## 20160208

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

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 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.
"I'm going to have a lot to catch up on."

"Anatomy of the eye! That's something my medical brain can wrap around."

"Thin lens ray tracings can be a simple, fast way to double-check your work while working on magnification calculations. Though only two rays are needed to find an image, three rays through the center of the lens and to each of the focal points help ensure measurements are accurate."

"In the case of a camera, focal length is fixed and image distance and object distance are inversely related. In the case of a human eyeball, image distance is fixed and focal length and object distance are directly related. The inability to focus on far away objects is called nearsightedness (myopia), and the inability to focus on near objects is called farsightedness (hyperopia, or presbyopia when it's caused by aging)."

"It was interesting how similar eyes are to the lens of cameras."

"The convention in ray tracings is for light to move from left-to-right, with the object located to the left of the lens, and the image (if real) would be located to the right of the lens. For this case the object distance do and the image distance di would both be positive. If the image is virtual, then it would be located to the left of the lens, and the image distance di would then be negative."

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.
"The magnification equation. I don't like it one bit. Also, the concept of the ray tracings and how it reflect what I'm actually 'seeing' in everyday life aren't connecting."

"I'm having trouble differentiating between a virtual image and a real image. Also having trouble matching up the ray tracings to the examples."

"I'm confused on how to determine if an image is real or virtual and how can we determine the correct ray tracing based off the type of lens and image?"

"I didn't find that much to be confusing. I've done thin lens rays before, getting back into ray tracings and using the equations is going well."

"I don't know if I find anything confusing right now. I'll know when I show up to class."

"This stuff is all really confusing. I need some more examples on the use of the thin lens equation, but mostly I need help with ray tracings. I can do them, but I can't interpret them. HELP!"

"I keep getting the enlarged, upright, virtual and real images confused. However, everything else so far makes sense, especially the linear magnification equation."

Identify the following thin lens parameters. (Only correct responses shown.)
Focal length: f [86%]
Magnification factor: m [86%]
Object distance: do [89%]
Object height: ho [81%]
Image distance: di [81%]
Image height: hi [86%]

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 [42%]
Lens-to-film distance: changes [47%]

For a model eye, identify which parameter(s) must change or remain constant in order to focus on different distance objects.
Focal length: changes [69%]
Lens-to-retina distance: remains constant [50%]

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

Lens: converging [61%] (Only converging lenses can make inverted images.)
Image: real [50%] (All inverted images are real.)
Ray tracing: 1 [36%]

Lens: diverging [39%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [42%] (All upright images are virtual.)
Ray tracing: 6 [25%]

Lens: converging [52%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [36%] (All upright images are virtual.)
Ray tracing: 4 [31%]

Lens: diverging [33%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [44%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [25%]

Lens: converging [31%] (Only converging lenses can make inverted images.)
Image: real [28%] (All inverted images are real, as the candle is upright, and the projected image is upside-down.)
Ray tracing: 1 [19%]

Lens: diverging [33%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [33%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [22%]

Lens: converging [47%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [47%] (All upright images are virtual.)
Ray tracing: 5 [25%]

Lens: converging [44%] (Only converging lenses can make inverted images.)
Image: real [39%] (All inverted images are real, as the slide is upside-down, making the projected image on the wall upright.)
Ray tracing: 2 [14%]

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 [78%]
Hyperopia: can see distant objects [78%]
Presbyopia: can see distant objects [67%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Will we have to memorize the rules regarding how rays behave for converging/diverging lenses (parallel ray redirects through primary F, etc.), or will that be given to us on the quiz?" (You should have them memorized! Well, if not already, then certainly in time for the quiz.)

"I feel like the more I reread this material the better I understand it, but I'm still pretty confused on the ray tracings."

"I don't understand any of this."

"As I was doing this assignment, my contacts lens were being a pain in the butt!"

"The ray tracings were easy but interpreting images to identify the lenses and positions of objects was challenging! In a fun sort of way."

"Chocolate is my best friend. I love it so much, sometimes I eat it." (Friends don't let friends eat friends.)