## 20140224

### Online reading assignment: interference

Physics 205B, spring semester 2014
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 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.
"Speakers are in-phase when they are wired with the same polarity. They are out-of-phase when wired with opposite polarities. Constructive interference happens if the wavelengths of the sound waves produced line up with one another. Destructive interference is when the wavelengths don't line up."

"Not much."

"I understand the difference between interference and diffraction. I remember learned a little bit about this in general chemistry. It's where interference deals with different waves that arrive at our vision in different phases and then diffraction is just the waves traveling around objects."

"I understand really well that constructive interference means the troughs and crests of both waves line up. Destructive interference means that the troughs and crests line up with the other waves crests and troughs. Constructive interference is a good sound while destructive will be silence or a quiet sound."

"For two in phase speakers where one wave travels a half-wavelength longer than the other, the path difference is (1/2)·λ, and as a result crests and troughs line up with the other speaker's troughs and crests: destructive interference."

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 equations were quite intimidating."

"All of the equations baffled me, and I am really not sure what I am supposed to take away with this knowledge. The explanations felt over my head. The example was like another language to me!"
"If in phase is called constructive interface why is there destructive too in in phase sources. I'm confused about why there is both constructive and destructive in both in phase and out of phase sources and how you can tell the difference besides looking at a stereo system and matching the wires by color. Other than that example in the blog I don't understand what the definition is of out of phase, I am assuming it is the same as destructive in which one wavelength is half a wavelength ahead of the second."

"I'm not really sure how to tell the difference between constructive and destructive interference. I think I'll be okay after some examples though."

Figure 25-4
Giambattista/Richardson/Richardson, Physics, 2/e, p. 926
McGraw-Hill, 2009

Identify the characteristics of the sources, waves, and interference type. (Only correct responses shown.)
Sources: in phase [32%]
Path length difference: odd number of half wavelengths [68%]
Interference: destructive [65%]

Figure u12lb11
Light Waves and Color--Lesson 3: The Path Difference
The Physics Classroom

Identify the characteristics of the sources, waves, and interference type. (Only correct responses shown.)
Sources: in phase [80%]
Path length difference: integer number of wavelengths [80%]
Interference: constructive [86%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"When designing stadiums for concerts do they take this into account and use physics to make sure the crowd hear the best constructive sounds?" (Well, maybe not for stadiums, but certainly for auditoriums and concert halls, especially taking into account reflections and reverberation.)

"When finding the path differences one can have constructive and destructive for either in phase sources and out of phase sources when doing a problem to find path difference--how can you tell when it is in phase or out of phase?" (The relative phase of the sources would be explicitly stated in a problem; or you would be given everything else, and be asked to determine the relative phase of the sources.)

"In a sound-canceling microphone, what happens when there are multiple waves from different frequencies?" (These typically can only destructively interfere with a constant amplitude, constant frequency ambient sound, such as a jet engine, etc., all other sounds would be unaffected.)

"I was a little confused about why constructive and destructive interference is different for in phase sources and out of phases sources. Can't you just apply the definition and know whether it's constructive or destructive?" (Yes, but "carefully.")

"m is any integer?" (Yes.)

"This is unrelated to this chapter but I heard about the Higgs boson and tried to Google it and am very confused but would like to know more about it." (You can't go wrong with Ph.D. Comics.)