Online reading assignment: corrective optics, magnifiers

Physics 205B, spring semester 2016
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 corrective optics and magnifiers.

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.
"A person who is near-sighted cannot see nearby objects at 25 cm and this is hyperopia, furthermore a person is given diverging lens to correct their vision. Oppositely, people who are far-sighted cannot see distances far away and this myopia, furthermore a converging lens is used to correct for this."

"Corrective lenses are designed to create upright virtual images at an optimal distance in front of the eye so that it can best see them. Diverging lenses correct for myopia by taking distant objects and creating a close-up virtual image, which becomes the object distance for the eye. Converging lenses correct for hyperopia by taking close-up objects and creating a virtual image farther away, which once again becomes the object distance for the eye."

"The angular size is not the actual size, it is a measure of how large an angle it subtends with your eye at the origin, and is a measure of how big something 'seems' from your viewpoint. When a converging lens with a focal length f is used a magnifier, the angular magnification is the ratio of the angular size as seen through the magnifier, compared to the angular size as seen with an unaided eye."

"Need help in class."

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 would like to go over angular magnification if possible."

"Why the nominal farthest distance for a myopia prescription is positive infinity. I understand how it works in regards to the thin lens equation, but the concept itself is puzzling."

"Could use some discussion on how m and M relate, if at all, and when to distinguish between the two."

"I have to do some more reading. Everything with lens I am having trouble with. When is the math coming, I am better at that sort of stuff!"

In general, a converging lens will produce virtual, upright images located __________ the original object.
closer than.   [0]
at the same distance as.  * [1]
farther than.  ************************** [26]
(Unsure/lost/guessing/help!)  *** [3]

In general, a diverging lens will produce virtual, upright images located __________ the original object.
closer than.   ************************ [24]
at the same distance as.  [3]
farther than.  ********* [9]
(Unsure/lost/guessing/help!)  *** [3]

Identify the type of lens used for these optics. (Only correct responses shown.)
Glasses/contacts to correct for myopia: diverging [72%]
Glasses/contacts to correct for hyperopia: converging [72%]
Glasses/contacts to correct for presbyopia: converging [64%]
Magnifying lenses: converging [69%]

State the units of optical power for lenses, and briefly describe the relationship between optical power P and focal length f.
"Diopters. P = 1/f, (+) values are converging lenses; (-) values are diverging lenses; and 0 is no power."

"Units are the inverse of focal length, so 1/m."

"Not sure how to answer this question."

Explain the difference between the two types of magnification, m and M.
"m is for linear magnification and M is for angular magnification."

"m is the magnification of the image, how large it is conpared to its size before. M is the angular magnification factor, and is a ratio of how much larger of an angular size the object now appears to be through the magnifier, compared to the naked eye."

"I honestly thought they were the same, I just assumed the bigger M meant a greater magnification than m."

Bringing something closer biggifies it. BIGGIFIES.

If an object is brought closer to your eye, its angular size will:
increase.  **************************** [28]
decrease.  ****** [6]
remain unchanged.  ** [2]
(Unsure/lost/guessing/help!)  *** [3]

When a converging lens is used as a simple magnifier, the object is placed at a distance p = __________ in front of (to the left of) the lens.
+∞.  ***** [5]
+25 cm (at your near point).  ************** [14]
+f (at the focal point of the lens).  ************ [12]
(Unsure/lost/guessing/help!)  ******** [8]

The ray tracing that best matches when a converging lens is used as a simple magnifier is:
#3.  ********* [9]
#4.  ********************** [22]
(Unsure/lost/guessing/help!)  ******** [8]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Is there any way you could recap everything in class? I feel like I understand little bits of information here and there but am not able to put them all together coherently to be able to fully understand the subject."

"I understand that we can calculate the prescription one needs by understanding his/her near point (NP) or far point (FP). We do this by using the two-step model, but how to plug in numbers and what they are is a mystery to me."

"I have over a week to study this material and do this assignment, and I end up doing it at 11:00 PM the night before it's due. Even I don't understand me."

"This stuff is hard, probably because I haven't thought about physics the last 12 days."

"Infinity seems to be getting thrown around pretty casually in this section."

"Ready for the next class."

"I need to get with it. I'm not bad at stuff. I swear!"

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