## 20150220

### Online reading assignment: corrective optics, magnifiers

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 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 two-lens system is what corrects visual defects for people; contacts or glasses coupled with the eyes. Ultimately, it's two lens equations that make a second image (q1) that acts as a second object (p2) so the eye can focus on the object (p1), but treat it as a second real upright image (q2). Knowing the near point or far point distances, contacts or glasses can be prescribed that correct these distances using the thin-lens equation."

"More about diverging and converging lenses--I had a lot of trouble with it because I didn't understand which images they produced."

"How optical instruments actually use a minimum of two lenses. Also, I learned a lot about how spectacular eyes are and how they function similar to a camera."

"The 1/p + 1/q = 1/f relationship is used to understand how cameras and eyes focus real images differently. A camera will have a set focal length, since the lens cannot change shape, so the image distance must change in order to focus on different object distances. Alternatively, the eye will adjust the lens thickness and focal length in response to the changing location of the object, keeping the image distance the same."

"Myopia requires a diverging lens to bring the images closer. While hyperopia uses a converging lens to push images away."

"I understand a bit more of the science behind knowing why a person would need bifocals. And I learned that in a two lens system we take it one lens at a time. I actually found this stuff really interesting as I wear glasses."

"Angular size 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. Angular modification 'M' is a numerical factor denoting how much larger the angular size of something appears as seen through a magnifier, compared to the naked eye. If a converging lens with 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 through an unaided eye.This is the ratio of the near point (25cm) to the focal length f."

"Essentially a lens produces a 'corrected' virtual image to compensate for the eye's deficiency, which the eyes pick up and interpret correctly. Essentially, by figuring out the type of deficiency, and plugging in the right values we can pretend to be optometrists and figure out the focal point correction, which then we can use to find the refractive power 1/f."

"We are dealing with two lenses now."

"Diving deeper into properties of lenses."

"The eye."

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'm mainly getting myself mixed-up with all the new variables."

"I'm lost on how to use the two-step model with the myopia and hyperopia prescription."

"I find interpreting ray tracing diagrams to distinguish between real and virtual images a really hard process."

"I had a hard time understanding the two lens system equations and how to use it in prescription cases. I also still have issues with the ray tracings."

"Overall, I understand the thin lens equation, however, I'm having difficulty applying the equation when given different values. This is where I need the greatest clarification."

"Lost me a little bit on the magnifiers."

In general, a converging lens will produce virtual, upright images located __________ the original object.
 closer than. ******* [7] at the same distance as. *** [3] farther than. ************************* [25] (Unsure/lost/guessing/help!) * [1]

In general, a diverging lens will produce virtual, upright images located __________ the original object.
 closer than. *************************** [27] at the same distance as. [0] farther than. ******** [8] (Unsure/lost/guessing/help!) * [1]

Identify the type of lens used for these optics. (Only correct responses shown.)
Glasses/contacts to correct for myopia: diverging [80%]
Glasses/contacts to correct for hyperopia: converging [83%]
Glasses/contacts to correct for presbyopia: converging [31%]
Magnifying lenses: converging [51%]

State the units of optical power for lenses, and briefly describe the relationship between optical power P and focal length f.
"Diopters. The optical power P is equal to the reciprocal of the focal length f."

"The unit of optical power for lenses is m-1. The optical power is equal to 1/f."

"I am not sure."

"€8-0 Freak out!"

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

"M stands for 'angular magnification' which is a numerical factor denoting how much larger the angular size of something appears as seen through a magnifier compared to with just the naked eye. The magnitude of the transverse magnification (m) is the ratio of the image size to the object size; the sign of m is determined by the orientation of the image."

"The angular magnification M (upper-case M, to distinguish it from linear magnification lower-case m) is a numerical factor denoting how much larger the angular size of something appears as seen through a magnifier, compared to with just the unaided eye. Little m is just image height divided by object height."

"m- is micro, M- is mega."

"I'm honestly lost as to what the difference is."

If an object is brought closer to your eye, its angular size will:
 increase. ********************************* [33] decrease. **** [4] remain unchanged. * [1] (Unsure/lost/guessing/help!) ** [2]

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.
 +∞. *** [3] +25 cm (at your near point). ***************** [17] +f (at the focal point of the lens). ********** [15] (Unsure/lost/guessing/help!) ***** [5]

The ray tracing that best matches when a converging lens is used as a simple magnifier is:
 #1. ******* [7] #2. ** [2] #3. *** [3] #4. ****** [6] #5. ********** [10] (Unsure/lost/guessing/help!) ******** [8]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I kinda forgot some of the stuff we learned in this chapter because of the nearly two weeks we had between classes and me not reviewing the material like most of the students in our class =]"

"I don't understand how someone could use bifocals, I think that would drive me crazy, but I guess you adapt." (No, as a bifocal wearer, you never really do. The bottoms of my eyes always seem like they're welling up with tears. #thesearenttearsimjustleakingfeelings)

"Look at this comment, just look at it." (What, through my bifocals? Oh wait, so that's what they're for.)

"I'm gonna study more."

"No comment." (You just did.)