|MadSci Network: Physics|
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Question: I was wondering if someone could help me to get a clearer picture of why some elements, tungsten for instance, produce a continuous spectrum whilst others, zinc/cadmium, produce line spectra. I am unsure how I could possibly predict whether an element would produce one or the other type of spectrum. As both types depend on electron transitions from higher to lower energy levels within their atoms, then I don't see how there are two different results. It is something to do with the temperature of the substance whose spectrum I am measuring? Tungsten I know has a very high melting point and can therefore withstand very high temperatures. However, surely its atoms contain only specific energy levels and thus only specific transitions can occur, thus resulting in specific lines. This is obviously not the case!“
There are several pointers to the answer to your question in the question itself. You mention the high melting point of tungsten and this property and the low volatility of the metal even when white hot makes it most suitable for use in light bulbs. The emission from such bulbs which we call incandescence is, as you say, continuous with a simple single-peaked spectral shape. The ability to predict the shape of this curve was first demonstrated by Plank and was a major triumph for the new quantum theory. Plank’s model of the “ideal black-body” dealt with the energy states associated with thermal energy, that is the vibration of atoms and molecules within the glowing body. As you point out, the emission of electromagnetic radiation accompanies a change in energy state and if the body is hot enough the changes in energy state are sufficient large for the emitted radiation to be in the visible region. The important point here in relation to your question is that this incandescent behaviour is almost independent of the emitting substance and depends mainly on temperature. ( In the case of the “ideal black body” it is only dependent on temperature but in practice materials deviate from this predictable behaviour. ) Any metal including zinc and cadmium will behave at least approximately in a similar way to tungsten if they could be heated to the same temperature. They can be, including the two you mention, but not as lamp filaments since their melting points are low at 419 degrees C and 321 degrees C respectively. Compared with tungsten’s 3410 degrees C. Their incandescent emission would be mainly infra red or lower at these temperatures. However, if they were heated in their molten state to a high enough temperature they would also glow with incandesecnce. Of course they also emit light under other circumstances, for example when they are in an atomic state but at high enough temperatures for their upper electronic excited states to be populated. This is the case when the metals are in a flame or plasma in which case the emissions spectra are made up of lines, usually broadened depending on the pressure, essentially as discrete lines. This is the case with tungsten too. I found the following site which stated: “Tungsten is quantified by AES ( Atomic Emission Spectroscopy ) using the emission line at 207.91 nm”.
The instrument they used was one which used plasma not just heat to vaporise the samples. Often atomic emission instruments use an electrically heated tungsten coil to vaporise the sample because tungsten has such a low volatility at high temperatures. So, it is indeed the case that tungsten emits at discrete wavelengths under the right circumstances. Under other circumstances it behaves like many other materials in producing incandescent radiation including zinc and cadmium. In other words all three metals you mentioned and most others behave in both ways producing either discrete line emissions or continuous incandescent emissions depending on the circumstances.
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