## 20180207

### Online reading assignment: corrective optics, magnifiers

Physics 205B, spring semester 2018
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.
"Using a two-lens system you can correct for common visual effects. Diopters can be (+) for converging lens and (–) for diverging lens."

"Diverging lenses are used to correct for myopia and that converging lenses are used to correct for hyperopia."

"For two lens systems, the image produced from the first lens is treated as the object for the second lens. I also understand a little bit about how optometrists prescribe corrective glasses to people and the difference between focal length and diopters. Finally, I have a decent understanding of angular magnification works by relating the distance away from the object to the angle at which you are seeing it."

"If you can't see both close and far things, get bifocals."

"I understand what angular size and magnification are. I also get that a 'magnifier' focuses rather than magnifies."

"The two-step model is used in corrective vision as the contact lenses produce a virtual intermediate image at a distance that the eye can focus on. This becomes object 2 for the second lens--the eye--as it turns it into a real image on the retina. If the far point is less than ∞ and/or near point is greater than 0.25 m, then an individual needs corrective optics. Bifocals for have converging lenses for myopia (looking straight) and diverging lenses for hyperopia/presbyopia (looking down for reading)."

"That 'magnifiers' aren't really magnifying, they're playing with angular magnification. By using a magnifying glass you aren't making an object bigger but by placing the object at the focal point of the lens in creates a virtual image that makes it look bigger/closer to you."

"I understood the process of why two lenses are sometimes needed. The first lens produces an intermediate image that the second lens then has to make into the final product. This reminded me of polarized vs non polarized light going through filters--if polarized light passes through two filters, the way it passes through the first directly affects the way it presents through the second filter."

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.
"This is all relatively confusing, as it is a lot of new information to take in. I need in-class reinforcement and repetition as usual :)"

"The contacts/glasses stuff confused me. From the slides I was uncertain how to contact the thin lens equation with the two step model."

"Some parts at the end of the online presentation were a bit confusing. The last few pictures didn't help."

"Angular magnification."

"I found the concept of 'projecting to infinity' confusing. It is hard to comprehend such an abstract idea."

In general, a converging lens will produce virtual, upright images located __________ the original object.
 closer than. ****** [6] at the same distance as. ** [2] farther than. ******************** [20] (Unsure/lost/guessing/help!) * [1]

In general, a diverging lens will produce virtual, upright images located __________ the original object.
 closer than. ******************** [20] at the same distance as. ** [2] farther than. ****** [6] (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 [72%]
Glasses/contacts to correct for hyperopia: converging [72%]
Glasses/contacts to correct for presbyopia: converging [31%]
Magnifying lenses: converging [70%]

State the units of refractive power for lenses, and briefly describe the relationship between refractive power P and focal length f.
"The units of refractive power are P, in units of diopters. This is the inverse of the focal length f, which is measured in meters."

"I have no idea."

Explain the difference between the two types of magnification, m and M.
"Angular magnification M is a value that describes how much larger the angular size of an object appears to be through a magnifier compared to an unaided eye. Meanwhile the linear magnification m is the ratio of image length to object length measured."

"m = linear magnification, while M = angular magnification which tells you how much larger the angular magnification is when looking at an object through a magnifier."

"Unsure."

A nominal, relaxed eye is set to focus on objects located at:
 +∞. *** [3] +25 cm (at your near point). ****** [6] +f (at the focal point of the lens). [15] (Unsure/lost/guessing/help!) ***** [5]

If an object is brought closer to your eye, its angular size will:
 increase. *********************** [23] decrease. ***** [5] remain unchanged. * [1] (Unsure/lost/guessing/help!) [0]

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.
 +∞. * [1] +25 cm (at your near point). ******** [13] +f (at the focal point of the lens). ******** [8] (Unsure/lost/guessing/help!) ******* [7]

The ray tracing that best matches when a converging lens is used as a simple magnifier is:
 #3. ******** [8] #4. ************** [14] (Unsure/lost/guessing/help!) ******* [7]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can we talk about angular magnification?"

"It seems like for presbyopia you would need both types of lenses? But isn't presbyopia similar to hyperopia?" (Yes, presbyopia is basically age-onset hyperopia; even if you can see near now, you will eventually lose that ability as you get older. So if you don't need glasses/contacts now, you'll need reading glasses if you live long enough. However, if you have glasses/contacts to be able to see far (myopia or nearsightedness), you'll need bifocals to also see near when you get old. (If you're still young and have glasses/contacts to be able to see near (hyperopia), you'll just need stronger reading glasses to see near when you get old.)

"I am sorry, I am not there yet. My answers look crazy. Trying to catch up already."

"I feel like everything seems to straightforward in class but then I get to these assignments and I feel so lost :("

"Thank you for all of your hard work in simplifying the text's material on your website (and making it fun)."

"My schedule right now is making things very hard to learn outside of class. The downside to doing the lecture oupside of class class is it can be hard to find the time or space to do this."

"Why does the object's height not mater when we did the ray tracings in lab? Won't that change the distance that the image is from the lens?" (In the thin lens equation (and for ray tracings), the object distance and image distance only depend on each other (and the focal length) of the lens:

(1/do) + (1/di) + (1/f),

and the object height and image height do not affect the object distance and image distance. However, the object height and image height are related to the object distance and image distance:

(hi/ho) = –(di/do).

Graphically, making the object arbitrarily large or small on a ray tracing just makes the resulting image proportionally large or small as well (stretching or scrunching the vertical dimensions of your ray tracing), but the object distance and image distance (the horizontal dimensions of your ray tracing) are unaffected.)