of visible light) it should be possible for free electrons to absorb and emit such energies , providing there is some mechanism in place enabling them to immediately re-absorb the missing energy. Thus electrical energy is conveyed by photons and not by electrons . This possibility is also examined in the second thread referred to in this post
“Sometimes a concept is baffling not because it is profound but because it’s wrong.”