AP Optics Test

I have just finished grading your test and the results are somewhere between discouraging and disgraceful. They show the persistence of trying to memorize meaningless formulae without understanding and a lack of attention to what was done in class. I suppose I share the blame for this since while my notes emphasize the ray diagrams, they also analyze the situations using the 1/f =1/do + 1/di formula. I tried to rectify that in class, but I guess without much success.

The subject can be largely mastered by drawing three rays, and the use of two sets of similar triangles. It is a simple subject, at least to the depth required of you.

Many of you got correct answers by plugging into a formula which was completely unjustified based on your diagrams. No matter how many times I showed you the reason why images get larger or smaller only a few of you used the distance from the object to the focal point as the major consideration.

Many of you do not understand that images are formed where rays from a part of the object converge with other rays from the same part, or at least appear to do so. An unreasonably large number of you cannot identify the three rays we used and their paths.

An unacceptably large number of you didn't even locate an image although the questions specifically asked you to do so.

Many of you determined that the image height of 24 cm was diminished from an object height of 12 cm. Thinking that 24 is smaller than 12 is a new level of misunderstanding.

Most of you quoted another formula involving inverse sin rather than think that

ncrystal sin theta critical = nglass sin 90deg so ncrystal = nglass/ sin theta critical.

Too often, you are still answering examples instead of solving problems.

I have updated the notes and summary although they still use the formula too much. I urge you to use hi/ho = f/|do-f| for converging mirrors and lenses and hi/ho = f/|do+f| for diverging lens and mirrors. Many of you used f/|do+f| for converging which is wrong.

http://h1.ripway.com/DrCherdack/NotesonGeometricOptics4-10.doc
http://h1.ripway.com/DrCherdack/SummaryofGeoOpticsrev4-10.doc