Online reading assignment: images produced by lenses, thin lens equations, cameras and eyes

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 examples of images produced by lenses, thin lens equations and camera and eyes.


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.

"Pretty confident doing ray tracing when I have a reliable straightedge available."

"Real objects and images will give positive d_o and d_i distance values. In comparison, virtual images give rise to a negative d_i (this makes sense, as the image has to be projected back behind the lens, whereas the positive cases meant the image is in front of the lens)."

"Lenses are starting to make sense; ray tracings are somewhat easier now."

"Cameras and eyes are both examples of converging lenses that make real images. Converging lenses is the only lens to make inverted real images. Diverging lenses are the only lens that makes diminished upright images."

"Camera lenses and eyes have some unique characteristics and similarities. They can be modeled as single converging lenses that project real images. Yet the difference is that a camera focuses on a distant object by adjusting image distance (the lenses moving inward/outward), while eyes are have obviously been evolved to use muscles to change the focal length to focus on objects."

"Nearsightedness or myopia means a person can see an object up close but struggles to see an object farther away. For farsightedness (hyperopia) and elderly vision (presbyopia) a person can see an object farther away but struggles to see an object up close. For presbyopia this is mostly due to aging."

"As you get older your near point get further and further away so to read you would have to hold the object further away."

"I think I have finally started to grasp how to do all of the ray tracings."

"I have had no time this weekend to read the book or online notes. So technically I can't understand any of it."

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 really like some more explanation on how to use the lens equations. From the lecture material I wasn't convinced I knew who to use them correctly."

"Telling the difference between the lenses."

"I'm still not really good at ray tracings."

"I get confused when, if I don't do my lines just right, I don't get an intersection. It makes it difficult to make determinations about the image."

"I really am not sure how to tell (visually) when an object is real or virtual. I just don't understand the rules for telling when the image of an object is real or virtual and so some clarification and detail would be greatly appreciated."

"I don't quite understand the equation for a thin lens and what its used for."

"I need a bunch of more practice with converging and diverging lenses, it is all really confusing. More problems similar to the worksheet we have been doing together in class would be beneficial. I think it may be a matter of understanding the directions for each respective type of lens."

Identify the following thin lens parameters. (Only correct responses shown.)

Focal length: f [84%]
Magnification factor: m [87%]
Object distance: d_o [87%]
Object height: h_o [87%]
Image distance: d_i [84%]
Image height: h_i [87%]

For a simple camera, identify which parameter(s) must change or remain constant in order to focus on different distance objects.

Focal length: remains constant [65%]
Lens-to-film distance: changes [68%]

For a model eye, identify which parameter(s) must change or remain constant in order to focus on different distance objects.

Focal length: changes [58%]
Lens-to-retina distance: remains constant [48%]

Identify the type of lens, image, and example ray tracing produced in the online reading assignment examples. (Only correct responses shown.)

Lens: converging [74%] (Only converging lenses can make inverted images.)
Image: real [71%] (All inverted images are real.)
Ray tracing: 1 [42%]

Lens: diverging [55%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [52%] (All upright images are virtual.)
Ray tracing: 6 [23%]

Lens: converging [55%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [48%] (All upright images are virtual.)
Ray tracing: 4 [26%]

Lens: diverging [48%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [48%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [23%]

Lens: converging [52%] (Only converging lenses can make inverted images.)
Image: real [48%] (All inverted images are real, as the candle is upright, and the projected image is upside-down.)
Ray tracing: 1 [23%]

Lens: diverging [52%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [45%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [23%]

Lens: converging [58%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [32%] (All upright images are virtual.)
Ray tracing: 5 [23%]

Lens: converging [52%] (Only converging lenses can make inverted images.)
Image: real [48%] (All inverted images are real, as the slide is upside-down, making the projected image on the wall upright.)
Ray tracing: 2 [16%]

A person with no vision defects can see both nearby and distance objects. Identify what can be seen by a person with the following vision defects. (Only correct responses shown.)

Myopia: can see nearby objects [81%]
Hyperopia: can see distant objects [84%]
Presbyopia: can see distant objects [65%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"I could still use more help with ray tracings."

"I am not great at ray tracing. Could you go over it in class please!"

"Can we talk about when looking at the presentation examples how we can tell whether the image is real or virtual?"

"How do they reshape the curvature of the eye during surgery? Like scrape it?" (That's the general idea. They peel back the outer layer of your cornea, and remove some of the inner layers, then put the outer layer back over it. Another method is to make very fine incisions so your corner will "collapse" downwards a little. Afterwards the curvature of your eye will be different after it heals. Either way, pretty crazy stuff.)