|
|
|
 |
|
|
|
AP Physics Featured Question: Optics Experiment
|
|
| |
by Greg Jacobs
Woodberry Forest School
Woodberry, Virginia
|
|
| | Introduction
On the 2003 AP Physics B Exam, free-response question 4 asked for a description of an experiment that could produce an image of a candle. This featured question is similar but will likely catch many students in a trap: the image produced must be virtual—no screens allowed.
Geometric optics is one of the simplest topics in the AP Physics B curriculum. No heavy mental lifting is required; the concepts of ray tracing are simple enough, and the mathematics of the mirror equation were covered back in seventh grade. Most everyone has some sort of kinesthetic experience with mirrors and lenses, and teachers can easily demonstrate their properties.
That said, there's an awful lot of material to memorize. Students must be able to instantly recognize what kinds of images each optical instrument can produce, and they must immediately be able to translate the meanings of the signs on optics quantities. On this topic more than any other, a solid grasp of facts can lead to success.
One of the common mistakes students made on the 2003 exam question was to ignore the experimental focus of this problem. Students who were comfortable with the mathematics and facts of the mirror and magnification equations seemed to use calculations as a crutch: "Ooh, here's a concave mirror problem, I know what equations to use here. Whew." It was easy to fail to appreciate that the question statement required an experiment. In 2003, a pure calculation of the correct object and image distances without a practical description earned no credit.
Okay, then, short of standing over your class with a whip when they take the exam, how do you teach them to approach AP experimental questions, well, experimentally?* No doubt practice with this and other AP-style experimental problems helps. Check out some of the follow-up ideas for more thoughts.
Though this question is primarily experimental in nature, it is nevertheless important that students also be able to make the calculation of image and object distance. I've included that aspect of the problem in the "Follow-Up Quiz."
Correlation to the Topic Outline in the Course Description
IV.C.1. Reflection and refraction
IV.C.2. Mirrors
IV.C.3. Lenses
"Laboratory and experimental situations," p. 18
Problem as a Homework Assignment
In the laboratory, you have a lit candle, 4 cm high, and a variety of mirrors. You must create a virtual image of the candle that is 8 cm high.
- Describe one method of producing this image. In your description, be sure to indicate quantitatively the parameters of the mirror you used, where relative to the mirror you placed the candle, and how you experimentally verified that the image height is 8 cm high.
- Draw a ray diagram of your experimental setup from part (a) that includes the locations of the candle, the mirror, and the image.
Problem Posed in AP Format
In your physics lab, you have a concave mirror with radius of curvature r = 60 cm. You must determine experimentally the location of a lit candle such that the mirror will produce a virtual image that is two times the height of the lit candle.
You have an optical bench, which is a long straight track as shown above. Objects in holders can be attached at any location along the bench. In addition to the concave mirror and the lit candle in the holders, you also have the equipment listed under (b).
- Briefly list the steps in your procedure that will lead you to the location of the lit candle that produces the desired image. Include definitions of any parameter that you will measure.
- On the list of equipment below, place check marks besides each additional piece of equipment you will need to do this experiment.
______ convex mirror in holder
______ concave lens in holder
______ convex lens in holder
______ meter stick
______ ruler
______ screen in holder
- On the scale below, draw a ray diagram of your lab setup in part (a) to show the locations of the candle, the mirror, and the image.
- Check the appropriate spaces below to indicate the characteristics of your image:
______ upright
______ larger than object
______ inverted
______ smaller than object
- Which, if any, of the following optical instruments could also produce a virtual image of the candle that is two times the candle's height? Check all that apply, and then justify your choice(s).
______ convex mirror
______ concave lens
______ convex lens
______ plane mirror
|
|
|
|
|
|
Course Home Pages