## 20170206

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

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 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.
"What is meant by 'real' and 'virtual' images, and that what causes an image to be virtual is that its perceived location is illusory. That near/farsightedness is caused by the eye's inability to properly refract incoming light."

"The image distance from the lens determines if an image is virtual being located to the left of the lens, and the image distance di would then be negative. Linear magnification can be either positive or negative, depending if the image is upright or inverted when comparing it to the original object."

"Cameras differ from eyes in that cameras adjust the distance from the lens to the retina in order to focus while retinas adjust the focal point. The distance between the lens and the object can affect the magnification and how focused an object can be."

"How to use the formulas to find the distances of the objects and the heights of the images."

"It is somewhat common knowledge that the eyes record an image upside-down and that the brain reads it as if it is right-side up; however, it is interesting to now be able to apply that to physics and classifying the image as real, inverted, and diminished. The reading on accommodation and how the ciliary muscles loosen/tense to focus on images relates to our previous lab where we measured the near point of students."

"Cameras and eyes both consist of a single converging lens projecting a real image. The focal length of an eye changes, while the focal length of a camera does not."

"Linear magnification is the ratio of the image height compared to the object height. m is positive if the image is upright and negative if the image is inverted. It is also equal to image vs. object distance ratio (multiplied by –1)."

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 thin lens equations, when comparing them for an eye and for a camera, and I would like to see more on that. I had a hard time trying to figure out exactly how accommodation in the eye would work."

"I still do not understand what qualifies an object as real or virtual. It seems like this can only be determined based on the math (i.e. whether or not di is positive)."

"I am a little confused determining if an image is diminished or enlarged."

Identify the following thin lens parameters. (Only correct responses shown.)
Focal length: f [89%]
Magnification factor: m [93%]
Object distance: do [93%]
Object height: ho [89%]
Image distance: di [93%]
Image height: hi [89%]

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 [59%]
Lens-to-film distance: changes [63%]

For a model eye, identify which parameter(s) must change or remain constant in order to focus on different distance objects.
Focal length: changes [74%]
Lens-to-retina distance: remains constant [74%]

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 [37%] (All inverted images are real.)
Ray tracing: 1 [33%]

Lens: diverging [33%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [26%] (All upright images are virtual.)
Ray tracing: 6 [11%]

Lens: converging [67%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [48%] (All upright images are virtual.)
Ray tracing: 4 [22%]

Lens: diverging [37%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [33%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [11%]

Lens: converging [44%] (Only converging lenses can make inverted images.)
Image: real [26%] (All inverted images are real, as the candle is upright, and the projected image is upside-down.)
Ray tracing: 1 [19%]

Lens: diverging [41%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [52%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [14%]

Lens: converging [56%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [26%] (All upright images are virtual.)
Ray tracing: 5 [19%]

Lens: converging [41%] (Only converging lenses can make inverted images.)
Image: real [44%] (All inverted images are real, as the slide is upside-down, making the projected image on the wall upright.)
Ray tracing: 2 [15%]

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 [85%]
Presbyopia: can see distant objects [74%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Doing a few examples from above to help clarify them would be much appreciated."

"How do we know an object is real or virtual without knowing the image distance?" (For a converging lens, it would depend on the object distance--if the object distance is greater than the focal length, you get a real image; and if the object distance is less than the focal length, you get a virtual image. For a diverging lens, it doesn't depend on the object distance, you always get a virtual image!)

"I'm sorry for the unsure/lost/guessing responses--I hope you understand this ridiculous behavior."

"What is the average age that people start to develop presbyopia? (According to the American Academy of Ophthalmology, "the average age of those first reporting symptoms of presbyopia is between 42 and 44 years of age with a complete loss of accommodation typically occurring between the ages of 50-55 years.")