## 20170210

### Online reading assignment: corrective optics, magnifiers

Physics 205B, spring semester 2017
Cuesta College, San Luis Obispo, CA

Students have a bi-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.
"More about diverging and converging lenses. The diverging lenses will bring an image closer while converging lenses will push an image farther away."

"What I what understood from the blog was the two-step model. I understand how the first image is used as the object for the second image."

"Corrective lenses adjust vision based on an individual's far point and/or near point. The corrective lens adjusts the distance of an object one attempts to see so that it matches their far point and/or near point. The eyes can then focus on the object after it has been 'moved' to the appropriate distance."

"I understand this section pretty well. It is interesting for me to read about glasses (I have worn glasses since I was 3!) I will have to look for my prescription and see what the number is now!"

"This section discussed the different kinds of lenses in contacts and glasses. The contact lens is the first lens, and the eye is the second lens in this two lens system. The focal length of contact lenses are not specified, instead they are rated in terms of refractive power P. In order to figure out the focal point using contacts we have to do calculations."

"The angular size is a measure of how big something seems from your view point. The maximum angular size of an object as seen by an unaided eye is when the object is placed 25 cm away."

"The closer an object, the larger it is perceived and the larger the angle. However, there is a certain extent to which an object can be brought closer in which if it goes too close, it cannot be brought into focus."

"I like how with two-lens systems you can just take it step-by-step. Intermediate image becomes an object, then final image, there's not some strange thing that happens between. It makes sense how a distant do corresponds with a small theta, and a near do corresponds with a larger θ."

"The more we dive into the subject of refraction of light, the more it all makes sense. Now we are able to use angular magnification to figure out why we see larger images for smaller objects at closer distances than larger objects at further distances."

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.
"What I didn't understand from the reading was the magnifiers. I did not understand the mignifying progress."

"I don't quite understand the idea of being able to focus 'at infinity.' I think it means everything can be focused on. Also, if the image produced by lens 1 is virtual, how can the eyes see it through lens 2?"

"I'm a little confused about myopia and how we correct it."

"What I did not understand from this assignment was the part of the two-step model--I just don't understand the flow of it."

"I think calculating the two-step model for lenses is a little tricky for me. I get a little mixed up on it. It would be very helpful to see an example in class and then it'll probably click. It might be the myopia and different kinds I'm getting mixed up on."

"Kind of weird that magnifying lenses do not magnify, but it makes more sense now."

In general, a converging lens will produce virtual, upright images located __________ the original object.
 closer than. ****** [6] at the same distance as. * [1] farther than. ******************* [19] (Unsure/lost/guessing/help!) * [1]

In general, a diverging lens will produce virtual, upright images located __________ the original object.
 closer than. ****************** [18] 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 [70%]
Glasses/contacts to correct for hyperopia: converging [78%]
Glasses/contacts to correct for presbyopia: converging [44%]
Magnifying lenses: converging [81%]

State the units of optical power for lenses, and briefly describe the relationship between optical power P and focal length f.
"The units of optical power for lenses is the inverse of meters and there is an inverse relationship between optical power and focal length, as focal length goes down, optical power goes up and vice versa."

"The optical power is measure in diopters, or P, which is the inverse of the focal length or (1/f)."

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

"Magnification M is the angular size of the image produced by a magnifying glass divided by a reference angular size. Magnification m is a linear measure determined by dividing image height by object height."

"m is virtual and M is real?"

"I am not sure. Will need help clarifying."

If an object is brought closer to your eye, its angular size will:
 increase. ********************** [22] decrease. ** [2] 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 do = __________ in front of (to the left of) the lens.
 +∞. *** [3] +25 cm (at your near point). ************ [12] +f (at the focal point of the lens). ********* [9] (Unsure/lost/guessing/help!) *** [3]

The ray tracing that best matches when a converging lens is used as a simple magnifier is:
 #3. ******** [8] #4. *************** [15] (Unsure/lost/guessing/help!) **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"With polarization, why do we multiply the fraction of first filter to the cos2θ of the second filter when we start with unpolarized light, but not when we start with polarized light and only go through one filter?" (If you have polarized light going through one filter, then the fraction transmitted would be cos2θ. If you have unpolarized light going through a filter, then (1/2) of that light would make it through, and it would then become polarized. If you have unpolarized light going through two filters, then (1/2) of that unpolarized light goes through the first filter making it polarized, and then cos2θ of that polarized light goes through the second filter.)

"A review of ray tracings would be helpful."

"Wearing contacts and glasses myself, this topic is super-interesting to me because I never knew that both were creating a virtual (intermediate) image!"

"It's cool how things become so simple once you understand the formulas for them and the relationships they have with the world."

"I'll be able to remember the difference between the two types of magnification because little m means linear. Little = linear (they both start with L's)."

"I thought I understood this until I began to try to visualize and rationalize what I've learned compared to my experience with a magnifying glass. This stuff takes time to familiarize."

"I am confused why all these math equations implement an inverse? I am confused why the equations cannot work if they are 'un-inversed.'" (Order of operations--inverses (exponent power of –1) go first.)

"I hope there is a lot of explaining going on in lecture... because this stuff is kind of crazy."