Transcript Modifying the Optics Laboratory for Greater Conceptual

Modifying the Optics Laboratory for
Greater Conceptual Understanding
Timothy T. Grove
Mark F. Masters
Indiana University Purdue University Fort Wayne
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.
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
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.
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.
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.
Lab Schedule
Week 1
Week 2
Lab Intro (cleaning optics)
Point and extended sources. Adapted from “Tutorials in
Week 3
Week 4
Week 5
Week 6
Week 7
Week 8
Week 9
Week 10
Week 11
Week 12
Week 13-16
Refraction
Image formation
Lens 1 (real image formation)
Lens 2 (predicting image location)
Lens 3 (virtual images)
Reflections and alignment
Curved mirrors vs. lenses
Polarization 1 (Linear polarizers and half-wave plates)
Polarization 2 (Quarter-wave plates and elliptic polar.)
Polarization 3 (Polarization and reflections)
Optics Project
Physics,” McDermott and Schaffer
Lab Write Ups available at http://users.ipfw.edu/masters/
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.
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)
Example Fragment 1
Light Source
200mm
Focal
length
Lens
centimeter
ruled screen
Webcam
Monitor
?????
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.
Light Source
200mm
Lens
Iris
f
do
Webcam
di
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.
Point Source
Color filter
Lens
200mm
Lens-less
Webcam
Monitor
1.50m
?????
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.
Fragment 2 continued
Set up the following using a 200mm focal length concave spherical
mirror. Try to keep the angle  as small as possible.
200mm
focal length

Point Source
Color filter
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.
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
QuartzHalogen
Lamp
50mm
lens
632.8nm
filter
/4 plate
Linear
Polarizer in
hand-turned
mount
Photometer
detector
Laser
First /4
plate
Second /4
plate
Linear Polarizer
in hand-turned
mount
Photometer
detector
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 cuvettes to
make a half wave or quarter wave plate?
 Build a spectrograph.
 Frustrated total internal reflection.
 Static Fourier Transform Spectroscopy.
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