Modifying the Optics Laboratory for Greater Conceptual Understanding

1. Modifying the Optics Laboratory for Greater Conceptual Understanding Timothy T. Grove Mark F. Masters Indiana University Purdue University Fort Wayne

2. Background… • Five years ago, our optics lab had not been updated for at least 20 years. • The equipment was literally falling apart and poorly organized. • The laboratory exercises explicitly told students to perform tasks in a manner similar to following a recipe in a cookbook. • Our observations of the students were that they going through the motions and not really understanding the basic optics (they often parroted the instructors statements). • We had recently completed successful revisions of our introductory labs and modern physics lab.

3. Our goals… • We want students discovering optical physics through experimentation (not the usual approach for advanced labs) • We will directly confront the students’ misconceptions. • Students will predict and test their predictions as opposed to following cookbook like procedures. • We also want students to develop greater independence in the laboratory. • We want students to use their new understanding of optics to accomplish a project with minimal instructor involvement • The students design their own experiment (equipment layout, analysis, etc.) • This requires familiarity with the equipment

4. Some of the hurdles we had to work around • Giving too many instructions runs counter to fostering student independence. • To perform a reasonably complex optical experiment, students must have certain basic skills, such as… • Handling/cleaning optics • Use of lenses • Knowledge of imaging systems • Using mirrors for alignment • Maintaining polarization after reflections • Use of wave plates and other polarization optics. • Not all investigations lend themselves to this approach; we had to select certain key topics.

5. Topics specifically covered in Lab • Point and extended sources • What is an image? • Real and virtual images • Image location: virtual images really do form behind the optic. • Does an image require a screen to be visible? • Point to point correspondence of images and objects • All rays do NOT pass through focal points • Why one would use a mirror rather than a lens • Polarization of light, ½ and ¼ wave polarizers. • The differences between circularly polarized and unpolarized light.

6. Some topics not covered in Lab • Interference and Interferometry • This was a painful exclusion (at least for me). • Our initial attempts to cover this topic in lab lead to far too much cookbook-like instruction. • Ultimately, we found it best to cover this in the lecture portion of the course with hands-on demo equipment; some later optics projects could be along these lines. • Fiber optics • Once more, we found our attempts lead to far too much cookbook-like instruction. • Again, this was covered in the lecture portion with hands-on demonstrations.

7. Lab Schedule Week 1 Lab Intro (cleaning optics) Week 2 Point and extended sources. Adapted from “Tutorials in Physics,” McDermott and Schaffer Week 3 Refraction Week 4 Image formation Week 5 Lens 1 (real image formation) Week 6 Lens 2 (predicting image location) Week 7 Lens 3 (virtual images) Week 8 Reflections and alignment Week 9 Curved mirrors vs. lenses Week 10 Polarization 1 (Linear polarizers and half-wave plates) Week 11 Polarization 2 (Quarter-wave plates and elliptic polar.) Week 12 Polarization 3 (Polarization and reflections) Week 13-16 Optics Project Lab Write Ups available at http://users.ipfw.edu/masters/

8. Some of the “unusual” methods we developed • Extensive use of webcams • Webcams are inexpensive and are easily connected to most computers • With webcams students can record and print out pictures of images. • This makes it easier to deduct points for missed observations such as whether the image was inverted or not. (Students find it harder to complain when there is concrete evidence in front of them). • Examining how images form on webcams • We first remove the lens that is attached to the webcam. Then students can experiment with where images form relative to webcam’s optical detector. • Using webcams to locate where the image forms • By using a lens-less webcam and a 200mm lens a fixed distance away from the webcam, there is a small depth of field for the webcam.

9. More of our “unusual” methods • In order to shorten the duration of the laborious task of turning a linear polarizer and then determining the elliptical polarization, we interface a rotation stage to a computer through a program we named “Polarama 2.1”. Polarama determines the shape of the light’s elliptical polarization. The method used in the program is explained in the investigations (i.e., the program is not a black box)

10. 200mm Focal length Lens centimeter ruled screen Webcam Light Source Monitor ????? Light Source 200mm Lens Iris Webcam f di do Example Fragment 1 Consider what would happen if we add an iris to the above set-up and remove the cm ruled screen. A student suggests that an iris will have a minimal effect if it is placed at the focal point of the 200mm lens (see the sketch shown below). The student explains that this works because all the rays must pass through the focal point. Do you agree or disagree with this students reasoning? EXPLAIN YOUR THOUGHTS.

11. Lens 200mm Color filter Lens-less Webcam Point Source Monitor 1.50m ????? Example Fragment 2 Set up the following. Use the Pasco point source with a strip of “translucent” tape over the hole. Then attach a pinhole over the “translucent” tape to act as a point source. Use the 50mm diameter, 200mm focal length lens. • Record the distance from the lens to the camera for the following circumstances. Make sure you readjust the webcam for an optimally focused image each time. Note: you may have to adjust the shutter speed of the camera so that it doesn’t saturate. • The students now insert a red, green, blue and no filter and observe how they must move the webcam to get a focused image.

12. Lens-less Webcam 200mm focal length Monitor  ????? Point Source Color filter 1.50m Fragment 2 continued Set up the following using a 200mm focal length concave spherical mirror. Try to keep the angle  as small as possible. Record the distance from the lens to the camera for the following circumstances. Make sure you readjust the focus each time. Note: you may have to adjust the shutter speed of the camera so that it doesn’t saturate.

13. Laser Second /4 plate First /4plate Linear Polarizer in hand-turned mount Photometer detector Photometer detector 632.8nmfilter Linear Polarizer in hand-turned mount 50mm lens /4 plate Quartz-Halogen Lamp Polarization Investigations • Use of a polarizing beam splitting cube to diagnose linear polarizations • Discovery based activities to determine the effects caused by half-wave and quarter-wave plates. • Examination of the differences between unpolarized light and circularly polarized light

14. Optics Projects • We assign the projects to students. • In our previous lab renovations, we found that it takes time and continued effort to get students to perform an independent experiment (at least a reasonable one). • Due to limited numbers of students, we can only offer optics lab every other year. • As a result we could conceivably have sophomores and seniors in the same class. • Examples • Case study: Can one use diluted corn syrup in acrylic cuvettesto make a half wave or quarter wave plate? • Build a spectrograph. • Frustrated total internal reflection. • Static Fourier Transform Spectroscopy.

15. Conclusion • We developed a set of labs geared to an upper level optics course • The labs are discovery based and focus on various student misconceptions • These labs are available at http://users.ipfw.edu/masters/ and through comPADRE