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MERLOT II


    

Peer Review


Physlet Problems: Optics

by Wolfgang Christian
 

Ratings

Overall Rating:

4 stars
Content Quality: 4 stars
Effectiveness: 4 stars
Ease of Use: 4 stars
Reviewed: Jun 11, 2002 by Physics
Overview: This is a collection of physics problems relating to optics and part of the
resource CD for the href="http://webphysics.davidson.edu/physletprob/Default.htm">Physlet Book.

The site consists of four example problems:



  1. A parallel beam may refract through or be confined within a dielectric region. The student can measure the index of refraction and the critical angle using angle readouts.

  2. A point light source is reflected from a focusing mirror. The student
    places the source to facilitate a measurement of the focal length.

  3. A point light source is located in front of a microscope. The student
    measures the object distance for focussing by the microscope by adjusting the
    source to give a parallel beam at the eyepice.

  4. A "ripple tank" display of waves diffracting through a double-slit aperture
    that is located outside of the viewport. The student measures properties of the
    diffracted waves to determine the slit separation.

Learning Goals: Illustrate concepts relating to the study of optics.
Target Student Population: Lower Level Undergraduate
Prerequisite Knowledge or Skills: Study of geometric and wave optics.
Type of Material: Simulation. Java Applet
Recommended Uses: On-line homework problems, Just-in-time teaching resource, tutorial, lecture
demo.
Technical Requirements: Intermittent crash and lockup on Netscape 4.75 under Windows 95. Can not run on
Macs.

Evaluation and Observation

Content Quality

Rating: 4 stars
Strengths: The problems in this group cannot be worked by "plug-and-chug". They require a
real understanding of the underlying concepts. As such, they significantly
extend the effectiveness of most printed textbooks. Problem 1 does a nice job at
illustrating the concept of total internal reflection, and can also be used to
illustrate fiber optics. The effect of an optical eyepiece is illustrated well
in problem 3.


In each of the problems students must actually interact with the simulation to
investigate both qualitative and quantitative concepts. In the first
simulation qualitative aspects of refraction and total internal reflection are
clearly displayed. In simulation two one can demonstrate what is meant by focal
position. The third simulation displays imaging using a two lens system. The
ripple tank gives a good visualization of the interference phenomenon.

Concerns: The angle measurements used are not conventional for using Snell's law and may
need some explanation for the students. In the first simulation the outer index
must be assumed as n=1.00. In the second and third simulations, the applet
displays the position of the source rather than distance from the lens. The
answer to problem 9.9.3 is given as "x = 0.43cm". This should be "object
distance = 0.43cm" since x is the reported in the applet window as position
relative to an origin not located at the objective lens.


The ripple tank should have more discussion about what information is being
displayed (amplitude, not time averaged intensity) and how that relates to the
equations typically used to discuss double slit interference. This is a
confusing subject for students. There are concerns over the intended use of
the ripple tank. The methods that can be used to determine the answer to the
question seem to have problems. First, precise measurements are difficult so
answers will be rough approximations at best. Second is the method. One
possible solution is to use d*sin(angle) along the first order fringe, assume
this is a straight line and use point slope method to get the angle, and work
back the source separation. This would work IF AND ONLY IF one is far away so
that first order fringes lie along a straight line. This demo is not far from
sources! A second method is to work with pythagorian theorem to solve for slit
separation, which seems too involved for introductory course students.


Potential Effectiveness as a Teaching Tool

Rating: 4 stars
Strengths: The first simulation is very effective for giving students a Snell's law
assignment that includes an interactive visualization. The second simulation
will reinforce the idea of what is meant by "focal length". The concepts
gained in the third simulation are very similar to those of the second, image
formation in a slightly more complicated system. The ripple tank is a good
visual learning tool.


Each problem in this grouping is an excellent example of how physlets can be
used to promote active learning. Students must make "measurements" from the
computer screen. More importantly, they must leverage their understanding of
the underlying concepts to create a measurement strategy.

Concerns: Precise quantitative measurements are difficult to make. Without additional
careful instructions or guidance, students will have difficulty using
quantitative features that are available in all the simulations. Without a
path difference indicator, it is difficult to make precise measurements of even
wavelength. Simulation four has possible solution methods that are either not
consistent with the simulation itself, or may be quite difficult for the
intended user.


Students that understand how to find and read the javascript source can examine
this to find the answers.


Ease of Use for Both Students and Faculty

Rating: 4 stars
Strengths: It is very easy to quickly display the qualitative features of refraction and
total internal reflection in the first simulation. In general, by pointing and
clicking, it is easy to learn the type of interactions that are available in
physlets.


For the most part, the applet controls and readouts are intuitive. Identifying
which elements are adjustable and how best to adjust them is part of the
discovery process.

Concerns: In the first simulation it is difficult to have fine control on the angles.
Thus it is difficult to approach an incident angle of 90 degrees to investigate
total internal reflection. While easy to learn what to do, it is often
difficult to do (measure or move items) with sufficient precision.


For example problem 1, the transmitted beam cannot be adjusted very close to an
angle of 90 degrees, and this may confuse students.


Other Issues and Comments: These are examples that instructors can use to create their own learning
materials. Each problem page contains the answer which should of course be
removed in a locally hosted page. The href="http://webphysics.davidson.edu/physletprob/Default.htm">Physlet
Book
has information for instructors wishing to use physlets in their own course pages.