## 20170224

### Online reading assignment: double-slit interference

Physics 205B, spring semester 2017
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.
"I know the difference between maxima and minima."

"That the path length difference relation tells us where the maxima or minima will be."

"With the double-slit interference, the source phase differences don't matter, but that just their path differences do, we are wanting to know how much longer the wave from one source travels than the wave from the other source. I also understood that in a double-slit interference condition the constructive is maxima, and a condition that is destructive is minima."

"Double slit interference is two side-by-side in-phase sources. Phase differences don't matter with this and only the path matters. When calculating this we locate where these two sources interfere constructive (maxima) or destructively (minima)."

"A really small difference in distances can put waves in interference with each other; it is interesting how much of a difference it can be."

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 am not understanding the flow chart for the constructive and destructive conditions. How am I able to tell which pathway I am to choose for the condition I am given?"

"How to figure out the difference between the minima and maxima for double-slit interference."

"Path length differences. I'm just not fully understanding on how to find it."

"I don't really understand what double-slit interference is."

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

"The difference between the two is that the maxima is when there is a constructive interference or when the path difference is a multiple of the wavelength. A minima happens when there is a destructive interference or when the path difference is either an odd number of half wavelengths."

"No idea."

"I'm trying to understand it."

"Maxima and minima is the minimum and maximum of wavelength?"

Match the double-slit parameter with its symbol. (Only correct responses shown.)
Distance between slits: d [77%]
Any positive or negative whole number: m [77%]
Distance from slits to a projection screen: L [41%]
Wavelength of light passing through both slits: λ [100%]
Difference in paths for light passing through both slits: d·sinθ [50%]
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 [95%]
Path difference: integer number of wavelengths [91%]
Interference: constructive [91%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)
Sources: in phase [59%]
Path difference: odd number of half wavelengths [86%]
Interference: destructive [77%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)
Sources: in phase [73%]
Path difference: integer number of wavelengths [82%]
Interference: constructive [68%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can we discuss the difference between maxima and minima in double-slit interference?

"Do we call it maxima/minima only when we're talking about interfering waves?" (Well, yes, but this works for both interfering sound waves (loud/quiet regions) or interfering light waves (bright/dark regions).)

"Petition for more long weekends!"

"Did you ride any emus over the long weekend?" (No, but Mrs. P-dog and I went snowshoeing up in the mountains.)