July 14, 1996
Each person will have a Project Star Spectrometer to observe with. There is a holographic diffraction grating mounted in a plastic circular sandwich at one end; this is the end that you will look through. Note that the circular disk can be rotated until you get a nice spectrum which lies between the two sets of numerical markings that you see when you look through the grating; the bottom numbers represent the wavelength of light in nanometers (nm); the top numbers represent the energy of the light in electron volts (ev). The spectrometers are quite fragile; please treat them with care.
At the opposite end of the spectrometer is the all-important slit, which you can see by looking down the rectangular hole on the right side. You can also see the slit by looking through the front of the spectrometer toward the extreme right corner. Try this, because I want you to notice if there is also another tiny unintended slit even to the right of that; the manufacturers did not do a great job in guarding against stray light entry... you may need to place your finger over this unintended light entry if you are looking at a faint source. Because the slit is where the light enters, you always want to point the slit directly at the light source.
On the top face of the spectrometer is a list of lines of various elements that you might see in the course of this lab. This list may help you identify the composition of a particular light source.
Before starting, a quiz on the reading that you were supposed to do for today; without books! List the 3 types of spectra, with a description of each, and what type of source(s) produce each of the three types. Your group can consult verbally on this quiz, although everyone should write in his/her own words (in the lab book!) without looking at another's paper. As soon as you are done with the quiz, bring it to me so I can check it and then you can start the lab. Write down everything you do in this lab directly into the lab book. Write in pen, and do not erase (or completely obliterate) anything that you write, even if it is in error. You cannot rewrite later the data that you gather in this (or any other) lab.
SECTION 1: Bryan 448 (front Physics Lab)
1) Active range of color vision
Determine the range of YOUR color vision (in nm) with the spectrometer. If it's a sunny day you can look at the blue sky (but NOT directly at the sun, right?); the spectrum will be fairly faint, and you may have to try covering up that stray light entry place mentioned earlier. You could also look at a frosted incandescent bulb that will be used in the next part. Don't agonize over this measurement; within 10 or 20 nm is good enough. Record the minimum and maximum wavelength seeable.
Did everyone in your group have the some active range of color vision, to within experimental uncertainty?
2) Changes in color and spectrum by a light source of variable temperature.
Attach a dimmer switch to a frosted incandescent light bulb in order to vary the temperature and brightness of the bulb. Start with the bulb at its brightest and gradually decrease the intensity. How does the color of the filament change (without using the spectrometer)? (With the spectrometer) how does the spectrum change?
3) Look at the spectrum of a color computer monitor showing a white screen. Reproduce the spectrum as best you can in your lab book. How is it different from the spectrum of sunlight and the incandescent bulb?
4) Look at and record the spectrum of the light from a butane lighter. Comparison to the spectra already observed above?
SECTION 2: Bryan 448 (front Physics Lab)
Look at each of the 4 gas discharge sources individually. In order to see the wavelength or energy scales, you may need some room light. And you'll have to be close up to the light source in order to see enough light from it to make a spectrum.... be careful to neither stick a finger into a live part of the power source nor knock over the power source. Try not to leave the power on for more than one minute at a time as the bulbs become extremely hot.
Try to draw a representative spectrum (you might estimate the wavelength of 3 or 4 of the brightest lines in each source; if there are many lines in a certain part of the spectrum, you might note that too) in your notes for each light source; it would be nice if you were organized and had each spectrum spanning the same horizontal distance and aligned vertically one over the other. It would also be nice to have the four spectra large enough so that the entire set covered the whole page.
Your ultimate goal is to try to identify each source using the wall chart. Use either the chart in Giancoli (p. 634), the wall chart hanging on the bulletin board, or the wavelength list on the spectrometer face to identify the element. Neither the Giancoli or wall charts will have the intensities of the lines correct, however!
The spectrometers are not perfectly calibrated; therefore the values you estimate for the wavelengths may be incorrect by up to 10 nm but not more, I suspect. For identification purposes, matching the pattern's brightest lines (to those given in Giancoli or on the wall chart) is as important as matching specific numeric values of the wavelength.
Check with the instructor at this point to make sure that you have correctly identified the 4 unknown sources.
When successful, use your work to identify the gas present in the fluorescent lights on the hall. Explain the basis of your identification.
SECTION 3: Bryan 442-444 (Astro classroom)
Use one of the overhead projectors (with an attached holographic grating) to produce a spectrum on one of the three walls of the classroom; focus the spectrum onto the projection screen or a large piece of white paper taped onto the wall. Remember that a slit is necessary to produce a spectrum. Color filters and colored liquids are also available for use here.
1) Before placing the red color filter over the slit, predict (in writing of course) what effect this will have on the appearance of the spectrum. Then write what actually happens. (Three columns, labeled with the filter color, your prediction, and then the actual results would be nice, huh?). By the way, experiment with the width of the slit.... what happens when the slit width increases? what happens if the slit width becomes too small?
In turn, predict and then try the blue and the green filters. Then try magenta, cyan, and yellow.
2) Now try the bottles of colored fluids (plastic bottles first; glass last). Again experiment with the slit width. How does the absorption produced by the liquids differ from that produced by the color filters?
Should the slit width be adjusted in the same manner as it was for the color filters?
3) Now use the spectrometer to look at the spectrum of sunlight. Aim the spectrometer at bright blue sky or a bright cloud (not directly at the sun). Can you see faint dark lines? Record their wavelengths in a table along with any element identifications you make. For id purposes, remember the table on the front of the spectrometer; the solar spectrum is also displayed on the wall chart associated with SECTION 2, in a separate chart on the bulletin board (with origins of the lines identified), and in Giancoli (p. 634).
4) Determine the wavelength of yellow light by using the overhead-with-grating and the interference condition for a diffraction grating. There are 750 lines/mm engraved on the grating
You will also need a meter stick or measuring tape.
SECTION 4: Bryan 457 (Astronomy Storage Room)
Each light&color box has a light source with two leads that plug into a 12-volt ac transformer. Also provided are color filters that can be inserted into one or more of the three slots surrounding the internal light source. The movable side walls can then be adjusted so that different color regions overlap on the flat table top. To see these color regions, put a blank sheet of white paper underneath the front of the light box. Cover the back exit (opposite the light source) with one of the pieces of opaque black plastic that contain one or more slits to prevent stray light from bothering other people's experiment. Warning: the light gets extremely hot when left on for a long period of time.
1) Color addition of lights of the additive primaries ( = red, blue, green)
Place these three color filters in the three slots surrounding the light bulb. Adjust the mirrors on the two movable doors so that two or more regions of color overlap.
(WARNING: some of the color transparency sets do not have quite perfect colors; if your results seem suspicious, let me know immediately.)
Identify the color of the following sums: R+B; R+G; B+G; R+B+G.
2) Color addition of lights of the subtractive primaries ( = magenta, cyan, and yellow)
Identify the color of the following sums: M+C; C+Y; Y+M; Y+M+C.
Show why each sum is as observed using one of the rules from part 1; further hint: what would you get from W(hite)+B ?
3) Color subtraction with the primary subtractive colors
You now have seen two demonstrations that R+B+G = White as long as we are talking about adding light. It should be fairly easy to deduce what a cyan filter does to white light. Write two different sentences for what happens when white light passes through a cyan filter; one sentence may use the words blue and green; the other sentence may not.
We could write similar sentences for the behavior of a Y or an M filter.
Now predict what you should occur if white light passes through 2 filters back-to-back, e.g., W-C-Y or W-Y-M or W-M-C or W-C-M-Y.
After making predictions (in writing of course), experiment to see if you are correct.
4) Color subtraction with the primary additive colors
Write (as in the previous section) two sentences that describe what happens when white light goes through a red filter; one sentence can use the word red; the other sentence can't.
Now predict (and then test): W-R-G or W-B-G or W-R-B.
SUMMARY QUESTIONS
1) Which light sources in this lab produced a continuous spectrum? (I think there were at least 5?) What did all of these sources have in common (besides emitting a continuous spectrum)?
2) Most all of these continuous spectrum light sources produced something that we might fairly call "white light." What other source of white light that you looked at in this lab did NOT produce a continuous spectrum?
3) List the light sources or combinations that you observed that are examples of the other two Kirchhoff laws.
4) Did the spectrum of a computer monitor (Section 1, Number 3) resemble that of anything else that you looked at in the lab? (Is the physical state of the light source in the spectrum similar to anything else in the lab?)
5) Consider all the sources of dark line spectra. In some cases you saw narrow dark lines and in other cases you saw dark bands; there were even some intermediate cases. Can you see any general trend in the property of the absorbers as you go from narrow lines to broad lines to bands?
6) Did you see any violations of Kirchhoff's laws in the lab? Which?
7a) Can a Blue and a Yellow transparency (i.e., filter) be used with light&color box to produce Green? If so, describe how/why and then test your hypothesis.
7b) Can these same transparencies (Blue & Yellow) be used with the light&color box to produce White. If so. describe how/why and then test your hypothesis.
7c) If you shine Blue light on a Yellow piece of clothing, what color will it look? Is this experiment the same as one of the 2 previously described experiments? If so, which? Why?
8) In Section 4 you saw how both the additive and subtractive colors could be used in both additive and subtractive ways. How are color slides produced? How about color printing (e.g., newspaper comics or glossy magazines)? For each process, which colors are used and in which (additive or subtractive) way. To really be convincing, you should be able to explain how an additive primary color (e.g., blue) is produced in a slide or in a comic strip and also how a subtractive primary (e.g., yellow) is produced. For this part, you will have access to color transparencies (which can be "taken apart") and newspaper comics (which can be studies with a hand lens). You will also be expected to do some outside research/reading.