- Computers are needed for lab this week
- Accepted values for this experiment:
- dactual = 0.0351 cm
- dwall ≈ 0.65 cm to 0.68 cm
- Lslide = 4.760 cm
- Lwall ≈ 90 – 95 cm (depends on projector placement, but will be <1.0 m)
- λred = 6.328×10-5 cm = 632.8 nm (I generally accept any wavelength of 600 nm or longer, ~5%)
- The distance from the grating to the wall, D, will vary depending on the setup used and the position of grating on the bench. Approximate distances:
- Back wall: ~5 to 5.2 m
- Movable table: ~4.4 m
- Side wall: ~3 m
- Important note: If students have the distance to the side wall as 3.10 m (310 cm), most likely the distance was 3.01 m (301 cm). This should be checked
- Be sure to emphasize during the pre-lab that notation is important, so D ≠ d
- Lslide measurement tip: Place slide on the laptop display to use as a light table
- All the windows are blocked, and all lights will need to be turned off while students are marking the interference spots on the wall. After they are finished, turn on just the back bank of lights (closest to the windows) so that there is subdued lighting on the wall where their Cornell slide will be projected
- Jeff will tape a piece of white printer paper to the wall; it is easier to see the slit pattern for measuring dwall. It also shows students the correct pair of slits to measure
- The lasers should be aimed at the center of a cinder block on the wall, so that students aren't marking points in the mortar lines between blocks. The laser and Cornell slide can be elevated if the laser is too close to a mortar line, or there is some obstruction in the lab; use a textbook or the vernier caliper cases
- Students should hang their target sheet horizontally on the wall. The distances, xn are measured with a ruler (to 0.1 cm), not the vernier calipers
- They must mark n = 0 before removing the paper from the wall!
- Briefly explain the operation of the laser (e.g. how to turn On/Off) and talk about laser safety
- Also note that eyeglass wearers should take extra care if they're marking the spots on the wall. It's easy to catch a reflection of the laser in the corner of your glasses while facing the wall
- A mid-1940's vintage Spencer-American Optical Model GK Delineascope is used to project an image of the aperture slide on the wall; the power switch is on the right side, just above the rear support foot. Note that the wall rack holding the banana-plug wires might need to be removed
- If you have a very large group, you might want to have some measure their gratings before marking the spots on the wall
- The Delineascope is wicked hot when running, so don't turn it on until it's needed, and then leave it turned on. Turn it off only when all students have finished measurements and have an acceptable value for the slit separation, d!
- Don't leave the Delineascope running unattended for long, and unplug it when you turn it off. Ask Jeff about the other Delineascope that caught fire!
- Students must place their slide in the Delineascope as shown in the instructions (details below); in this manner, their slits are centered on the bottom of the image, and Lslide is to the right so that it clears the coat rack. Slide loading procedure:
- Face wall
- While facing the wall, and reading the label on the glass slide, turn 90° counterclockwise
- Insert slide into holder on left side of projector
- Push the slide holder completely into the projector
- Students will measure the separation of the slits on the wall (dwall) by carefully following the procedure in the instructions, measuring on the left and right side of the slits. The uncertainty in dwall is calculated as the difference between this left and right measurement, so it's imperative that they close the vernier calipers between measurements. You should mention to them the limitations of using the vernier calipers in this manner; they can't 'clamp down' on something for the measurements!
- Common student screw-ups:
- They still don't know how to read the vernier calipers! dwall should be around 0.6 cm; if they get 0.4 or 2 cm, they're seriously confused
- You need to keep an eye on them at all times: they will project the laser through the wrong aperture, or they will measure the wrong pair of slits when the slide is projected on the wall. *sigh*
- They will create a KaleidaGraph plot of xn vs n. This graph will be linear; they should know to use the 'Linear w/Uncertainties' fit, and not 'Linear thru Origin' or the plain, built-in linear fit – but they don't. If their graph is a 'V' shape, they forget that xn should have positive and negative values!
- Their xn vs n graph should be a nice straight line. If it's wavy or has some 'sawtooth' sections, then they screwed up numbering the spots. Either they didn't skip numbers for blank (minima) areas, or they skipped the wrong number. They also don't always mark n = 0 on the paper, or measure xn as carefully as they should. They should fix these problems, but sometimes this experiment is the week before spring break, so most just won't give a damn
- The slope of the graph should be <1 cm (≈0.5 for the side benches, to 0.9 cm to the back of the room, depending on D); if they get the slope > 1 cm, then they probably plotted n vs. xn!
- When everyone is finished, tip over all the flashlights. Students will frequently leave them turned on and place them face-down on the bench, draining the batteries
- Jeff has a spreadsheet to check student results. Laser wavelength and uncertainty are calculated from their basic measurements, KaleidaGraph slope and Standard Error
- The %diff between d and dactual is calculated in the spreadsheet, as well as λ and λred. These %diffs should be the same; if not, students likely made an error in their measurement of D.
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