20180226

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.

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