## 20160226

### Online reading assignment: double-slit interference

Physics 205B, spring semester 2016
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.
"Paying close attention to the diagrams. We're looking at waves at various angles to see how it affects interference."

"When there is two side-by-side in phase wave sources--because the wave sources are in phase, what we are concerned about in this case are path differences only. In double-slit interference a condition that is constructive is maxima, and a condition that is destructive is minima."

"Since light is a wave it travels through slits creating dark areas and bright areas depending on how it has interfered with other waves of the same wavelength."

"NOTHING! HELP!"

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 don't understand the logic of path length differences. I'm not sure how to how to explain or walk through setting the equation up."

"This seemed pretty straightforward, even from just skimming it."

"I don't get how this is mathematically quantified."

"I am having trouble on deciding if something is in phase or out of phase. Also another thing i am not understanding is if it is destructive or constructive. I tried the chart from the previous blog but i am still a little bit confused."

Explain the difference between "maxima" and "minima" in double-slit interference.
"Maxima are points of constructive interference, and minima are points of destructive interference."

"Maxima is the location where light from two individual sources are reinforcing each other and correspond to points of brightness. Minima as where two individual sources are destroying each other and correspond to points of darkness."

"Meow."

"Minima = constructive interference, maxima = destructive interference."

Match the double-slit parameter with its symbol. (Only correct responses shown.)
Distance between slits: d [64%]
Any positive or negative whole number: m [82%]
Distance from slits to a projection screen: L [41%]
Wavelength of light passing through both slits: λ [86%]
Difference in paths for light passing through both slits: d·sinθ [50%]
Position along screen, as measured from the centerline: y [50%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)
Sources: in phase [94%]
Path difference: integer number of wavelengths [78%]
Interference: constructive [89%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)
Sources: in phase [53%]
Path difference: odd number of half wavelengths [56%]
Interference: destructive [72%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)
Sources: in phase [83%]
Path difference: integer number of wavelengths [50%]
Interference: constructive [56%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I didn't think you could add angles into this topic. How do we determine the angle off to the side of the dashed center line? Will it just be given or should we know how to find it?" (Typically there are three related parameters: slit spacing d, maxima (or minima) angle θ, and wavelength λ. If you are given any two, you should be able to solve for the third.)

"I get the constructive/destructive subway map. That's cool." (Cool. Cool cool cool.)

"Everything is confusing and overwhelming at this point. I'm hoping none of this stuff is on Quiz 2 or else I'm in major trouble!" (Quiz 2 only covers lenses, interference is on Quiz 3.)

""These assignments are a lot harder to do when you haven't had time to read the book."

"What would happen if the detector moved from θ = +23° to θ = –23°?" (It would still be destructive interference, as the diagram would just be flipped. #isitworthit #letmeworkit #iputmythingdownflipitandreverseit)

"Missy Elliott has been stuck in my head since Monday. Thanks P-dog." (#tiesreverdnatipilfnwodgnihtymtupi)