20200219

Online reading assignment: optical instruments

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 optical instruments and interference.


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.
"Magnifiers make objects appear bigger than they are by increasing the angular size of an object. An object that is farther away appears smaller because the angular size is smaller."

"From this section I learned that telescopes and microscopes have a different set of lenses in order to either magnify an object that is small or an object that is far away. The different lenses in both of the instruments allow each to provide different functions."

"Ray tracings for both microscopes and telescopes create real intermediate images because the rays actually converge. I am beginning to understand that placing the object just outside the focal point of the objective will result in an enlarged image."

"Microscopes use two converging lenses to zoom in on objects that are very small. The first lens, or the objective, creates a real, enlarged image in front of the second lens. This image is then used as the object for the eyepiece, which translates to an upright, virtual view of the original object."

"During microscope magnification, the focal length of the objective lens is decreased and the focal length of the eyepiece is also decreased. Conversely, in telescope magnification, the focal length of the objective lens is increased while the focal length of the eyepiece lens in decreased."

"I understand how a microscope works before reading the text since I had to understand it for biology but now I did learn about the equation for it. I didn't know that it even had an equation before so it was interesting to learn."

"The objective makes an image just inside the eyepiece focal length. The eyepiece projects this first image to an infinite distance so a relaxed eye can see the final image."

"In a compound microscope or telescope, the angular magnification is the angular size of the final image divided by the reference angular size. A difference between them is that the image in a telescope is first diminished before being magnified while a compound microscope enlarges twice."

"I now understand how a compound microscope is designed to work, something I have never thought about before in all of my years of using them in school. They work by having an additional converging lens (objective) 'pre-magnify' the image before it passes through the magnifying glass (eyepiece), ultimately increasing the angular magnification."

"I understand the microscope parts because of taking biology. I understand that the 'tube length' for microscopes is defined as the distance measured between the objective and eyepiece focal points. the 'barrel length' for telescopes is defined as the distance measured between the objective to the eyepiece lenses, which is the same as the sum of their focal points."

"Microscopes and telescopes have some similarities such as the placement of an objective at the front leading to an eyepiece in the back. Moreover, both instruments create the same ray tracing. The main difference between these two instruments is the location of the physical object. For example, the object for a microscope is located just outside of the focal point of the objective while the telescope showcases an object extremely far away."

"There is a negative sign in both the microscope and telescope magnification equations. The negative sign for both denotes that the image produced will be inverted."

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 don't quite know why but memorizing the ray tracings is still a little difficult."

"I would like a little more information on these ray tracings and how to memorize the differences from the ones we've previously done."

"I do not understand how to draw a telescope ray tracing. I also do not understand why when talking about magnification, that only focal distance is taken into consideration."

"I found confusing the formulas used to calculate the magnifications. Each formula was different depending on the optical instrument."

"I do not fully understand the ray tracings behind the microscope and telescope concepts. I am also unsure how to utilize the equations."

"I would like to go over the angular magnification equations in class. I am a little bit confused about why the magnification factors are negative, primarily."

"I'm not really confused on anything because there wasn't much in the reading, which is what usually confuses me but telescope and microscope are similar. The equations are also distinct enough not to get them confused."

"I believe I understood this topic."

Identify the type for each of these lenses. (Only correct responses shown.)
Microscope objective: converging [94%]
Microscope eyepiece: converging [71%]
Telescope objective: converging [79%]
Telescope eyepiece: converging [59%]


Identify the ray tracing for each of these lenses. (Only correct responses shown.)
Microscope objective: ray tracing 2 [74%]
Microscope eyepiece: ray tracing 3 (or 4) [44%]
Telescope objective: ray tracing 3 (reversed) [0%]
Telescope eyepiece: ray tracing 3 (or 4) [44%]

For the microscope equation, 'L' is the distance between the objective and eyepiece lenses, and 'N' refers to the near point, which is assumed to be the nominal 25 cm value.

A (compound) microscope should have a __________ focal length objective lens and a ___________ focal length eyepiece lens in order to maximize its angular magnification.
short; short.  *************** [15]
short; long.  ************** [14]
long; short.  * [1]
long; long.  [0]
(Unsure/lost/guessing/help!)  **** [4]

A telescope should have a __________ focal length objective lens and a ___________ focal length eyepiece lens in order to maximize its angular magnification.
short; short.  [0]
short; long.  ** [2]
long; short.  ********************** [22]
long; long.  ****** [6]
(Unsure/lost/guessing/help!)  **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"A little extra help with these two-part ray tracings would be awesome!"

"Please explain the initial and final images produced by a microscope."

"May we review how to compare real-life examples to ray tracings?"

"Can we review how to actually use the equations for the magnification of the microscope and telescope please?"

"Cool concepts, but a tad difficult."

"Why are we learning about microscopes and telescopes?" (We're just finishing up the optics section of this course with some cool stuff.)

"Why can't we use the same angular magnification formula for both the microscope and telescope?" (Even though both the microscope and telescope use an eyepiece the same way (to magnify the images made but the objective lenses), the objective lens for a microscope looks at an object very close to it, just outside its focal point, while the objective lens for a telescope looks at an object very far away, essentially out at infinity. So the differences between the microscope and telescope equations object distances primarily result from the different uses of the objective lenses.)

"When looking at the ray tracing, the ray tracing to the eyepiece looks like converging lens tracing #3. However, the rays never intersect on this ray tracing. How is this possible?" (Your relaxed eye is set to look at things infinitely far away. The rays from an object at infinity are parallel as they hit your eye. So an eyepiece will take an object at its focal point and its rays will be parallel (which would normally not produce an image), but your eye will take those parallel rays as something it thinks is infinitely far away, and easily focus those rays to interpret it as being from a distant location (but apparently with a very large angular size.)

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