Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2020
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03Cv1d



Sections 30882, 30883 results

0- 6 :
7-12 :
13-18 :	**** [low = 15]
19-24 :	***************
25-30 :	*************** [mean = 24.2 +/- 4.2] [high = 30]

Online reading assignment: double-slit interference

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 double-slit 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.

"For in-phase sources, the difference in path length will be a whole number of wavelengths for constructive interference. For destructive interference the path difference will be (a whole number plus) a half of a wavelength."

"The presentation did a good job at conveying the path differences for the double slits. It was defined clearly."

"I understand from this section that the maxima and minima of a double-slit interference means whether or not it is constructive or destructive. this is determined by various variables of a double-slit interference like the wavelength, distance between slits, etc."

"Young's double-slit experiment showed how two monochromatic light sources could interfere constructively and destructively. This was shown by the alternating dark and light fringes on the screen he used."

"The difference in lengths that parallel waves travel is equal to their separation distance d between multiplied by sinθ. The inner wave also travels farther."

"In double-slit interference θ is measured from the horizontal and ranges from –90° to +90° with 0° deg being horizontal. The equation dsinθ is used to find the difference in path lengths traveled by two sets of waves. Whether the resulting wave is constructive or deconstructive depends on the whether there is a whole or and a half difference in path lengths."

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.

"Maxima and minima were confusing. double-slit interference was more difficult to understand."

"Definitely the picking the correct information to match the equation. All these symbols are confusing."

"The 'Train of Pain' is still a little confusing, just need to a little more with it."

"I understand the equations and the idea of the path lengths pretty well, but I might need some more practice using and applying them."

"I would benefit from homework examples that have to do with calculating the maxima and minima values to acquire a greater understanding of their relationship to the central bright spot."

"I pretty much understood everything, especially the basics. I might need some polishing of some concepts but that's it."

"Nothing at this time."

Explain the difference between "maxima" and "minima" in double-slit interference.

"The resulting constructive interference is the maxima, and the resulting destructive interference is the minima."

"A maxima is where it is brightest and a minima is where it is least bright."

"Maxima refers to the bright fringes while minima refers to the dark fringes."

Match the double-slit parameter with its symbol. (Only correct responses shown.)

Distance between slits: d [74%]
Any positive or negative whole number: m [74%]
Distance from slits to a projection screen: L [49%]
Wavelength of light passing through both slits: λ [83%]
Difference in paths for light passing through both slits: d·sinθ [51%]
Position along screen, as measured from the centerline: y [46%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [97%]
Path difference: integer number of wavelengths [100%]
Interference: constructive [97%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [89%]
Path difference: odd number of half wavelengths [94%]
Interference: destructive [94%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [97%]
Path difference: integer number of wavelengths [77%]
Interference: constructive [74%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Would like to review these last two diagrams in or after class."

"Are you going to go over this in class?" (Yes.)

"Will we be doing the double-slit experiment in lab?" (Yes.)

"Since the light that passes through the double slits is from a source that is side-by-side and in-phase with the same wavelength, won't the light passing through the two slits always be in phase?" (Well, yes, unless one wave has to travel a further distance than the other wave in order for them to meet at a receiver.)

"I'm not sure how to use the formula ∆l = dsinθ. In which case will the path difference be a whole wavelength or half a wavelength?" (Well, just calculate ∆l for a given θ direction, then see if it is equal to m·λ or (m + 1/2)·λ.)

"I think that in regards to Young's double-slit experiment, when two light paths interfere constructively they create a bright fringe, and when they interfere destructively they produce a dark fringe... is that correct?" (Yes.)

"I was a little confused whether the double-slit experiment only works when the light sources are side-by-side. If you have two separate sources of light that are constructive but that differ in distance does the double-slit experiment still work?" (Yes, but the sources must have exactly the same wavelength, and be in phase with each other. For visible light the most practical way to do this is to have a laser illuminate two side-by-side slits simultaneously.)

"I would benefit from some practice problems."

"Going to try my hardest to make it tomorrow."

Online reading assignment: interference

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 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.

"I understand that if two sound speakers are wired the same, then they will be in phase which results in constructive interference and the sound will be louder. If they are not wired the same, then they will be out of phase which results in destructive interference and a more quiet sound."

"If two waves are in phase and have a whole length difference, they will produce constructive interference. If the waves are out of phase or have a half wavelength difference, destructive interference is produced."

"The principle of linear superposition states that the resultant disturbance is the sum of the disturbances from the individual waves. I also understand that interference alters the brightness of light and loudness of sound."

"This paragraph from the presentation helped me understand the basics: 'Here we have two speakers, which are our sources of two sound waves. Since they are plugged into the same frequency source, they will generate sound waves of the same frequency f (which is depends only on the source), same speed v (which depends only on the medium), and thus the same wavelength λ (which depends on both f and v). If the speakers are wired the same way--red and black wires to red and black plugs--then they will oscillate in phase, with both speaker cones moving forward and backwards in unison.'"

"I understood the in-phase and out-of-phase part, and how to tell the difference. I also understood what is constructive and destructive."

"The way two wave sources are emitted determines whether the resulting wave will be constructive or deconstructive. In-phase waves have the same velocity, wavelength, and frequency, also in-phase waves are traveling in the same direction and can form a constructive of bigger resulting wave. Out-Phase waves do not align crest to trough and they do not have the same velocity, frequency or wavelength and the resulting wave is smaller or no wave."

"My primary take-away from this set of slides is the difference between constructive interference (music playing from the neighbor's stereo), and destructive interference (silence, or quieted volume). When the crests and troughs of the two wavelengths line up, constructive interference occurs; however, if those wavelengths are not aligned, destructive interference takes place."

"There was not much that I understood from this presentation."

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 need a little more detail about when the interfering wave examples have a whole wavelength path length difference or half-wavelength path length difference."

"Completely grasping the constructive/destructive conditions flowchart."

"I found the flowchart of constructive/destructive conditions to be overwhelming especially because I don't fully understand the solid lines vs. the dashed lines. I could also use a bit of clarification on what path difference is."

"I would like a little more understand on phase differences. As well as how path length difference works."

"At first, the path length difference was confusing to understand because I did not comprehend the equations. However, after seeing some visual examples, I now understand and can observe whether a wave will be constructive or destructive."

"The difference between constructive and destructive seemed confusing. Its relationship with waves was easier to understand with the diagrams."

"I found a lot confusing with these concepts. I don't understand the difference between constructive and destructive waves. I also don't how changing the distance makes the two so different. I am also confused what they are considered when the waves are in unison, but the distance is changed."

"How does this work for radio waves? Please give examples of constructive, destructive and non-coherent sources. The book only uses visible light as an example."

"This isn't very confusing to me."

Classify the various interfering wave examples. (Only correct responses shown.)

1: in phase sources [91%]; whole wavelength path difference [74%]; constructive [97%]
2: in phase sources [14%]; half wavelength path difference [86%]; destructive [89%]
3: in phase sources [86%]; whole wavelength path difference [77%]; constructive [89%]
4: out of phase sources [86%]; whole wavelength path difference [40%]; destructive [94%]
5: out of phase sources [23%]; half wavelength path difference [49%]; constructive [86%]
6: out of phase sources [89%]; whole wavelength path difference [54%]; destructive [97%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Finally lenses are over. I don't wear glasses so it was a difficult set of chapters." (I'm not so sure that this topic was any easier for student who do wear corrective optics.)

"When you change the volume on a stereo how is more sound produced? Are more sound waves generated?" (Well, "more" in the sense that the speaker cone moves in and out with a larger range of motion (larger amplitude), so more air is pushed in and out, and a greater range of pressure fluctuations result.)
"What does m stand for in the constructive/destructive conditions equation?" (Any whole positive/negative number (0, ±1, ±2, etc.).)

"Why in the world could I not find anything about path length differences?"

"May we review situation #5 specifically?"

"I just need a brief discussion about these ideas and I think I will be good on this topic."

"The wave interference problems don't seem too difficult, just need some practice."

"In that case given the two sources had the same total length and each crest and trough matched, I assumed there would be no length difference; yet, that was not an option?" (A case where there is no path length difference is the covered under the case of having a path length difference of a whole (number) of wavelengths, in this case the whole number would be zero.)

"If two sound waves are not exactly a half-wavelength off from each other, would there still be destructive interference?" (Yes, but they would not completely cancel each other out. The resulting loudness would be quieter than a single wave, but not completely quiet.)

"Will we be using noise-canceling headphones?" (That is a great application of destructive interference, but no. You can always purchase a set for yourself, though.)

"I was confused about coherent and non-coherent light sources of light. What makes a laser coherent compared to light bulbs which are incoherent?" (Basically, the light from a laser has a constant amplitude, single wavelength that starts off with a definite phase, while light from an incandescent bulb is a range of different wavelengths that can vary in amplitude and phase. It's a bit more complicated than that, but for our purposes we'll start from this working definition.)

Physics midterm question: destructively interfering out-of-phase radio transmitters

Physics 205B Midterm 1, spring semester 2019
Cuesta College, San Luis Obispo, CA

Two vertical radio transmitters broadcast at the same wavelength, and are spaced 6.0 m apart along the east-west direction. A receiver held by a Physics 205B student located to the east of both transmitters detects a destructive interference signal, and a receiver held by another Physics 205B student located to the north also detects a destructive signal. Show (a) why the transmitters must be out-of-phase sources, and (b) find a plausible numerical value for the wavelength. Explain your reasoning using the properties of source phases, path lengths, and interference.

Solution and grading rubric:

• p:
  Correct. Discusses/demonstrates:
  1. that the radio transmitters are out of phase, as there is no path length difference for the student located to the north, and destructive interference (as is the case here) can only occur if the sources are out of phase; and
  2. for the student located to the east, for these out of phase sources to interfere destructively the path length difference must be a whole number of wavelengths, and since the path length difference is 6.0 m, plausible (non-zero) wavelength values would be 6.0 m, 3.0 m, 1.5 m, etc.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least discusses why (1) transmitters are out of phase, but (2) does not use the correct destructive interference condition (whole number of wavelengths) for out of phase sources for the student located to the east.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at discussing source phases, path lengths, and interference.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of discussing source phases, path lengths, and interference.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm01Ft6G
p: 13 students
r: 4 students
t: 13 students
v: 12 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 5250), identifying the correct path length difference condition for two sources that are out of phase:
Another sample "p" response (from student 1810), with a graphical representation of the interfering waves:

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2019
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03b3Am



Sections 30882, 30883 results

0- 6 :	** [low = 3]
7-12 :	**
13-18 :	*********
19-24 :	***************** [mean = 21.5 +/- 6.0]
25-30 :	************ [high = 30]

Online reading assignment: double-slit interference

Physics 205B, spring semester 2019
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 double-slit 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.

"How the maxima and minima angles can give us constructive or destructive interference."

"Constructive interference leads to a bright fringe. Destructive interference leads to dark fringe. Intensity varies among the bright fringes; the central fringe has the greatest intensity."

"For two waves with in-phase sources and a distance of d apart from each other, we can approximate their path length difference by using the formula d·sinθ measured from the center line."

"When two waves are leaving from separate slits in phase they can be traveling parallel where θ = 0° and they would stay in phase. If the angle at which both waves are traveling is not zero then one wave is traveling a longer distance and thus can make the waves destructive interference. Constructive or destructive interference depends on how far apart the waves are from each other and the angle at which they are going toward the object."

"A detector can be used to detect the interference between two in phase sources that are side-by-side. The sources interfere at a detector that when moved side to side, create a path difference of the sources that can detect whether the sources interfere constructively or destructively. As the angle that the waves are moved changes, either by half a wavelength or a whole wavelength, the interference changes. "

"When light from two in-phase sources pass through slits, the two 'path lengths' of light from the slit to the point where they meet can relate in two important ways. If the lengths are equal or differ by a whole wavelength, constructive interference will occur when the two lightwaves meet and form a maxima. If the lengths differ by half of a wavelength, destructive interference will occur when the lightwaves meet and form a minima."

"Honestly I'm still kind of lost on this. I know we are looking for the difference in path length, but the equations are a bit difficult."

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'm a little confused on what the variables are and how they relate to the pictures in the presentation."

"I don't understand the connection between maxima/minima angles and interference."

"I'm still not quite sure about what minima and maxima mean and how that related to constructive and destructive interference."

"I found the double-slit interference to be a little confusing where m is used as an integer used to describe how many wavelengths difference the waves are. It was confusing at first but after thinking it through and a little practice it made a lot more sense."

"I find it hard to understand the equations. Perhaps a tutorial in class may help."

"How do we know when to use each path length difference equation for the maximas and minimas?"

"I need practice applying the ∆l = d·sinθ equation. I also need to better understand the principle of a diffraction grating."

"Not confused much but would like to see examples."

"I am still a bit confused about how to apply the path length difference approximation in problem-solving. I feel like I understand the principles, different waves are interfering, but I do not understand why this is useful."

"Might be some confusion on putting all the pieces together."

"Not too much is super-confusing to me."

"I didn't really find anything very confusing I think I've got a pretty good understanding of the material. "

Explain the difference between "maxima" and "minima" in double-slit interference.

"'Maxima' refers to constructive interference and 'minima' refers to destructive interference, regarding double-slit interference of in-phase waves."

"Maxima is dark and minima is bright."

"It seems that maxima is a constructive interference and the minima is destructive."

"No idea--but I know calculus maximas and minimas."

Match the double-slit parameter with its symbol. (Only correct responses shown.)

Distance between slits: d [85%]
Any positive or negative whole number: m [85%]
Distance from slits to a projection screen: L [67%]
Wavelength of light passing through both slits: λ [92%]
Difference in paths for light passing through both slits: d·sinθ [54%]
Position along screen, as measured from the centerline: y [62%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [95%]
Path difference: integer number of wavelengths [87%]
Interference: constructive [97%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [74%]
Path difference: odd number of half wavelengths [82%]
Interference: destructive [90%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [90%]
Path difference: integer number of wavelengths [67%]
Interference: constructive [79%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"No question today, this all seems to make sense to me."

"How did I do on my ray tracings for the quiz?" (Have you checked your quiz scores online?)

"This was very helpful in making the path length differences and how the two wavelengths form the light and dark spots 'click' for me."

"Is there a way to mathematically check these examples above?" (Before every class, click on the "review previous reading assignment responses" link to see the correct answers for each reading assignment question.)

"I'd like to know how to do the interference from the path length differences." (You'll get to see this in lab.)

"Okay this might be dumb but I don't really understand what the double-slit interference is. I don't understand what this interference is supposed to represent." (It's a way of dividing a single beam of light into two (or more) separate beams of light, that can interfere with each other constructive (or destructively) in certain directions. Next week's lab is where we shoot a laser beam through a CD-RW disc, such that it passes through many adjacent tracks to separate into many separate beams of light that will interfere constructively (or destructively) at certain places on a screen.)

"Sometimes I get tripped up on the math, so the trigonometry confuses me a bit. I would love to see something drawn to scale where the parallel lines from the two slits end up creating a bright spot together." (We'll get to "act out" how two waves interfere constructively or destructively for double-slit interference.)

"Is there any way you can make the individual lab reports less time consuming and tedious, especially the procedure section? :(" (Although the procedure section can get tedious if you detail every step that you did in lab, there is a skill in summarizing that in a succinct manner (usually with diagrams) that I would like you to practice this semester. Shorter is better, but not too short.)

Online reading assignment: interference

Physics 205B, spring semester 2019
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 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.

"The principle of linear superposition, states that the resultant disturbance is the sum of the disturbances from the individual waves, which can be constructive or destructive interference."

"Two waves combine when they arrive at the same point. They reinforce each other during constructive interference or cancel each other through destructive interference."

"If two waves constructively interfere, their crests and troughs line up creating a bigger wave. In contrast, if two waves destructively interfere, the crests and troughs are contrary and they cancel out each other. Also whether two waves are constructive or destructive not only depends on the source phases of the waves, but also on the path length difference."

"I like how visual this lecture was. It seemed much easier to comprehend seeing the waves and the differences in their peaks and troughs which then made it easier to understand that when they align they are constructive and when they do not they are destructive."

"In phase sources result in constructive interference and out of phase sources result in destructive interference. For two in phase sources, if one wave travels a whole wavelength longer than the other, then constructive interference occurs; if instead one wave is a half wavelength longer than the other, then destructive interference occurs. Out of phase sources have an opposite result."

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 mathematical equations regarding the conditions; I would like those to be explained in-depth."

"The equations are confusing. Applying things to the diagrams needs more elaboration."

"This section is pretty straightforward."

"No questions today but I think I am going to like this section."
"I think I understand this section but I feel like I'm missing something."

Classify the various interfering wave examples. (Only correct responses shown.)

1: in phase sources [85%]; whole wavelength path difference [62%]; constructive [91%]
2: in phase sources [35%]; half wavelength path difference [74%]; destructive [85%]
3: in phase sources [76%]; whole wavelength path difference [71%]; constructive [85%]
4: out of phase sources [79%]; whole wavelength path difference [38%]; destructive [85%]
5: out of phase sources [35%]; half wavelength path difference [71%]; constructive [79%]
6: out of phase sources [82%]; whole wavelength path difference [62%]; destructive [88%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"I need to better understand the source phase and path length differences. I will go over the presentation preview to better understand when a wave is considered to be constructive or destructive when the sources are out of phase."

"Okay, it was going smoothly up until the in phase and out of phase questions."

"I am not so sure about in phase vs out of phase sources so some clarification on that would be helpful."

"I'm a little confused on constructive/destructive differences. How can waves be in phase when one travels one-half of a wavelength longer than the other? I think I've only half got this concept down." ("In phase" or "out of phase" refers to how the two waves start off in relation to each other; either they both start off making crests (or troughs) together, so these two sources are said to be "in phase" with each other (regardless of how their waves travel after that); or one source makes a crest while the other source makes a trough (or vice versa), so these two sources are said to be "out of phase" with each other (regardless of how their waves travel after that).)

"I love the double-slit experiment! To me it's where what we think we know about the universe really starts to lose footing!" (We'll get there next week. #toosoon.)

Physics midterm question: second maxima angles not possible

Physics 205B Midterm 1, spring semester 2018
Cuesta College, San Luis Obispo, CA

A green laser (wavelength 550 nm) illuminates a grating with a unknown spacing between adjacent slits, producing an interference pattern with a first maxima angles of ±32°. (Drawing is not to scale.) Discuss why it is not possible for second maxima angles to be produced by this grating. Explain your reasoning using the properties of source phases, path lengths, and interference.

Solution and grading rubric:

• p:
  Correct. Discusses/demonstrates that the second maxima angle is not possible, by:
  1. solving for the spacing d between grating slits, given the first maxima angle and the wavelength λ of the laser; then
  2. solving for the second maxima angle results in a domain error for the inverse sine function, and interprets this as meaning that the there is no defined second maxima angle.
  (It is possible to use d = λ/sin(32°) from the first maxima equation and insert it into the second equation without explicitly numerically solving for d, such that θ = sin⁻¹(2/sin(32°) for the second maxima angle results in a domain error.)
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Typically has math errors.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. Uses minima equation and/or compounded math errors.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying properties of source phases, path lengths, and interference.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying properties of source phases, path lengths, and interference.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm01cVdP
p: 29 students
r: 2 students
t: 3 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1929):
Another sample "p" response (from student 3481):

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2018
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03Ch4R



Sections 30882, 30883 results

0- 6 :
7-12 :	** [low = 9]
13-18 :	****
19-24 :	************ [mean = 23.3 +/- 4.9]
25-30 :	************** [high = 30]

Online reading assignment: double-slit interference

Physics 205B, spring semester 2018
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 double-slit 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.

"The general introduction to double-slit interference; more specifically the modified 'Train of Pain.'"

"We are able to find out if constructive or destructive interference occurs at a detector when two in-phase light waves are situated so that they are offset at an angle, which therefore also produces a difference in length. We can use an equation to find maxima/minima, angle, and length as they are related to each other."

"When comparing two waves that are both in phase, we're trying to figure out how much longer it takes for one wave to travel than the other. The path difference, how many wavelengths, etc."

"When considering path differences we can make the assumption that the detector is sufficiently far from the source of waves and that the two waves (d apart) travel along a parallel path. This makes it so that the location of the detector can be specified by a specific angle."

"Double-slit interference is basically a case in which waves from two side-by-side in phase sources interfere in both a constructive or destructive manner. In this lecture, source phases do not matter since we are looking at only in-phase sources, so our main focus of attention is looking at the path differences. The minima are where the paths interfere destructively, and the maxima are where the paths interfere constructively. There exists a trigonometric equation to determine how long a path is relative to another path as well."

"When comparing two waves that are both in phase, we're trying to figure out how much longer it takes for one wave to travel than the other. The path difference, how many wavelengths, etc."

"When we change the path of two waves, it affects the distance the waves have to travel to reach a destination. The equation delta ∆L = d·sinθ will tell if the interference is constructive (maxima) or deconstructive (minima)."

"The maxima and minima angles can give us constructive or destructive interference. This interference could be the product of waves having a path difference of a whole or half wavelength."

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.

"Nothing was really confusing so far. I think the only thing harder to understand for me was the diagrams and the angle itself how that is found."

"How the altered distance of the two parallel waves effects their in/out of phase properties."

"I need more help on applying the ∆L = d·sinθ equation."

"I seemed to have understood this alright. Some lecture time would help me though."

"I'm confused about when a path difference would be destructive. My assumption is that the waves are constructive when the angle is at 0 degrees and destructive when the angle is larger or smaller than zero because these cases require that one path be shorter than the other."

"The path difference is really confusing to me. I think because it is harder to 'picture' in my head as I think it out."

"I don't understand the principle of a diffraction grating."

"Could you go over the equations?"

"I really don't find this presentation confusing. But how do we know and test which angles will be destructive and constructive?"

"I was pretty lost on finding the maxima and minima. I understood how you can manipulate delta l to be able to plug in wavelengths and m and then solve for an angle theta, but I don't know how this angle really comes into play in the big picture of it all."

"I really don't understand how path length differences are calculated."

"I didn't find anything too confusing in this lesson."

Explain the difference between "maxima" and "minima" in double-slit interference.

"'Maxima' is constructive interference and 'minima' is destructive interference."

"When two in-phase sources in a double-slit interfere at maxima this is constructive interference, and when they interfere at a minima there is destructive interference."

Match the double-slit parameter with its symbol. (Only correct responses shown.)

Distance between slits: d [74%]
Any positive or negative whole number: m [83%]
Distance from slits to a projection screen: L [55%]
Wavelength of light passing through both slits: λ [97%]
Difference in paths for light passing through both slits: d·sinθ [62%]
Position along screen, as measured from the centerline: y [55%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [100%]
Path difference: integer number of wavelengths [97%]
Interference: constructive [97%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [61%]
Path difference: odd number of half wavelengths [94%]
Interference: destructive [94%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [90%]
Path difference: integer number of wavelengths [74%]
Interference: constructive [84%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Review the above three diagrams from this assignment?"

"I could never go wrong with some more practice with the types of problems above, hopefully we have some in class :) ?"

"What is y? I put 'position along the screen as measured from the centerline.' Not sure I see what we are talking about."

"I am sorry I could not think of anything funny to write or ask this time, I will do better next time I promise.

Physics quiz question: double slit spacing

Physics 205B Quiz 3, spring semester 2016
Cuesta College, San Luis Obispo, CA

A red laser (wavelength 632 nm) illuminates two slits, producing a first maximum located at angle of 27° on a screen located 1.75 m away. The two slits are spaced __________ apart.
(A) 0.29 µm.
(B) 0.70 µm.
(C) 1.1 µm.
(D) 1.4 µm.

Correct answer (highlight to unhide): (D)

The first maxima angle θ for double-slit (and grating) interference is given by relation:

d·sinθ = m·λ,

where m = 1, such that:

d = λ/sinθ = (632×10^–9 nm)/sin(27°) = 1.39209...×10^–6 m,

or to two significant figures, 1.4 µm.

(Response (A) is λ·sinθ; response (B) is λ/(2·sinθ); and response (C) is λ·L.)

Sections 30882, 30883
Exam code: quiz03Ccf7
(A) : 0 students
(B) : 4 students
(C) : 4 students
(D) : 32 students

Success level: 80%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.11

Physics quiz question: moving screen away from single slit

Physics 205B Quiz 3, spring semester 2012
Cuesta College, San Luis Obispo, CA

Light from a red laser illuminates single slit to form a diffraction pattern on a distant screen. If the distance from the slit to the screen is increased, the width of the central maximum on the screen would:
(A) increase.
(B) remain the same.
(C) decrease.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

From the first minima equation (which uses the angle that measures one-half of the width of the central maximum on the screen), where W is the width of the slit:

W·sinθ = (1)·λ,

such that the "spread angle" is given by:

θ = sin^–1(λ/W).

Since the wavelength λ and the slit width W remain constant, the "spread angle" θ is unchanged if the distance from the slit to the screen in increased.

However, from trigonometry, the relation between the angle θ and the adjacent leg L (distance from the single slit to the screen) and the opposite leg y (one-half of the width of the central maximum on the screen) is given by:

tanθ = y/L,

where:

θ = tan^–1(y/L),

and since the distance L from the slit to the screen is increased, while the "spread angle" θ remains constant (as λ and W are constant), then y, which is one-half of the width of the central maximum on the screen must increase, and correspondingly the width of the entire central maximum on the screen would increase as well.

Section 30822
Exam code: quiz03gR7L
(A) : 21 students
(B) : 3 students
(C) : 3 students
(D) : 0 students

Success level: 78%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.71

Physics quiz question: diffraction through single slits of different widths

Physics 205B Quiz 3, spring semester 2014
Cuesta College, San Luis Obispo, CA

Light from a green laser pointer (wavelength 532 nm) illuminates a single slit. If the slit is made narrower than __________, the first diffraction minima will not be visible anywhere on a screen.
(A) 266 nm.
(B) 532 nm.
(C) 797 nm.
(D) 1,064 nm.

Correct answer (highlight to unhide): (B)

From the first minima equation (which uses the angle that measures one-half of the width of the central maximum on the screen), where W is the width of the slit:

W·sinθ = (1)·λ,

such that the "spread angle" is given by:

θ = sin^–1(λ/W).

Since both responses (C) and (D) have slit openings greater than 532 nm, they will produce a first minimum fringe at θ = 41.9° and 30°, respectively.

Response (B) has a slit opening that is equal to the 532 nm wavelength, such that θ = 90°, which is the boundary case where an opening slightly larger than 532 nm that would result in a spread angle slightly less than 90° (and thus be visible on a screen), and an opening slightly smaller than 532 nm would result in an undefined spread angle, as the inverse sine function would operate on a value greater than 1, so there would be no first minimum fringe at any angle (or on a screen) if the slit opening W is less than 532 nm.

Response (A) is already too narrow to produce a first diffraction minima, as it is already smaller than 532 nm.

Sections 30822, 30883
Exam code: quiz03cDvD
(A) : 11 students
(B) : 28 students
(C) : 0 students
(D) : 1 student

Success level: 70%
Discrimination index (Aubrecht & Aubrecht, 1983): –0.05

Physics quiz question: laser diffraction of hair strands

Physics 205B Quiz 3, spring semester 2016
Cuesta College, San Luis Obispo, CA

A Physics 205B student uses a red laser (wavelength 632 nm) to separately illuminate two different strands of hair. The __________ strand of hair will produce a wider central diffraction maximum.
(A) thinner.
(B) thicker.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

From the first minima equation (which uses the angle that measures one-half of the width of the central maximum on the screen), where W is the width of the obstacle (strand of hair):

W·sinθ = (1)·λ,

such that the "spread angle" is given by:

θ = sin^–1(λ/W),

so for a given wavelength of 632 nm, the thinner hair strand (smaller width W) would result in a larger "spread angle" θ.

Sections 30822, 30883
Exam code: quiz03Ccf7
(A) : 29 students
(B) : 11 students
(C) : 0 students
(D) : 0 students

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50

Online reading assignment: interference

Physics 205B, spring semester 2018
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 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.

"When two light waves arrive the same point they combine. they can reinforce each other during constructive interference, or cancel each other through destructive interference."

"With constructive interference the waves are in sync and reinforce each other and with destructive interference the waves cancel each other out. There are also different types of source phases and path length differences that can affect whether a wave is constructive or destructive."

"I understand that when sound waves troughs and crests line up (move in unison) there is constructive interference. On the other hand, when the troughs and crests don't line up (out of unison) there is 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.

"I found distinguishing between in phase and out of phase sources slightly confusing. I'm not entirely sure whether a wave is considered to be constructive or destructive when the sources are out of phase. I just need some examples and I think I'll be able to figure it out."

"I don't understand the source phase and path length differences."

"I don't understand how to determine the path length differences."

"I have a good understanding of this material. Maybe some in-class examples of constructive and destructive interference would be helpful."

"I didn't find anything really confusing this lesson."

Classify the various interfering wave examples. (Only correct responses shown.)

1: in phase sources [92%]; whole wavelength path difference [96%]; constructive [92%]
2: in phase sources [63%]; half wavelength path difference [96%]; destructive [92%]
3: in phase sources [83%]; whole wavelength path difference [83%]; constructive [92%]
4: out of phase sources [83%]; whole wavelength path difference [67%]; destructive [88%]
5: out of phase sources [42%]; half wavelength path difference [63%]; constructive [83%]
6: out of phase sources [92%]; whole wavelength path difference [42%]; destructive [88%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Are you enjoying the Olympics?" (Only while watching Mrs. P-dog's cousin compete in slopestyle and big air snowboarding.)

I think I got these diagrams...but sometimes I think I understand something and it later turns out that I'm entirely wrong."

"Lab due next Monday?" (No school next Monday. So, they're due next next Monday.)

Physics final exam problem: destructive interference angles in one quadrant

Physics 205B Final Exam, spring semester 2017
Cuesta College, San Luis Obispo, CA

Two vertical radio transmitters broadcast in phase at the same wavelength of 1.2 m, and are spaced a certain apart along the east-west direction. A Physics 205B student holding a receiver starts from due south of the transmitters, and detects three different locations with destructive interference signals before finally reaching due east of the transmitters. Determine a plausible separation distance (in m) between the transmitters. Explain your reasoning using the properties of source phases, path lengths, and interference.

Solution and grading rubric:

• p:
  Correct. Discusses/demonstrates that three minima locations will be found in the range θ = 0° (due south) to 90° (due west) by using one of two approaches:
  1. using the destructive interference condition d⋅sinθ = (m + 1/2)⋅λ, where m = 0, 1, 2, ..., finds a plausible separation distance d such that the third minima (m = 2) will be within θ = 90°, but the fourth minima (m = 3) is outside of θ = 90° (i.e., 3.0 m ≤ d ≤ 4.8 m); or
  2. using the constructive interference condition d⋅sinθ = m⋅λ, where m = 0, 1, 2, ..., finds the separation distance d such that the third maxima (m = 3) will be at θ = 90°; which allows for the m = 0, 1, and 2 minima to exist within that range (i.e., d = 3.6 m).
• r:
  Nearly correct, but includes minor math errors. May have claimed equally spaced minima angles at θ = 30°, 60° and 90° to find a plausible separation distance d using θ = 30° for the first minima angle.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Garbled attempt at applying properties of source phases, path lengths, and interference.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at applying properties of source phases, path lengths, and interference.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: finalmR3x
p: 3 students
r: 4 students
t: 6 students
v: 7 students
x: 4 students
y: 2 students
z: 0 students

A sample "p" response (from student 0428), finding the maximum possible separation distance: