Spectra
I. Thermal Radiation
Solids and dense gases give off a continuous spectrum of electromagnetic radiation simply due to the thermal motion of the atoms and molecules jostling each other about. For example, a chunk of lead is heated to 1,000 degrees Kelvin, then 2,000 degrees Kelvin, then 3,000 degrees Kelvin. The amount of electromagnetic radiation given off at each wavelength of the spectrum is measured, using a light meter. The following results are obtained:
Wavelength
(Angstroms) |
AMOUNT OF EM RADIATION EMITTED |
| at 1,000 K |
at 2,000 K |
at 3,000 K |
| 4,000 |
8.7 X 10-7 |
56 |
23,000 |
| 5,000 |
3.8 X 10-4 |
675 |
82,000 |
| 6,000 |
0.018 |
2986 |
162,000 |
| 7,000 |
0.26 |
7660 |
236,000 |
| 8,000 |
1.8 |
14,000 |
285,000 |
| 10,000 |
21 |
28,000 |
312,000 |
| 15,000 |
336 |
41,000 |
210,000 |
| 20,000 |
879 |
33,000 |
117,000 |
| 30,000 |
1283 |
15,000 |
39,000 |
| 40,000 |
1030 |
7249 |
16,000 |
a) Graph the amount of EM radiation emitted versus wavelength for each
temperature all on one plot. (So you should have 3 curves, one for
each temperature, on your graph. Put wavelengths on the horizontal
axis (from 4,000 to 40,000 angstroms), and amount of radiation emitted
on the vertical axis (from 0 to 350,000 ergs/cm2/s/angstrom).
b) Referring to Figure 4-3 on p. 48 of your text, label the type of
electromagnetic radiation that corresponds with the wavelengths on your
horizontal axis (for example, "x-rays", "blue visible light", "radio").
c) The peak of the curve shows the wavelength at which most of the
radiation is being emitted. At which temperature would you most likely
be able to see radiation with your naked eye? Explain your answer.
d) From the three curves you plotted, at which temperature does the
lead give off the most radiation at ALL wavelengths?
e) Describe how your graph would be different if a chunk of aluminum
were heated instead of a chunk of lead. Note that aluminum has 13
protons and lead has 82 protons.
II. Line Radiation
a) The above section described radiation from a solid chunk of
material. The spectrum of a thin gas (many examples of which are in
the spectroscopy lab) is very different from the thermal radiation
spectrum of a solid or dense gas. How is the spectrum different?
![[Atom image]](./images/Atom2.gif)
b) Describe how a photon would interact with the atom drawn at right to
create:
i) an absorption line
ii) an emission line
c) The above hydrogen atom absorbs a photon which has just the right
amount of energy to kick the electron from the 2nd energy level to the
3rd. When this energized atom relaxes, how will the wavelength of the
emitted photon compare with the wavelength of the photon which was
absorbed?
d) The spectrum from a star has continuous thermal radiation with
absorption lines. Explain how this could be (a drawing may be
helpful).
III. Stars
a) Suppose you observe three stars with both a red and a blue filter.
Star A is brighter in the blue than in the red. Star B is brighter in
the red than in the blue and Star C is equally bright in both the red
and the blue. With this information, put the three stars in order of
increasing temperature. Explain briefly how you got your answer.
b) Suppose you see two stars with the same color--the peak of the
thermal radiation curve is at the same wavelength. Yet one star
appears 100 times brighter than the other. What can you conclude?