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

Physics 205B, spring semester 2020
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.

"Converging lenses cause light rays to converge toward a focal point, while diverging lenses cause light to diverge (spread out). Converging lenses can produce either real or virtual images, while diverging lenses can only produce virtual images. The thin lens and magnification equations help us determine the location, size, and nature of the image a lens produces."

"I understand the image tracings a little bit better. I understand how the height and what side of object the image is on effects the the orientation and size of the image. I also understand that if the lines can be traced back that makes the image virtual."

"When drawing ray tracings, rays move from left-to-right with the object located on the left of the lens. Once the tracing is complete, if it is a virtual image, then the image would be located on the left of the lens. If the image is real then the image will be located to the right of the lens."

"The magnification equation uses the ratio of image height to object height, or the negative value of image distance to object distance. Regardless of whether or not the image is enlarged or diminished, an upright image will always have a positive height value and an inverted image will have a negative height value."

"How the thin lens equation and magnification equation are derived. I understand the basics of the thin-lens and magnification formula. I understand how near point is corrected for someone who is farsighted and how the far point is corrected using a diverging lens for someone who is nearsighted."

"This section is still on converging and diverging lens, however, it talks about how cameras and eyes are similar because they are both converging lens which produce real images. Camera have a fixed focal length lens meaning on the image and object distance is the only thing influencing its equation. Eyes have fixed image length distance since the eye is constant. This means that only object distance and focal lens are then only factors that can change."

"The difference in the different 'sighted-nesses.' Myopia being nearsighted, meaning they can see things near to them better. Versus hyperopia being far-sighted meaning they can see things far to them better. I have heard of these terms before, but it is cool to know the physics behind 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.

"Real and virtual images are still giving me a little but of trouble. I am always unsure on my answers. Also comparing ray tracings was oddly difficult."

"I'm still struggling to understand how to distinguish between real/virtual, upright/inverted, and enlarged/diminished images."

"I'm still confused about what kind of image is produced through each lens, whether the image is real, virtual. I'm also not sure how to apply the thin lens equation."

"I'm still having trouble with the distinction of real image versus virtual image. Diverging lens are always virtual I believe but need more refreshing of these concepts."

"I do not understand the sign conventions for lenses, drawing ray tracings might help. The sign conventions were just given as a list in the book so it's hard to relate them to images being formed."

"How to use the thin lens and magnification equations correctly in-relation to examples. Need some example problems and discussion to process this quantitatively to the best of my ability."

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

Focal length: f [91%]
(Linear) magnification factor: m [89%]
Object distance: d_o [94%]
Object height: h_o [83%]
Image distance: d_i [86%]
Image height: h_i [94%]

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 [49%]
Lens-to-film distance: changes [60%]

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

Focal length: changes [80%]
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 [66%] (Only converging lenses can make inverted images.)
Image: real [63%] (All inverted images are real.)
Ray tracing: 1 [46%]

Lens: diverging [43%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [46%] (All upright images are virtual.)
Ray tracing: 6 [26%]

Lens: converging [60%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [46%] (All upright images are virtual.)
Ray tracing: 4 [20%]

Lens: diverging [49%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [49%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [31%]

Lens: converging [40%] (Only converging lenses can make inverted images.)
Image: real [40%] (All inverted images are real, as the candle is upright, and the projected image is upside-down.)
Ray tracing: 1 [11%]

Lens: diverging [51%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [54%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [28%]

Lens: converging [60%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [46%] (All upright images are virtual.)
Ray tracing: 5 [29%]

Lens: converging [54%] (Only converging lenses can make inverted images.)
Image: real [46%] (All inverted images are real, as the slide is upside-down, making the projected image on the wall upright.)
Ray tracing: 2 [26%]

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 [97%]
Hyperopia: can see distant objects [97%]
Presbyopia: can see distant objects [91%]

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

"Can we please go over real and virtual images again in the examples above? I wasn't sure how to identify the ray tracings."

"I really just don't understand how we can tell whether an image is produced from a converging or diverging lens. So confused!"

"I feel that I need a lot of help with the lenses and real-life examples because I feel lost."

"Is the difference between concave and convex lenses the same as the difference between converging and diverging lenses?" (Yes, exactly.)

"I'm really unsure of how to distinguish between what produces a real or virtual image just based on the object. I feel like I missed a blog presentation that explained this because I've looked through them and I can't seem to find the best resource to help me understand them. I can see on the worksheet where the light rays intersect (or don't) if they are real or virtual and also if they are upright or diminished. I just don't know how we can determine this just be knowing what object light isis passing through. Maybe I am missing something really obvious?" (We officially didn't have a super-specific reading or lecture on this, but we had a flowchart to determine whether an image or real or virtual, and this process is something we'll pick up from practice rather than from strict formal definitions.)

"I thought this chapter was very intriguing because of its relation to human anatomy."

"Is it accurate to say that you can see a real image without having to look directly through a converging lens but in order to see a virtual image you have to look through the lens that produced it?" (Yes. You can only project a real image onto a screen, as you'll be doing in lab this week. You can see both real or virtual images by looking back through the lens.)

"Interesting material, also it's just been so cold in the mornings lately!"