## 20150227

### Online reading assignment: double-slit interference

Physics 205B, spring semester 2015
Cuesta College, San Luis Obispo, CA

Students have a 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.
"Don't understand."

"With double slit interference, the source phase differences don't matter, just path differences, such as how much longer the wave from one source travels than the wave from the other source."

"Light from a single narrow slit spreads out primarily in directions perpendicular to the slit. Two narrow slits on the same screen interfere with one another."

"I understand what the path difference is and how to find it."

"I didn't really know (or even think about it) before, but now I know realize that there is a difference in distances that can effect the interference."

"How much longer the waves travel along the longer path (on the same path with phase differences) is given by ∆l = dsinθ. Then once you have figure out the difference in paths of the two waves you figure out if the wave interferance is constructive (maxima) or destructive (minima)."

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 still lost on double slit interference. How is it that we can have constructive and destructive waves if we have the same two sources?"

"Probably everything, as usual."

"Basically everything besides the two equations and the situations in which they are each used for. In all honesty, I'm sufficiently lost on this particular section. I know that this builds on the constructive and destructive interference information, but I'm lost on how it does so."

"No comment."

"Seems fairly straightforward."

Explain the difference between "maxima" and "minima" in double-slit interference.
"Maximas and minimas are intensities which differ due to the interferences it undergoes. Maxima intensity is produced by constructive interference and minima intensity is produced by destructive interference."

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

"Maxima is large and minima is small."

Match the double-slit parameter with its symbol. (Only correct responses shown.)
Distance between slits: d [74%]
Any positive or negative whole number: m [71%]
Distance from slits to a projection screen: D [54%]
Wavelength of light passing through both slits: λ [80%]
Difference in paths for light passing through both slits: d·sinθ [66%]
Position along screen, as measured from the centerline: x [54%]

As defined for double-slit interference, the range for possible θ angles is:
 –180° to +180°. ** [2] –90° to +90°. ********************* [21] 0° to 180°. ****** [6] l0° to 360°. * [1] (Unsure/lost/guessing/help!) ***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"How do you recommend we study in order to succeed on the upcoming quiz on Monday?" We'll have a practice quiz (taken from last year's class), and that should tell you what you need to focus between now and Monday.)

"I don't see or understand the point of the double slit idea yet."

"Please go over the variables more in-depth and what they really mean." (We'll do that as we go over examples in class, and highlight some technical details you'll encounter in next week's laboratory.)

"Good grief I am so confused. I would have never thought this concept so far in class would be so hard. You are the smartest man I know to understand this." (I'm still having to learn a lot about physics education research (a relatively new field), in order to understand how to better teach this stuff.)

"This has nothing to do with this week's reading assignment, but can you please clarify what the focal point is? And how there is a primary and a secondary focal point?" (For a converging lens, its primary focal point is where incoming parallel light rays are brought together and eventually meet. For a diverging lens, its primary focal point is where incoming parallel light rays appear to spread out away from. The secondary focal points are because lenses are symmetric, so there must be another corresponding focal point spaced out equally on other side.)

"So...we're almost halfway through the semester." (Shh--don't rush it. This is week six, and there are 17-18 weeks in a semester.)