## 20170227

### Online reading assignment: diffraction, charges and materials

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 diffraction and charges and materials.

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.
"Diffraction is the spread of light from a single slit."

"Due to the diffraction minima equation, if there is a smaller slit opening, it would result in an increase of the spread of the diffracted waves and if you make the slit opening larger, it would result in decreasing the spread of the diffracted waves."

"Electrons in an insulator are constrained to their atoms, while a conductor has atoms that are more free to move around."

"When a neutral insulator (the electrons are pretty fixed in place) is placed near an object that has a charge there is a net attraction depending no matter what the charge is of the object. If a conductor (the electrons are way more free-moving) is placed near a charged object the electrons move away if it is negative or towards the object if it is positively charged."

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 why it is that the smaller slit gets a larger spread or vise versa."

"The mobility of electrons part was a bit confusing for me."

"I am having trouble understanding how a neutral insulator or conductor has always has a net attraction towards a charged object, regardless if it is negatively or positively charged.

Match the single slit parameter with its symbol. (Only correct responses shown.)
Width of a single slit: W [96%]
Any positive or negative non-zero whole number: m [76%]
Distance from the slit to a projection screen: L [84%]
Wavelength of light passing through the slit: λ [96%]
Direction, as measured from the centerline: θ [60%]
Position along screen, as measured from the centerline: y [48%]

A gas pump fire is inadvertently caused by the sudden discharge of an electrical spark. Describe when and how the woman became electrically charged.
"She became electrically charged after she got in and out of her car. This likely occurred as she adjusted her sweater because her hands were rubbing the electrically neutral fabric."

"What kind of sorcery is this?!? But I think she was able to transfer a change in electrons from clothes to spark a fire."

"When she is getting out of the car and as she slides across the seat she becomes charged."

"The lady became statically charged when she sat in her car. She could have de-charged by touching something metal to ground herself."

"This is one of my biggest fears, so I always make sure to touch something metal before I begin pumping gas

Electrically neutral polystyrene "packing peanuts" would be attracted to a cat that has __________ charge.
 a positive. * [1] a negative. * [1] either a positive or a negative. ********************** [22] zero (neutral). [0] (Unsure/lost/guessing/help!) * [1]

An electrically neutral aluminum soda can would be attracted to a balloon that has __________ charge.
 a positive. [0] a negative. ** [2] either a positive or a negative. *********************** [23] zero (neutral). [0] (Unsure/lost/guessing/help!) [0]

Electrically neutral polar molecules in a water stream would be attracted to a comb that has __________ charge.
 a positive. * [1] a negative. ** [2] either a positive or a negative. ********************* [21] zero (neutral). * [1] (Unsure/lost/guessing/help!) [0]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Very cool to see different types of everyday things that people use or do and see physics out of it!"

"Practicing diffraction minima examples would be helpful."

"I did not feel as comfortable with the diffraction minima part of the blog. Why is the 'm' value equal to 1? Also, do minimas get categorized with a number like maximas do?" (Mathematically using m = 0 in the diffraction minima equation would give you a dark spot at θ = 0°, which is not what happens physically as the center is always bright, so we "disallow" using m = 0. Using m = 1 gives you the first minimas (dark spots) on either side of the central spot, which is effectively the "spread" of light on the screen. There are other dark (and faint light) spots farther out from the center, and so the locations of these (relatively unimportant) dark spots are given by higher values of m = 2, 3, 4, etc., until you get to θ = 90°.)

"I am slightly confused about using a hair in lab to diffract laser light--will it diffract light the same way as if it was going through a slit of the same size?" (Yes, due to Babinet's principle, light spreading out through a small opening will create the approximately same diffraction pattern as light wrapping around a small obstacle. If you think about light going through a small opening, it is curving around each side of the opening, causing light to spread out from the center. So when you look at light encountering a small obstacle, it will curve around each edge of the obstacle (and cross paths), effectively causing light to spread out from the center (ignoring most of the direct light that passes on either side of the obstacle that doesn't "wrap around" the edges of the obstacle).)