## 20140226

### Online reading assignment: double-slit interference

Physics 205B, spring semester 2014
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.
"Based on the reading and the presentation, I understood that the double slit experiment demonstrates the wave nature of light. The path difference starts out in phase but travels different paths to reach the screen, and so the pattern presented on the screen shows the alterations between constructive interference and destructive interference."

"This chapter was pretty straightforward about describing double slit in phase interferences and the multiple parts of it such as maxima and minima, different lengths, wavelengths and angles."

"I feel that I understand the basics on the information presented in the textbook and the blog, such as what the maxima and minima are and what the difference is between d and D."

"By dispersing the light rays through two slits causes the light to act constructively and destructively depending on the distance each wave of light has to travel until it meets the other at a given point. When the distance allows for a whole integral multiple of the original wavelength, the light is constructive and more intense. When the waves are related by an odd number of 1/2 wavelengths, the light waves are destructive to each other and cancel each other out."

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 think I know what D is. Or how to find it."

"I found most of the information presented in the textbook and blog to be confusing and could use some further explanations or lecture on this material. I am mainly confused on how to apply this information to problems or why we would."

"All the numbers seem reasonable enough, but for some reason, I can't really see the conceptual picture if I was to be asked a question pertaining to this section of the chapter. It shouldn't be too big a deal, but I'm usually able to figure things out much faster if I can paint a picture in my head of what's going on. Think just more pictures would help remedy this."

"I found the variables to be confusing after considering the reading and example questions in the book. I am familiar with what they stand for but would benefit from examples so I understand what the definitions and variables really mean with respect to the slits and screen."

"I did not understand a lot from the reading and the presentation. The only thing I really understand is how to find the path length difference ∆l using d·sinθ."

"I am confused as to when to use the double-slit interference equation. I also don't fully understand the Young's double-slit experiment that is explained in the textbook."

Explain the difference between "maxima" and "minima" in double-slit interference.
"The difference between maxima and minima is that maxima is the max intensity at the screen which is produced by a constructive interference while minima is the minimum or zero intensity at the screen produced by a destructive interference."

"Either constructive (maxima) or destructive (minima) interference occurs when we solve for different θ angles."

"Maxima interference is constructive and is at points where the light rays have a path difference of whole integral wavelengths. Minima are deemed by destructive behavior by waves with odd integrals of 1/2 wavelengths."

Explain the difference between the two distances in double-slit interference, d and D.
"The centers of the slits are separated by a distance of d, and the separation of the slits from the screen are a distance of D."

As defined for double-slit interference, the range for possible θ angles is:
 –180° to +180°. ** [2] –90° to +90°. ************************** [26] 0° to 180°. * [1] l0° to 360°. [0] (Unsure/lost/guessing/help!) ***** [5]

Describe what x is used to measure in double-slit interference.
"It is used to measure the position on the screen."

"x is used to measure the distance between the center maxima fringe and the adjacent bright fringe."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"If the light wave is passing through a vertical slit, does that mean that only half of the light intensity is making it past the first screen?" (That would only be true for polarizers with "slits" that are approximately the same size as the wavelength of light passing through. For visible light, the sizes of the slits are immensely larger than the wavelengths, so the slit would behave as a conventional "window" rather than a polarizer.)