20190304

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.)

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