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

Physics 205B, spring semester 2019
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.

"I understand image distance is a positive value when an image is formed on the on the right side of the lens by a real object. A negative number for image distance is when the image is formed to the left of the lens by a real object."

"I think I understand the difference between converging and diverging lenses. With diverging, you will always get a virtual image. With converging, if it is upright, it is virtual, and if it is inverted, it is real. This makes sense when thinking about eyes because they are converging lenses, so the image that we see is real, and it is displayed as inverted on the backs of our eyes (our retinas)."

"When looking at object and image distances, we can infer that if the object is located to the left of the focal lens, the image is real if light allows the image to moved to the right of the lens. If the image is virtual then that image is located to the left. We can also examine these relationships with an equation that relates object distance to image distance with the constant focal length."

"Thin lens equations can give us useful information like wether it's a converging or diverting lens depending on the f being positive or negative. Or if it's a real or virtual image depending on object distance or image distance is positive or negative."

"Converging lenses make inverted images, and all inverted images are real. Diverging lenses make upright, diminished images, and all upright images are virtual."

"Upright images make a positive magnification and inverted images make a negative magnification. For a camera to focus, the focal length remains fixed and distance is manipulated. For an eye to focus, the distance is constant and the focal length is manipulated. Myopia is nearsighted, which means you cannot see far objects and hyperopia is farsighted, which means you cannot see close objects."

"This was a cool one. I liked the example of eye problems, it made the lens stuff easier to understand. I was a little confused on "increasing image distance" for thin lenses of cameras until I read about how how eyes have a fixed image distance. It made sense because cameras can move their lenses to make an image closer, but you cant exactly move your eyes out of your head to do the same."

"The sum of the inverses of object distance and image distance is equal to the inverse of the lens focal length. Converging lenses have a positive focal length (produce real images) and diverging lenses have a negative focal length (produce virtual images). The magnification created by a lens is the ratio between image distance and object distance (the sign of this ratio is "flipped" for magnification)."

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'm having trouble still with ray drawings and light refraction which is making it difficult to solve problems like on the homework."

"I'm still working on understanding how to identify a real image versus a virtual image and the other parameters that go with it. I'll be spending time with a physics tutor this week to grasp this concept. I'm able to draw the rays, interpreting them is where I'm struggling."

"I am a little confused still on how to determine the difference between a real and virtual image."

"I am trying but I am not sure I understand how to read the ray drawings."

"It is hard for me to understand what focal length is. I understand that it can change the relationship between the object and image distance, but I do not understand how."

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

Focal length: f [95%]
Magnification factor: m [95%]
Object distance: d_o [89%]
Object height: h_o [87%]
Image distance: d_i [95%]
Image height: h_i [92%]

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 [57%]

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

Focal length: changes [59%]
Lens-to-retina distance: remains constant [57%]

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

Lens: converging [62%] (Only converging lenses can make inverted images.)
Image: real [57%] (All inverted images are real.)
Ray tracing: 1 [30%]

Lens: diverging [61%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [38%] (All upright images are virtual.)
Ray tracing: 6 [14%]

Lens: converging [35%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [43%] (All upright images are virtual.)
Ray tracing: 4 [16%]

Lens: diverging [41%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [49%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [22%]

Lens: converging [41%] (Only converging lenses can make inverted images.)
Image: real [35%] (All inverted images are real, as the candle is upright, and the projected image is upside-down.)
Ray tracing: 1 [16%]

Lens: diverging [43%] (Only diverging lenses can make upright, diminished images.)
Image: virtual [46%] (All upright images are virtual.)
Ray tracing: 7, 8, 9 or 10 [19%]

Lens: converging [41%] (Only converging lenses can make upright, enlarged images.)
Image: virtual [30%] (All upright images are virtual.)
Ray tracing: 5 [19%]

Lens: converging [43%] (Only converging lenses can make inverted images.)
Image: real [32%] (All inverted images are real, as the slide is upside-down, making the projected image on the wall upright.)
Ray tracing: 2 [8%]

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 [81%]
Presbyopia: can see distant objects [76%]

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

"I'm completely lost about identifying the types of lenses and and if the image is real and I have no idea what the ray tracing number is."

"Please make this make more sense."

"I am pretty lost on how to tell whether things are seen through a converging/diverging lens."

"I'm still confused on real versus virtual images."

"Is text on a page considered real or virtual?" (The text on the page is an object. If you are looking at it with your naked eye, it will form a real image on the retina on the back of your eye.)

"I didn't really find anything confusing about the reading, but I think it would be beneficial to do some problems with the thin-lens equation to get a better understanding of it."

"This section seemed pretty clear and easy to understand, but I was confused when you stated that a camera initially focused on a distant object must move its lens outwards in order to focus on a nearby object. Does that mean that the lens gets smaller when it needs to see an object nearby?" (The lens remains the same size/shape, but the lens move outwards from its housing (increasing the image distance d_i as the object distance d_o decreases.)

"Since each of these vision defects are caused by an abnormal curvature of the eye, how does the curature change as say your vision gets worse?"