## 20150206

Physics 205B, spring semester 2015
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on lenses.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"The principal rays and how to draw them. I also understand how to find out if the object is upright/inverted, enlarged/diminished and real/virtual."

"There are two types of lenses, converging lenses, and diverging lenses. Converging lenses will take parallel rays of light and bring them to a focus at a point in space. Diverging lenses will take parallel rays of light and defocus them away from a point in space."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I am not sure what generates a real or virtual image and when it is upright or enlarged."

"The ray tracings, and am hoping that you would be able to go over them in class. The tracings do not make sense to me at all and I have no idea how to draw them or let alone name any of them."

Complete the online reading assignment ray tracings as best as you can. Identify the type of image produced for each ray tracing. (Only correct responses shown.)
Converging lens 1: real image [90%]
Converging lens 2: real image [83%]
Converging lens 3: (no image produced) [57%]
Converging lens 4: virtual image [52%]
Converging lens 5: virtual image [50%]
Diverging lens 6: virtual image [63%]
Diverging lens 7: virtual image [78%]
Diverging lens 8: virtual image [71%]
Diverging lens 9: virtual image [78%]
Diverging lens 10: virtual image [68%]

Complete the online reading assignment ray tracings as best as you can. Identify the image orientation and size produced for each ray tracing. (Only correct responses shown.)
Converging lens 1: inverted, diminished [55%]
Converging lens 2: inverted, enlarged [45%]
Converging lens 3: (no image produced) [59%]
Converging lens 4: upright, enlarged [49%]
Converging lens 5: upright, enlarged [34%]
Diverging lens 6: upright, diminished [58%]
Diverging lens 7: upright, diminished [57%]
Diverging lens 8: upright, diminished [48%]
Diverging lens 9: upright, diminished [50%]
Diverging lens 10: upright, diminished [43%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"How do you know if an image is enlarged or diminished compared to the object on a ray tracing?" (With a ruler. ALWAYS USE A RULER.)

"Do diverging lens always create upright, diminished virtual images?" (Yes, for any object placed in front of it. That is one of the key patterns to these ray tracing examples.)

"It seems like since the objects on the right side are the intersection of actual rays, they are real; but the images on the left are from extrapolated rays, so they are virtual? Is that correct?" (Yes, you've found another one of the ray tracing patterns.)

"I am not comfortable with figuring out if an image is virtual or real. The same goes for knowing exactly when an object is appears upside-down or not? Could there be a combination of where half the image is right side up and the other half is upside down?" (No. An image must be a complete, whole version of the object, so it must be entirely upright or inverted.)

"Drawing the rays is confusing! How do you tell if an image is real or virtual? I hope we go over magnification and inversion because I'm really confused." (I will do my best to get you unconfused. This stuff isn't really that hard.)

"I wanted to put the freaked-out guy for these questions but there was no option." (Here you go: ≡:o .)