P-dog's blog: boring but important

20130212

Physics quiz question: vertically polarized light, different polarizer sets

Physics 205B Quiz 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 22.45

Polarized light is incident on two sets of ideal polarizers.

More light will be transmitted through:
(A) polarizer set 1.
(B) polarizer set 2.
(C) (Both polarizer sets will have the same non-zero amount of light transmitted through them.)
(D) (Both polarizer sets will have no light transmitted through them.)

Correct answer (highlight to unhide): (A)

For Malus' law, the fraction of polarized light passing through a polarizer is given by cos²θ, where θ is the angle between the original polarization and the transmission axis of the polarizer.

For polarizer set 1, the fraction of light passing through the first polarizer is cos²(0°) = 1, and will still have a vertical polarization. The fraction of this light passing through the second polarizer is cos²(15°) = 0.93301270189, or 0.93 to two significant figures, and have a polarization that is at a clockwise angle of 15° with respect to the vertical. Thus the fraction of the original light that passes through both polarizers of polarizer set 1 is the product cos²(0°)·cos²(15°) = 0.93.

For polarizer set 2, the fraction of light passing through the first polarizer is cos²(15°) = 0.93, and the resulting light that passes through will have a polarization that is at a clockwise angle of 15° with respect to the vertical. The fraction of this light passing through the second polarizer is cos²(15°) = 0.93 (and will again have a vertical polarization). Thus the fraction of the original light that passes through both polarizers of polarizer set 2 is the product cos²(15°)·cos²(15°) = 0.87051270189, or 0.87 to two significant figures.

Section 30882
Exam code: quiz01b3Es
(A) : 22 students
(B) : 2 students
(C) : 11 students
(D) : 0 students

Success level: 63%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.54

Physics quiz question: polarization by scattering

Physics 205B Quiz 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 22.6

If the sun is directly overhead, and you are looking at the south horizon, you would observe light that is:
(A) partially polarized horizontally, north-south.
(B) partially polarized horizontally, east-west.
(C) partially polarized vertically (up-down).
(D) randomly polarized (unpolarized).

Correct answer (highlight to unhide): (B)

Unpolarized light from the overhead sun, intercepted by an atmospheric particle at the south horizon, makes the particle vibrate horizontally north-south and vibrate horizontally east-west. From your viewpoint, you would only see the east-west horizontal polarization, as the north-south horizontal polarization would be longitudinal instead of transverse, thus the east-west horizontal polarization would dominate.

Section 30882
Exam code: quiz01b3Es
(A) : 5 students
(B) : 20 students
(C) : 9 students
(D) : 1 student

Success level: 57%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.72

Physics quiz question: reception from dipole antenna

Physics 205B Quiz 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 22.1

A radio station is located due north of your location. It uses an electric dipole antenna oriented east-west. In order to maximize reception of this broadcast at your location, you need to orient an electric dipole receiving antenna:
(A) vertically.
(B) horizontally north-south.
(C) horizontally east-west.
(D) (No reception is possible using an electric dipole antenna.)

Correct answer (highlight to unhide): (C)

This electric dipole antenna will broadcast radio signals in directions perpendicular to its orientation, and none along its ends. Since the antenna is oriented east-west and located north of the receiving antenna, then east-west horizontally polarized radio waves will be received at your location.

Section 30882
Exam code: quiz01b3Es
(A) : 4 students
(B) : 1 student
(C) : 29 students
(D) : 1 student

Success level: 83%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.26

Physics quiz question: index of refraction of NaCl

Physics 205B Quiz 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 22.18

Light of wavelength 550 nm in air (index of refraction 1.000) with an incident angle of 45.0° is transmitted at an angle of 27.3° into sodium chloride (NaCl). (Drawing is not to scale.) The index of refraction of sodium chloride is:
(A) 1.38.
(B) 1.54.
(C) 1.65.
(D) 1.93.

Correct answer (highlight to unhide): (B)

From Snell's law, where air is medium 1 and NaCl is medium 2:

n₁·sinθ₁ = n₂·sinθ₂,

and solving for n₂:

n₂ = n₁·sinθ₁/sinθ₂ = (1.000)·sin(45.0°)/sin(27.3°) = 1.54171474555,

or to three significant figures, the index of refraction n₂ of NaCl is 1.54.

(Response (A) is 3·sin(27.3°); response (C) is (45.0°)/(27.3°); response (D) is 1/tan(27.3°).)

Section 30882
Exam code: quiz01b3Es
(A) : 2 students
(B) : 30 students
(C) : 2 students
(D) : 1 student

Success level: 86%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.27

Physics quiz question: As₂S₃ wavelength in air

Physics 205B Quiz 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 22.19(a)

"Orpiment 1"
Parent Géry
commons.wikimedia.org/wiki/File:Orpiment_1.JPG

Light of wavelength 218 nm in arsenic trisulfide (As₂S₃) (index of refraction[*] of 2.66) is incident at the interface between arsenic trisulfide and air (index of refraction 1.000). The wavelength of this light in air is:
(A) 82.0 nm.
(B) 218 nm.
(C) 362 nm.
(D) 580 nm.

[*] nvlpubs.nist.gov/nistpubs/jres/59/jresv59n2p83_A1b.pdf.

Correct answer (highlight to unhide): (D)

The relations between the index of refraction and the speed of light, for arsenic trisulfide (medium 1) and for air (medium 2) are:

n₁ = c/v₁,

n₂ = c/v₂,

where the given (or assumed to be known) quantities, unknown quantities, and quantities to be explicitly solved for are denoted. Also the relations between wavelength, speed, and frequency are:

λ₁ = v₁/f₁,

λ₂ = v₂/f₂.

However, the frequency of the light in arsenic trisulfide is the same as the frequency it has in air, such that:

f₁ = f₂,

(v₁/λ₁) = (v₂/λ₂),

λ₂ = λ₁·(v₂/v₁),

λ₂ = λ₁·((c/n₂)/(c/n₁)),

λ₂ = λ₁·(n₁/n₂) = (218 nm)·(2.66/1.000) = 580 nm.

(Response (A) is (218 nm)/(2.66); response (C) is (218 nm)·(1.66).)

Section 30882
Exam code: quiz01b3Es
(A) : 6 students
(B) : 3 students
(C) : 4 students
(D) : 21 students
(No response) : 1 student

Success level: 60%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.91

Physics quiz archive: electromagnetic waves, reflection/refraction

Physics 205B Quiz 1, spring semester 2013
Cuesta College, San Luis Obispo, CA
Section 30882, version 1
Exam code: quiz01b3Es

Section 30882 results

0- 6 :	* [low = 6]
7-12 :	****
13-18 :	**************
19-24 :	************ [mean = 19.1 +/- 5.9]
25-30 :	**** [high = 30]

Astronomy current events question: ridges on Mars

Astronomy 210L, spring semester 2013
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)

Kevin Stacey, "Ridges on Mars Suggest Ancient Flowing Water," January 29, 2013
http://news.brown.edu/pressreleases/2013/01/marswater

According to Brown University researchers, __________ may be evidence of water that had once flowed beneath the surface of Mars.
(A) long collapsed tunnels and canals.
(B) networks of ridges in impact craters.
(C) mineral-rich spherules called "blueberries."
(D) underground pockets of dry ice.
(E) layers and layers of sedimentary rocks.

Correct answer: (B)

Student responses
Sections 30678, 30679, 30680
(A) : 6 students
(B) : 35 students
(C) : 5 students
(D) : 5 students
(E) : 5 students

Astronomy current events question: NWA 7325 meteorite from Mercury?

Kelly Beatty, "The First-Ever Meteorite from Mercury?," February 1, 2013
http://www.skyandtelescope.com/community/skyblog/newsblog/The-First-Ever-Meteorite-from-Mercury-189374981.html

A University of Washington researcher will announce that the NWA 7325 meteorite recently discovered in Morocco may be a fragment of Mercury, due to its:
(A) extremely high density.
(B) large embedded crystals.
(C) cosmic ray exposed crust.
(D) high-temperature core.
(E) magnesium-rich, but iron-free composition.

Correct answer: (E)

Student responses
Sections 30678, 30679, 30680
(A) : 5 students
(B) : 4 students
(C) : 0 students
(D) : 0 students
(E) : 47 students

Astronomy current events question: asteroid 2012 DA14 closest approach

Space.com editors, "Talk About Close! Asteroid to Give Earth Record-Setting Shave," February 1, 2013
http://www.nbcnews.com/id/50672151/ns/technology_and_science-space/

When asteroid 2012 DA14 makes it closest approach to Earth this month, NASA expects that it will:
(A) pass inside of GPS satellite orbits.
(B) be visible in the daytime sky.
(C) cause higher than normal tides.
(D) transit across the face of the moon.
(E) have developed a long, bright tail.

Correct answer: (A)

Student responses
Sections 30678, 30679, 30680
(A) : 39 students
(B) : 9 students
(C) : 2 students
(D) : 2 students
(E) : 4 students

Online reading assignment: history of astronomy, telescope powers (NC campus)

Astronomy 210, spring semester 2013
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 reviewing the history of astronomy, Kepler's and Newton's laws, and telescope powers.

Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.

"I like thinking of the days when Kepler and Newton were making their laws. It must been pretty exciting to create some of the first correct models of the solar system..."

"Learning how telescopes work was pretty badass. I'm looking forward to rating telescopes in class by their light gathering power, resolving power and magnifying power."

"How much controversy Galileo's telescopic discoveries created, and how some were offended by these discoveries."

"Galileo's true relationship with the telescope was really interesting because I had always been led to believe that he had invented it."

"That for about 1,500 years people accepted Aristotle's First Principle and Ptolemy's geocentric model as accurate descriptions of events in the universe. If people believed Aristotle was so great, why were his ideas about the universe so wrong?"

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"Some of the math involved with telescope powers seems a little tedious."

"How do I keep track of all the astronomers? I can't keep them straight! All that history is probably still the worst part about astronomy. Less history, more science!"

"It's strange that early astronomers used first principles and people just accepted them."

"Keplers's laws of planetary motion and Newton's three laws of motion. I understand the origins of both sets of laws, but I need some help understanding what each law means and how they relate to each other."

"The different types of telescopes and how some are better than the others, since wouldn't they all see the same object?"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"How did you come up with the website called waiferx.com? Does it mean something?" (Waifer X is one of the main characters of an online comic strip I drew 10-15 years ago, back when online comic strips was a thing before becoming a thing.)

"You ask us a question, this relationship is very one-sided." (That is why I'm trying to get you to ask (more) questions of me.)

"Learning a lot of things about astronomy is mind-blowing (in a good way)!" (Once that happens, then my job is officially done.)