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 total internal reflection and polarization.
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 think I'm starting to really understand Snell's law."
"If the incident angle results in an transmitted angle greater than 90°, then all of of the light is reflected back and you have total internal reflection (TIR). If the incident angle results in a transmitted angle that is 90° from the normal, then the incident angle is called the critical angle."
"During total internal reflection, where the incident angle in the medium with higher n is larger than the critical angle, light wouldn't be transmitted to the medium with smaller n. In this case, Snell's law doesn't apply."
"One of the things I understood was the difference between a polarized light source and an unpolarized light source. A polarized light source vibrates organized (up-and-down, side-to-side, diagonal-to-diagonal). An unpolarized light source vibrates randomly."
"If unpolarized horizontal and vertical rays pass through a polarizer, only the parellel rays will pass through the transmission axis and the perpendicular rays will be blocked."
"Sorry P-dog, but I am swamped."
"I honestly just did not get to this yet."
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 Snells law formula did kind of confuse me. I get a little confused with which parts go up when others go down."
"I found total internal reflection to be quite confusing. I had time figuring out what exactly the critical angle meant and how it tied into each scenario. In the homework I also had trouble with the 'plug-and-chug' questions and relating the index of refraction to a wavelength etc. and finding that the frequencies were equal to each other."
"What I don't understand is how the critical angle idea works. Why there? Why does it work?"
"The idea of a critical angle confused me a little bit. I may just have to go back and read it a little bit closer or read about it more in the textbook."
"What I found confusing from the reading was the application part of total internal reflection. I understand the part about the diamond and how it is cut a certain way to reach TIR. I do not understand about optical fiber and the see snake and how they are able to reach TIR."
"I don't understand the difference between horizontal polorized light and vertical polorized light. Also, I am not sure when transmitted if it's vertical or horizontal polarization from the antenna."
"Sound waves cannot become polarized because they travel longitudinally? I do not quite understand what this means. I think that it means the wave moves forward and backwards along a path outward from the source, but it is difficult for me to visualize. Also, light can be polarized because it is transverse? I would like to understand these concepts with more clarity."
"I didn't understand the section about polarizers and the fence post model. A discussion in class about this would be beneficial."
"I'm not confused about anything in particular."
If the incident angle of a light ray is less than the critical angle, the light ray will be:
reflected. *** [3] transmitted. *********************** [23] (Both of the above choices.) ******** [8] (Neither of the above choices.) [0] (Unsure/guessing/lost/help!) *** [3]
If the incident angle of a light ray is greater than the critical angle, the light ray will be:
reflected. ******************************* [31] transmitted. ** [2] (Both of the above choices.) * [1] (Neither of the above choices.) * [1] (Unsure/guessing/lost/help!) ** [2]
Total internal reflection is possible when a light ray in a __________ medium hits a boundary with a __________ medium.
faster; slower. ********* [9] slower; faster. ****************** [18] (Both of the above choices.) *** [3] (Unsure/guessing/lost/help!) ******* [7]
horizontally. ******* [7] vertically. ************************ [24] (Both of the above choices.) [0] (Neither of the above choices.) * [1] (Unsure/guessing/lost/help!) ***** [5]
horizontal. *********************** [23] vertical. ********* [9] (Both of the above choices.) * [1] (Neither of the above choices.) [0] (Unsure/guessing/lost/help!) **** [4]
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Frequency determines the type of wave and not wavelength? I thought different kinds of light have specific wavelengths." (If you always compare different types of light in the same medium (such as vacuum), then it doesn't matter if you compare different types of light in terms of different frequencies or different wavelengths. But for light going from one medium into another medium, the frequency will remain the same, while the wavelength will change. So it will make more sense to describe light in terms of frequency, as that will always stay the same no matter where that light goes, as an x-ray in air will remain an x-ray as it goes underwater (as its frequency stays the same), even though its wavelength will not stay the same.)
"Why does the energy of a wave increase as its frequency increases, and at what point is this energy enough to cause biological harm?" (It takes energy per time to continuously wiggle charges to produce light waves, and the faster you wiggle, then the more energy you put into the wave, so "faster wiggling" (higher frequency, smaller wavelength) light sends out more energy for a given amount of space. For radio waves, this is a low frequency, low energy wave that is "spaced out" a large distance (long wavelength), so it's not so dangerous. For infrared waves, this is a medium frequency, medium energy wave that is "more compact" (medium wavelength), so the cells in your body receive this energy, and (safely) warm up. For x-ray rays and even shorter frequency waves, these high frequency, high energy waves are "scrunched in" (small wavelengths), so dangerous amounts of energy can be absorbed by individual molecules or atoms in your cells and DNA, causing damage.)
"I understood most of Snell's law I think. It was nice having lab before lecture and then homework about it."
"I'm sure you said in lecture but we are calling refracted rays 'transmitted' rays right?" (Yes.)
"I'm slightly confused in the total internal reflection section. So when the incident angle is greater than the critical angle, it internally reflects so there's no transmitted angle?" (That is correct.)
"Could you go more in to detail about the three specific cases and how to use Snell's law with the weird case?" (We'll do that in the form of a flowchart.)
"Could you go into polarization a little bit before you let us loose on the worksheets?" (Yes, both today and next Monday.)
"Can light only exist vertically, diagonally, or horizontally polarized, or is it possible to send polarized light in a spiral?" ("Spirally polarized" light is what is known as circular polarization.)
"So both emitting and receiving antennae need to be with both horizontal or both vertical, would it just not work if there was one horizontal and one vertical?" (Correct!)
"No comment."
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