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Question:
I am in the state of disagreement with my physics teacher.
One of his tests the following question appeared:
A line spectrum is produced by an incandescent:
A) solid
B) liquid
C) gas
D) none of the above
I answered D and got the question wrong. I know solids do not produce a spectrum, but they do produce a full one. I do not know about liquids. And I did not think gases could be incandescent. After I got the test back I went to the library and looked in a enormous scientific manual and it said that incandescence was, "radiation being emitted by a HOT body." I really do not care if I am right or wrong now, I just want to know the correct answer. My teacher told me that the gases were "hot bodies," but I did not believe him. Who is right me or him?



Replies:
Atoms from different elements are different sizes. If you are familiar with the periodic table there are trends in size associated with an atoms position in the periodic table. Generally, atomic radius decreases from left to right in a single row of the table. Size increases from top to bottom of the periodic table.

gregory r bradburn


Your teacher is right as usual. A gas certainly can be "incandescent" -- the "body" part of the definition is just physics jargon for any arbitrary object, solid liquid or gas. The reason the answer is "C" is actually an area of physics that I personally do my work in - the electronic states of condensed systems. In a gas, the atoms or molecules are far enough apart from one another that every electron has only a small number of possible states, and the transitions between those states are what produces the lines in the spectrum. In the liquid or solid, the atoms are all very close to one another and the electron orbitals (of the valence electrons) merge producing a broad range of energy levels that the electrons can inhabit. So the spectrum from a solid or liquid has a wide range of energy levels and looks pretty much like a continuum in the visible (although different materials may do it stronger in some regions than others).

Arthur Smith



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