## 20140204

Physics 205B, spring semester 2014
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.
"I understand that converging lens refract light inward and are thickest at the center. Diverging lens bend light outward from the focal point and are thinnest at the center."

"An image is considered to be real when the light rays of a certain point on a certain object converge to a corresponding point on a certain image. It is considered virtual if light rays from a particular point appear as if they are diverged from that particular on that particular image, even though they actually didn't. I get that stuff."

"I understand the difference between a converging lens and a diverging lens. A converging lens takes parallel rays of light and makes them meet at a specific point in space. Diverging lenses do the opposite; they take parallel rays of light and spread them out at the focal point."

"Images are formed by reflection from mirrors and refraction from lenses. Lenses can be diverging or converging, meaning they 'focus' light or 'defocus' light, respectively. I can draw the two (or three) principal rays through a lens."

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.
"Not sure what upright/inverted or enlarged/diminished means."

"I have no idea how to draw the principal rays. I thought I understood, but then I looked at the key for the ray tracing homework and now I'm completely lost, I don't know how I'm supposed to know which line goes where and at what angle. I'm even more lost on how to identify the type of image produced for ray tracings and identifying the orientation and size of the image."

"I am having a hard time distinguishing between a real and a virtual image. I somewhat understand the mirror example, but I would not know how to apply these concepts to lenses. Can we please go over more examples?"

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 [83%]
Converging lens 2: real image [69%]
Converging lens 3: (no image produced) [49%]
Converging lens 4: virtual image [57%]
Converging lens 5: virtual image [57%]
Diverging lens 6: virtual image [69%]
Diverging lens 7: virtual image [69%]
Diverging lens 8: virtual image [71%]
Diverging lens 9: virtual image [63%]
Diverging lens 10: virtual image [49%]

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 [46%]
Converging lens 2: inverted, enlarged [34%]
Converging lens 3: (no image produced) [46%]
Converging lens 4: upright, enlarged [26%]
Converging lens 5: upright, enlarged [29%]
Diverging lens 6: upright, diminished [34%]
Diverging lens 7: upright, diminished [34%]
Diverging lens 8: upright, diminished [35%]
Diverging lens 9: upright, diminished [46%]
Diverging lens 10: upright, diminished [26%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Two antennae got married, the ceremony was only so-so, but the reception was fantastic."

"I am so completely lost. The book was incredibly confusing and I so I was hoping the blog presentation would help, and it did a little, but I still have no idea how to do the ray tracing homework."

"The ray tracing worksheet was very confusing for me, could you go over it?" (Yes.)

"Isn't divergent lens 10 image the same size as the object?" (Very nearly so--it's just a little smaller than the object.)