## 20160125

### Online reading assignment: electromagnetic waves, reflection and refraction

Physics 205B, spring semester 2016
Cuesta College, San Luis Obispo, CA

Students have a 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 electromagnetic waves and redirecting light (reflection and refraction), along with advice from students from the previous semester, and videos on the flipped class mode of instruction used in this course.

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.
"Electromagnetic waves are classified by their frequency. Index of refraction is used to gauge the speed of light in different media. Redirecting light can happen through reflection and refraction."

"I understand that the electromagnetic spectrum is mostly invisible to our eyes with the exception of visible light. Radio waves have the lowest frequency and Gamma rays have have the highest frequency. The two ways to redirect visible light are through refraction and reflection."

"Light is part of a larger spectrum of electromagnetic waves that include radio waves, x-rays etc. Light travels at different speeds and wavelengths (but same frequency) through different media. The index of refraction gives us info on how fast it travels in different mediums. The higher the index, the slower the medium. Light gets reflected at the same angle perpendicular to the surface. It is refracted at a different angle that can be calculated using Snell's law."

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.
"Mnemonic: Fast-to-slow, bend towards the normal.' Not sure what this means."

"Snell's law. I have no idea what I am doing with those thetas."

Consider light traveling either through air (nair = 1.0) or through water (nwater = 1.33). Light travels with the faster speed through:
 air. *********************************** [35] water. ** [2] (There is a tie.) [0] (Unsure/guessing/lost/help!) * [1]

A ray of light has an incident angle of 60° in air, and a transmitted angle of 36° in plastic. Determine what happens to each of the following parameters as the light passes from air into plastic.
(Only correct responses shown.)
speed v: decreases [79%]
frequency f: remains constant [66%]
wavelength λ: decreases [63%]

For the above example of light incident in air being transmitted into plastic, __________ has the greater index of refraction.
 air. ** [2] plastic. ******************************** [32] (There is a tie.) [0] (Unsure/guessing/lost/help!) **** [4]

A ray of light has an incident angle of 20° in plastic, and a transmitted angle of 29° in air. Determine what happens to each of the following parameters as the light passes from plastic into air.
(Only correct responses shown.)
speed v: increases [76%]
frequency f: remains constant [68%]
wavelength λ: increases [63%]

For the above example of light incident in plastic being transmitted into air, __________ has the greater index of refraction.
 air. ********************** [22] plastic. ************ [12] (There is a tie.) [0] (Unsure/guessing/lost/help!) **** [4]

State your preference for denoting the inverse sine operation.
 Arcsin. [0] sin-1. ************************************* [37] (No preference.) [0] (Unsure/guessing/lost/help!) * [1]

Explain what is different about homework in a flipped class, compared to a conventional class.
"We do our reading assignments, and read blogs at home before the next class begins and submit them online, then come to class and learn more about material based on how we did on the online assignments."

"Homework is different in a flipped class because it is more focused on attempting to understand the material on your own, then coming to class to ask questions and seek clarification."

"We work it out in class after studying the lectures."

Describe where/when most student learning should occur in a flipped class, compared to a conventional class.
"Most student learning should happen wherever you feel most comfortable I guess? You do the work at your convenience throughout the week, but class should be a place to hone your understanding."

"The learning should occur at home previewing lectures and material before class and then asking the instructor in class for clarification or help."

"Most student learning should occur at home and the information should be put into use in the classroom."

Pick one piece of student advice from the previous semester, and discuss why you agree (or disagree) with it.
"'Don't procrastinate, and keep up with the reading assignments and do the problems ahead of time.' I agree with this piece of advice because last semester I was trying to always catch up and it ended up getting me in the end."

"'Take notes on reading assignments and textbook reading, even if the material does not make any sense. Then show up to class, and add to the notes to finish any concepts that didn't make sense before. Take notes on all examples in class (this is one of the ways to study for quizzes and future midterms). Don't miss any classes, and practice, practice, practice + office hours!!' This piece of advice from the previous semester relates to me really well and it is something I already do."

"'Keep up with reading, studying material, quizzes and exams.' I found this piece of advice to be accurate because if you really want to understand all the material you have to keep up with reading the chapters and presentation blogs in order to understand how everything works. Also, the more practice problems you go over the better you will be at solving them and understanding them."

"'Solving more practice problems and not being afraid to ask questions!' This is the best advice I could give to other students because the practice problems provided by P-dog are the most similar to what you will find on quizzes/tests. And asking questions about topics you don't understand in my opinion is the only you will actually be able to understand confusing concepts, because re-reading things you don't understand will not make you figure out what you are trying to learn."

"'Do the damn homework: free points!' This was my own advice. While it is theoretically possible to just click submit on all the homework assignments without actually doing them, it's more likely that students will put at least some effort into the homework, thus making class time much more productive."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can we review the index of refraction in a little more detail? I'm still a little confused and I'm not sure if what I comprehend is correct."

"I'm confused about how the different electromagnetic waves are actually produced. The book briefly covered it, but I didn't get it. For example, the book states that infrared waves are created by the movement of molecules. Does that mean that critters that can see in infrared are seeing moving molecules, or are they seeing heat produced from the moving molecules? If they are seeing heat, doesn't that mean that infrared is a type of 'visible' light, which is described as light produced by heat?" (All electromagnetic waves are produced by oscillating charges. The different types are produced by different-sized objects, have different-sized wavelengths, and are detected by comparable-sized objects. So the only difference between all of the electromagnetic wave types is scale.
Radio waves are produced by electrons oscillating up and down large metal poles (antenna), have long wavelengths, and are detected by large metal poles as a "signal").
Visible light is produced by rapidly oscillating atoms and electrons, have a shorter wavelength, and are detected by oscillating atoms and electrons (the valence electrons in certain chemical bonds in your rods and cones). When these chemical bonds are broken, the presence of these fragments in your rods and cones trigger your neurons to perceive "brightness").
Infrared is between radio waves and visible light in scale--it is produced by vibrating molecules, and has a medium-sized wavelength, and is detected by vibrating molecules such as in your skin, which is what you perceive as "radiant heat.")

"Do materials that aren't clear still have a refractive index?" (Typically, no--if the material is opaque, then light can't travel through it, and then you can't define a light speed and a refractive index for that material. Some materials that look opaque will let some light through it if you cut a very thin sample of it, and in that case you could define a light speed and a refractive index for that material.)

"How does the light actually look like when it moves slower?" (Still pretty fast. Light always travels the same speed in the empty space between atoms, the trick to slowing light down is to make it spend more time being received and then re-transmitted at each atom (or crowd more atoms together).)

"I'm afraid of the disconnect that a flipped class might bring if someone falls behind slightly."

"This classroom 2401 feels weird to be in. I like the old classroom 2402 better. I feel disoriented and these little chair-desks are silly."

"Is there a new giant hidden planet beyond Pluto?" (#iwanttobelieve.)