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:
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.
A point light source is reflected from a focusing mirror. The student places the source to facilitate a measurement of the focal length.
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.
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.
Illustrate concepts relating to the study of optics.
Intermittent crash and lockup on Netscape 4.75 under Windows 95. Can not run on Macs.
Evaluation and Observation
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.
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
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.
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.
Ease of Use for Both Students and Faculty
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.
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.