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 corrective optics and magnifiers.
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 understood the presentation on myopia. I think it was super cool to see how we are entry-level optometrists after this."
"Refractive power is used by optometrists to measure the degree at which light is converged or diverged denoted by a (+) or (–) sign. It is also the inverse of the focal length."
"The combined total refractive power for two lenses is addition of each refractive power."
"Negative values for the focal length and refractive power mean that corrective lenses should be diverging, to correct for the person's myopia. Corrective lenses with a positive focal length and refractive power will be converging lenses, to correct for hyperopia."
"Magnifiers 'magnify' by increasing the angular size of the object thus making the object appear bigger to the eye. Increasing the distance between the object and the eye provides a smaller angular size and makes the object appear smaller."
"Angular size is the size of an object at a certain distance. The larger the object, the larger the angular size. Magnification isn't really magnifying but rather the lens allows the eye to be able to focus on the object at a distance where it other wise wouldn't have been able to."
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.
"Corrective optics. It all seems overwhelming, but simplification will help. Especially refractive power and power addition equations."
"Still figuring out how to work with the thin-lens equations. The steps for correcting myopia and hyperopia were a little confusing."
"I am a little confused on the important difference(s) between linear and angular magnification. I think that I get the basic idea of both but want to make sure I clearly understand how they differ."
"Maybe a little more clarification on the mathematics on angular magnification."
"I did not understand how the eye can perceive the penny to be bigger than the moon. I also did not understand the idea of doing multiple thin lens equations for two lens."
"A review of the equations for angular size and angular magnification would be very helpful. Also I am not really sure about what it exactly means when 'the image is at infinity.'"
"The math seems a little confusing to me right now per usual. But it's late and I'm sleepy."
In general, a converging lens will produce virtual, upright images located __________ the original object.
closer than. ******* [7] at the same distance as. ** [2] farther than. ********************** [22] (Unsure/lost/guessing/help!) * [1]
In general, a diverging lens will produce virtual, upright images located __________ the original object.
closer than. ************************** [26] at the same distance as. [0] farther than. ***** [5] (Unsure/lost/guessing/help!) * [1]
Identify the type of lens used for these optics. (Only correct responses shown.)
Glasses/contacts to correct for myopia: diverging [75%]
Glasses/contacts to correct for hyperopia: converging [78%]
Glasses/contacts to correct for presbyopia: converging [29%]
Magnifying lenses: converging [74%]
State the units of refractive power for lenses, and briefly describe the relationship between refractive power P and focal length f.
"Diopters. Refractive power is 1/f."
"The unit of refractive power for lenses is diopters, which is the inverse of the focal length."
"Units for P are meters–1."
"I don't understand what this is asking. "
Explain the difference between the two types of magnification, m and M.
"Linear magnification is 'm' and angular magnification is 'M.'"
"m is the ratio of the object's distance or height to the images distance or height at convergence of the light rays hi/ho or –di/do. M is how much the apparent size of something is change by a magnifying lens. The apparent size is an illusion and relative to the viewer."
"m is the ratio of image height to object height while M is a distinct ratio of angular size of some image produced by an optical instrument divided by the reference angular size of an object viewed without an optical instrument."
"I'm not sure what the answer is."
"Again, I am pretty lost."
"Don't remember."
A nominal, relaxed eye is set to focus on objects located at:
+∞. ************ [12] +25 cm (at your near point). *********** [11] +f (at the focal point of the lens). **** [4] (Unsure/lost/guessing/help!) ***** [5]
If an object is brought closer to your eye, its angular size will:
increase. **************************** [28] decrease. * [1] remain unchanged. [0] (Unsure/lost/guessing/help!) *** [3]
When a converging lens is used as a simple magnifier, the object is placed at a distance do = __________ in front of (to the left of) the lens.
+∞. **** [4] +25 cm (at your near point). ************* [13] +f (at the focal point of the lens). ********** [10] (Unsure/lost/guessing/help!) ***** [5]
The ray tracing that best matches when a converging lens is used as a simple magnifier is:
#3. * [1] #4. *************************** [27] (Unsure/lost/guessing/help!) **** [4]
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can you show a drawing demonstrating the difference between microscope and telescope lens placement and rays? I was confused with presbyopia and how that age-onset disorder works with myopia. (1) In that case someone would be unable to see far--myopia--while simultaneously being unable to see close--presbyopia? Also (2) can someone experience hyperopia and presbyopia or would the condition default to the latter? Finally, while partially off-topic, (3) is there a physics-based description of blindness?" ((1) Yes, someone with myopia (can see near, can't see far) who develops presbyopia (can't see near) would then require bifocals to correct both problems. (2) If you can't see near as a child, you have congenital hyperopia. If you could see near when you were young, but gradually lost that ability as you got older, then you have presbyopia, which is basically age-onset hyperopia, and they are both corrected with a (stronger) converging lens prescription. (3) Not really, I would think that's more of a biology/anatomy/physiology thing.)
"Can we go over problems or see these problems done? When I see these equations being used it makes it easier to understand."
"I have some questions about what a relaxed eye is meant to focus on."
"Please help to understand the angular size and tie all the equations to the ray tracing, this is getting complex."
"I'm not too confident on my answer for how close a simple converging lens used as a magnifier should be."
"I would love more examples of how to guess what kind of ray tracing would apply to real-life situations like a magnifier."
"I think angular magnification is the hardest part of this section, especially when comparing them to ray tracings." (It really is. More on angular magnification next week.)
"Can the human eye control the amount of light that comes in?" (It make less light come in (by making the pupil smaller, or by squinting your eyelids), but it can't make more light coming in than is already there.)
"Do you meditate? Moreover, if you do meditate, do you call it 'total internal reflection?' (Sorry I'm a couple chapters late on this joke.)" (I think my meditation chill is more like something called 'frustrated total internal reflection.' It's when total internal reflection is supposed to happen, but introducing an external object with a new index of refraction that barely touches that surface ruins the total internal reflection, allowing a transmitted ray to go through.)
"How long have you taught physics?" (My first physics class as an instructor was at UC-Davis 26 years ago, when I was still a graduate student.)
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