Online reading assignment: double-slit interference

Physics 205B, spring semester 2020
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.

"For in-phase sources, the difference in path length will be a whole number of wavelengths for constructive interference. For destructive interference the path difference will be (a whole number plus) a half of a wavelength."

"The presentation did a good job at conveying the path differences for the double slits. It was defined clearly."

"I understand from this section that the maxima and minima of a double-slit interference means whether or not it is constructive or destructive. this is determined by various variables of a double-slit interference like the wavelength, distance between slits, etc."

"Young's double-slit experiment showed how two monochromatic light sources could interfere constructively and destructively. This was shown by the alternating dark and light fringes on the screen he used."

"The difference in lengths that parallel waves travel is equal to their separation distance d between multiplied by sinθ. The inner wave also travels farther."

"In double-slit interference θ is measured from the horizontal and ranges from –90° to +90° with 0° deg being horizontal. The equation dsinθ is used to find the difference in path lengths traveled by two sets of waves. Whether the resulting wave is constructive or deconstructive depends on the whether there is a whole or and a half difference in path lengths."

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.

"Maxima and minima were confusing. double-slit interference was more difficult to understand."

"Definitely the picking the correct information to match the equation. All these symbols are confusing."

"The 'Train of Pain' is still a little confusing, just need to a little more with it."

"I understand the equations and the idea of the path lengths pretty well, but I might need some more practice using and applying them."

"I would benefit from homework examples that have to do with calculating the maxima and minima values to acquire a greater understanding of their relationship to the central bright spot."

"I pretty much understood everything, especially the basics. I might need some polishing of some concepts but that's it."

"Nothing at this time."

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

"The resulting constructive interference is the maxima, and the resulting destructive interference is the minima."

"A maxima is where it is brightest and a minima is where it is least bright."

"Maxima refers to the bright fringes while minima refers to the dark fringes."

Match the double-slit parameter with its symbol. (Only correct responses shown.)

Distance between slits: d [74%]
Any positive or negative whole number: m [74%]
Distance from slits to a projection screen: L [49%]
Wavelength of light passing through both slits: λ [83%]
Difference in paths for light passing through both slits: d·sinθ [51%]
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 [97%]
Path difference: integer number of wavelengths [100%]
Interference: constructive [97%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [89%]
Path difference: odd number of half wavelengths [94%]
Interference: destructive [94%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [97%]
Path difference: integer number of wavelengths [77%]
Interference: constructive [74%]

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

"Would like to review these last two diagrams in or after class."

"Are you going to go over this in class?" (Yes.)

"Will we be doing the double-slit experiment in lab?" (Yes.)

"Since the light that passes through the double slits is from a source that is side-by-side and in-phase with the same wavelength, won't the light passing through the two slits always be in phase?" (Well, yes, unless one wave has to travel a further distance than the other wave in order for them to meet at a receiver.)

"I'm not sure how to use the formula ∆l = dsinθ. In which case will the path difference be a whole wavelength or half a wavelength?" (Well, just calculate ∆l for a given θ direction, then see if it is equal to m·λ or (m + 1/2)·λ.)

"I think that in regards to Young's double-slit experiment, when two light paths interfere constructively they create a bright fringe, and when they interfere destructively they produce a dark fringe... is that correct?" (Yes.)

"I was a little confused whether the double-slit experiment only works when the light sources are side-by-side. If you have two separate sources of light that are constructive but that differ in distance does the double-slit experiment still work?" (Yes, but the sources must have exactly the same wavelength, and be in phase with each other. For visible light the most practical way to do this is to have a laser illuminate two side-by-side slits simultaneously.)

"I would benefit from some practice problems."

"Going to try my hardest to make it tomorrow."

Physics quiz question: comparing blue, yellow laser frequencies

Physics 205B Quiz 1, spring semester 2020
Cuesta College, San Luis Obispo, CA

"laser3.jpg"
©CrystaLaser
https://www.crystalaser.com/new/yellowlaser.html

Blue laser light has a wavelength of 445 nm light in air, while yellow laser light has a wavelength of 594 nm in air. The __________ laser light has a higher frequency in air.
(A) blue.
(B) yellow.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

Wave speed v depends on the properties of the medium. Frequency f depends on the properties of the source. These two parameters can be varied independently of each other.

The wavelength λ is the parameter dependent on both of the independent parameters:

λ = v/f.

Since the two laser light colors have the same speed v (as they travel through the same medium), then because of their different frequencies, they will have have different λ wavelengths (as this parameter depends on both the wave source and the properties of the medium):

λ₁ = v₁/f₁,

λ₂ = v₂/f₂,

where λ₁ < λ₂, as blue light (here, λ₁) has a much shorter wavelength than yellow light (here, λ₂). Since v₁ = v₂ (as both forms of laser light travel through the air), then f₁ > f₂, and thus blue laser light has a higher frequency than yellow laser light.

Sections 30882, 30883
Exam code: quiz01PxP7
(A) : 22 students
(B) : 8 students
(C) : 5 students
(D) : 0 students

Success level: 63%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67

Physics quiz archive: electromagnetic waves, polarization, reflection/refraction

Physics 205B Quiz 1, spring semester 2020
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz01PxP7



Sections 30882, 30883 results

0- 6 :	* [low = 6]
7-12 :	**
13-18 :	*********
19-24 :	*********** [mean = 21.9 +/- 5.9]
25-30 :	************ [high = 30]

Online reading assignment: polarization

Physics 205B, spring semester 2020
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 re-reading textbook chapters and reviewing presentations on polarization.


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.

"In order for TIR to take place, two things must take place: the index of refraction for the first surface must be greater than the second, and the angle incident angle is less than the critical angle."

"Horizontal and vertical antennae didn't make sense to me at first, but after seeing a visual drawing of them, I understand how waves can be transmitted and received in certain directions, but also have dead zones in specific areas."

"A vertical antenna can send messages to other vertical antennas, and has a blind spot on the top and bottom."

"The fencepost model describes how light polarized in a specific direction can pass through a polarizer. Malus's law allows us to calculate how much polarized light passes through a polarizer."

"With polarized light, if the transmission axis of polarizer is perpendicular, all light is blocked. Light with polarization parallel to the transmission axis passes."

"How polarizing sunglasses work. All the molecular antennas have their own direction of polarization in the sunglasses so only half of unpolarized light gets through polarized sunglasses. I also liked learning about liquid crystal displays and the polarization of calculator displays."

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 think I understand the theory behind polarization but concepts are new and a little hard to wrap my head around."

"Having issues visualizing Malus' Law."

"I need more practice on angles other than 90° and 0° or 180°."

"I am still having a hard time grasping Malus' law and I am confused about about how polarized light and the transmission axis of a polarizer works."

"When light passes through multiple polarizers it gets more confusing."

"Honestly it all seemed overwhelming and confusing. if the basics are covered I am pretty sure I'll understand it."

How much unpolarized light is transmitted through an ideal polarizer?

0%.	* [1]
50%.	********************************** [34]
100%.	* [1]
(Unsure/guessing/lost/help!)	* [1]

Unpolarized light shines through two ideal polarizing sheets, that have their transmission axes aligned at right angles to each other. How much of the original intensity is transmitted through these two sheets?

0%.	********************* [21]
50%.	****** [6]
100%.	****** [6]
(Unsure/guessing/lost/help!)	**** [4]

Unpolarized light is projected through the first (smaller) polarizer. If the transmission axis of this first polarizer is horizontal, then one-half of the unpolarized light will pass through, and the light that passes through is now horizontally polarized.

When the light that passed through the first polarizer is now completely blocked by the second polarizer, the transmission axis of the second polarizer is:

horizontal.	*** [3]
vertical.	************************ [24]
diagonal, upper left to lower right.	* [1]
diagonal, lower left to upper right.	*** [3]
(Unsure/guessing/lost/help!)	****** [6]

Polarizing sunglasses are typically manufactured with a vertical transmission axis (to filter out horizontally polarized glare while fishing or driving).

Light from the screen of the iPad (in portrait mode) is:

horizontally polarized.	******************* [19]
vertically polarized.	*************** [15]
diagonally (upper left to lower right) polarized.	[0]
diagonally (lower left to upper right) polarized.	[0]
unpolarized.	[0]
(Unsure/guessing/lost/help!)	*** [3]

Consider a Nexus 4 smartphone (again, held in portrait mode), and polarizing sunglasses (with a vertical transmission axis).

Light from the screen of the Nexus 4 is:

horizontally polarized.	***** [5]
vertically polarized.	********** [10]
diagonally (upper left to lower right) polarized.	*** [3]
diagonally (lower left to upper right) polarized.	************ [12]
unpolarized.	*** [3]
(Unsure/guessing/lost/help!)	**** [4]

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

"Please review answers to above questions."

"I think I need some clarification on the difference between polarized and unpolarized light in class if possible."

"Is diagonally polarized light a thing?" (Yes, definitely.)

"Why do screen manufacturers polarize screens in different ways? Why wouldn't there be an industry standard?"

"Does unpolarized light correspond to bright light? Can polarized light be as bright as unpolarized?" (Yes. Glare off of water or ice is mostly polarized, but polarizing sunglasses can help cut that down.)

"Book is here. Book is unread." (An unread book is a dead book.)

"Have you been to Pismo Preserve yet? The trails were really nice this weekend." (Not yet. #fomo.)

Physics midterm question: same transmitted fraction of unpolarized/polarized light

Physics 205B Midterm 1, spring semester 2019
Cuesta College, San Luis Obispo, CA

In case (a) unpolarized light is incident on a 45° diagonal polarizer, and a certain fraction of incident light is transmitted through it. For case (b) horizontal polarized light is incident on a polarizer with a transmission axis that can be rotated to any θ angle. Discuss what the θ angle should be in case (b) such that the same fraction of incident light is transmitted through it as in case (a). Explain your reasoning using the properties of light and polarization.

Solution and grading rubric:

• p:
  Discusses/demonstrates that:
  1. in case (a), the fraction of unpolarized light that passes through a polarizer (regardless of its transmission angle) is (1/2); and
  2. for case (b), the fraction of polarized light that passes through a polarizer is given by Malus' law: cos²(90° − θ), where the angle of interest is between the horizontally polarization of the incident light and the transmission angle of the polarizer; and
  3. for the fraction transmitted in case (b) to equal the fraction transmitted in case (a), sets cos²(90° − θ) = (1/2), and thus θ = 45°.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least understands what happens in case (a), and has some systematic approach to match this fraction for case (b).
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying the properties of light, polarizers, and polarization.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying the properties of light, polarizers, and polarization.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm01Ft6G
p: 16 students
r: 2 students
t: 14 students
v: 10 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7843):

Online reading assignment: double-slit interference

Physics 205B, spring semester 2019
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.

"How the maxima and minima angles can give us constructive or destructive interference."

"Constructive interference leads to a bright fringe. Destructive interference leads to dark fringe. Intensity varies among the bright fringes; the central fringe has the greatest intensity."

"For two waves with in-phase sources and a distance of d apart from each other, we can approximate their path length difference by using the formula d·sinθ measured from the center line."

"When two waves are leaving from separate slits in phase they can be traveling parallel where θ = 0° and they would stay in phase. If the angle at which both waves are traveling is not zero then one wave is traveling a longer distance and thus can make the waves destructive interference. Constructive or destructive interference depends on how far apart the waves are from each other and the angle at which they are going toward the object."

"A detector can be used to detect the interference between two in phase sources that are side-by-side. The sources interfere at a detector that when moved side to side, create a path difference of the sources that can detect whether the sources interfere constructively or destructively. As the angle that the waves are moved changes, either by half a wavelength or a whole wavelength, the interference changes. "

"When light from two in-phase sources pass through slits, the two 'path lengths' of light from the slit to the point where they meet can relate in two important ways. If the lengths are equal or differ by a whole wavelength, constructive interference will occur when the two lightwaves meet and form a maxima. If the lengths differ by half of a wavelength, destructive interference will occur when the lightwaves meet and form a minima."

"Honestly I'm still kind of lost on this. I know we are looking for the difference in path length, but the equations are a bit difficult."

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 a little confused on what the variables are and how they relate to the pictures in the presentation."

"I don't understand the connection between maxima/minima angles and interference."

"I'm still not quite sure about what minima and maxima mean and how that related to constructive and destructive interference."

"I found the double-slit interference to be a little confusing where m is used as an integer used to describe how many wavelengths difference the waves are. It was confusing at first but after thinking it through and a little practice it made a lot more sense."

"I find it hard to understand the equations. Perhaps a tutorial in class may help."

"How do we know when to use each path length difference equation for the maximas and minimas?"

"I need practice applying the ∆l = d·sinθ equation. I also need to better understand the principle of a diffraction grating."

"Not confused much but would like to see examples."

"I am still a bit confused about how to apply the path length difference approximation in problem-solving. I feel like I understand the principles, different waves are interfering, but I do not understand why this is useful."

"Might be some confusion on putting all the pieces together."

"Not too much is super-confusing to me."

"I didn't really find anything very confusing I think I've got a pretty good understanding of the material. "

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

"'Maxima' refers to constructive interference and 'minima' refers to destructive interference, regarding double-slit interference of in-phase waves."

"Maxima is dark and minima is bright."

"It seems that maxima is a constructive interference and the minima is destructive."

"No idea--but I know calculus maximas and minimas."

Match the double-slit parameter with its symbol. (Only correct responses shown.)

Distance between slits: d [85%]
Any positive or negative whole number: m [85%]
Distance from slits to a projection screen: L [67%]
Wavelength of light passing through both slits: λ [92%]
Difference in paths for light passing through both slits: d·sinθ [54%]
Position along screen, as measured from the centerline: y [62%]

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 [87%]
Interference: constructive [97%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [74%]
Path difference: odd number of half wavelengths [82%]
Interference: destructive [90%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [90%]
Path difference: integer number of wavelengths [67%]
Interference: constructive [79%]

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

"No question today, this all seems to make sense to me."

"How did I do on my ray tracings for the quiz?" (Have you checked your quiz scores online?)

"This was very helpful in making the path length differences and how the two wavelengths form the light and dark spots 'click' for me."

"Is there a way to mathematically check these examples above?" (Before every class, click on the "review previous reading assignment responses" link to see the correct answers for each reading assignment question.)

"I'd like to know how to do the interference from the path length differences." (You'll get to see this in lab.)

"Okay this might be dumb but I don't really understand what the double-slit interference is. I don't understand what this interference is supposed to represent." (It's a way of dividing a single beam of light into two (or more) separate beams of light, that can interfere with each other constructive (or destructively) in certain directions. Next week's lab is where we shoot a laser beam through a CD-RW disc, such that it passes through many adjacent tracks to separate into many separate beams of light that will interfere constructively (or destructively) at certain places on a screen.)

"Sometimes I get tripped up on the math, so the trigonometry confuses me a bit. I would love to see something drawn to scale where the parallel lines from the two slits end up creating a bright spot together." (We'll get to "act out" how two waves interfere constructively or destructively for double-slit interference.)

"Is there any way you can make the individual lab reports less time consuming and tedious, especially the procedure section? :(" (Although the procedure section can get tedious if you detail every step that you did in lab, there is a skill in summarizing that in a succinct manner (usually with diagrams) that I would like you to practice this semester. Shorter is better, but not too short.)

Physics quiz question: Google's Project Soli wavelengths

Physics 205B Quiz 1, spring semester 2019
Cuesta College, San Luis Obispo, CA

"Welcome to Project Soli"
Google ATAP
youtu.be/0QNiZfSsPc0

Google's Project Soli[*] is developing a sensor that detects hand gestures using radio waves, with frequencies between 57 GHz to 64 GHz in air. In air, the 57 GHz radio wave has __________ wavelength compared to the 64 GHz radio wave.
(A) a shorter.
(B) a longer.
(C) the same.
(D) (Not enough information is given.)

[*] rarstechnica.com/gadgets/2019/01/googles-project-soli-radar-gesture-chip-isnt-dead-gets-fcc-approval/.

Correct answer (highlight to unhide): (B)

Wave speed v depends on the properties of the medium. Frequency f depends on the properties of the source. These two parameters can be varied independently of each other.

The wavelength λ is the parameter dependent on both of the independent parameters:

λ = v/f.

Since the two radio waves have the same speed v (as they travel through the same medium), then because of their different frequencies, they will have have different λ wavelengths (as this parameter depends on both the wave source and the properties of the medium):

λ₁ = v₁/f₁,

λ₂ = v₂/f₂,

where v₁ = v₂ (as both waves travel through air). With f₁ = 57 GHz being lower than f₂ = 64 GHz, then λ₁ > λ₂, and so the 57 GHz radio wave has the longer wavelength.

Sections 30882, 30883
Exam code: quiz01aN7u
(A) : 6 students
(B) : 28 students
(C) : 2 students
(D) : 1 student

Success level: 76%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.63

Physics quiz archive: electromagnetic waves, polarization, reflection/refraction

Physics 205B Quiz 1, spring semester 2019
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz01aN7u




Sections 30882, 30883 results

0- 6 :	* [low = 3]
7-12 :	****
13-18 :	*******
19-24 :	***************** [mean = 20.8 +/- 5.7]
25-30 :	******** [high = 27]

Online reading assignment: polarization

Physics 205B, spring semester 2019
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 re-reading textbook chapters and reviewing presentations on polarization.


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.

"The direction of disturbances, perpendicular to the direction of the wave, determine the polarization of the wave. An example of polarized light source is an antenna."

"When atoms and electrons, sources of light, vibrate in different directions, they generate unpolarized light. In contrast with antennas, electrons have to move up and down the length."

"Waves that are linearly polarized can only pass through openings that are parallel to the direction of the rope vibrations. A polarizing material is called a polarizer."

"I felt like I really grasped the idea of the fencepost models we were given and how when unpolarized light is incident on a polarizer, only the light parallel to the polarizer will pass through.The light that passes through will have a polarization direction that matches the transmission of the polarizer."

"I understood that all light will pass through a polarizer if the transmission axis and polarization of the incident light are parallel. In contrast, if the polarization of the transmission axis and incident light are perpendicular, no light will pass through."

"Light that is perpendicular to a polarizer is not transmitted. The intensity of light that passes through a polarizer is related to the angle of the polarizer to the transmission axis of the light. The intensity of light transmitted is equal to the intensity of light entering the polarizer multiplied by the square of the cosine of the angle of the polarizer."

"Malus' law is a way to determine the intensity of polarized light after it goes through a polarizer and is filtered. Cos²θ of the unpolarized light going through a polarizer will give you the new intensity of the polarized light."

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 confused as to why east-west oriented antennae are best transmitted north-south. What causes this?"

"I'm not quite understanding how it is that electrons can move side to side along a horizontal antenna."

"I find it confusing when unpolarized light passes through several stacked polarizers ( how the polarization of the polarizers affects the light)."

"Unpolarized light could be a little confusing. I also found some of the questions a bit confusing."

"I do not fully understand how to structure information about polarizers to solve questions about the intensity of light. I would benefit from examples on how to use Malus' law."

"I don't quite understand the LCD screen section. I would like to understand more about how voltage works. It makes sense to me that light is one type of electromagnetic wave but I am confused about how it relates to the positive and negative charges on the transparent electrodes sandwiching the liquid crystal. I also don't quite understand how the liquid crystal works and if it is related to solid crystal."

How much unpolarized light is transmitted through an ideal polarizer?

0%.	**** [4]
50%.	*********************** [23]
100%.	***** [5]
(Unsure/guessing/lost/help!)	***** [5]

Unpolarized light shines through two ideal polarizing sheets, that have their transmission axes aligned at right angles to each other. How much of the original intensity is transmitted through these two sheets?

0%.	******************** [20]
50%.	******** [8]
100%.	**** [4]
(Unsure/guessing/lost/help!)	***** [5]

Unpolarized light is projected through the first (smaller) polarizer. If the transmission axis of this first polarizer is horizontal, then one-half of the unpolarized light will pass through, and the light that passes through is now horizontally polarized.

When the light that passed through the first polarizer is now completely blocked by the second polarizer, the transmission axis of the second polarizer is:

horizontal.	*** [3]
vertical.	**************** [26]
diagonal, upper left to lower right.	*** [3]
diagonal, lower left to upper right.	[0]
(Unsure/guessing/lost/help!)	***** [5]

Polarizing sunglasses are typically manufactured with a vertical transmission axis (to filter out horizontally polarized glare while fishing or driving).

Light from the screen of the iPad (in portrait mode) is:

horizontally polarized.	******************* [19]
vertically polarized.	********** [10]
diagonally (upper left to lower right) polarized.	* [1]
diagonally (lower left to upper right) polarized.	** [2]
unpolarized.	[0]
(Unsure/guessing/lost/help!)	***** [5]

Consider a Nexus 4 smartphone (again, held in portrait mode), and polarizing sunglasses (with a vertical transmission axis).

Light from the screen of the Nexus 4 is:

horizontally polarized.	******* [7]
vertically polarized.	********* [9]
diagonally (upper left to lower right) polarized.	***** [5]
diagonally (lower left to upper right) polarized.	[11]
unpolarized.	[0]
(Unsure/guessing/lost/help!)	***** [5]

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

"This section is making sense to me and I find it very interesting because I've always wondered what polarized sunglasses did. I think all of the concepts make sense to me so far but I do have a couple questions about polarized light. For one, I am having a hard time understanding the energy of light and basically how it is created. It just seems weird to me how antenna's shoot electrons up and down to create a wave out of thin air? What are these waves made of? How do they keep energy and are able to travel so far?"

"I like this section but I wish there was more explanation of how light waves are formed, and what they are. I understand the concepts but it would be cool to get deeper understanding of how it all works. For example if I wanted to create and antenna and start sending out waves I would have no idea where to start." (Wait until we get through electromagnetism, as light waves are just oscillating electromagnetic fields.)

"What makes a material capable of polarizing light?" (Something that looks like fencepost on the scale of light wavelengths. A set of parallel metal rods will polarize radio waves, while a sheet with long, parallel polyvinyl alcohol chains will polarize visible light.)

"If something is polarized, that means light only transmits in a single direction? If sunglasses are polarized, that means that only single direction of light can transmit through?" (Yes.)

"I am confused about the two polarizer filters lined up and how much unpolarized light is able to get through. If only 50% gets in through the first one and the second filter is lined up on the same axis then would 50% be the final amount that gets through and if the second filter is lined up on a perpendicular axis it would be 0%?"

"Could you go into these sunglasses examples especially the one with the Nexus smartphone?"

"I'm kind of struggling on this subject."

Physics midterm question: different light polarizations, same polarizer set

Physics 205B Midterm 1, spring semester 2018
Cuesta College, San Luis Obispo, CA

A set of two polarizers has either (a) horizontally polarized light, or (b) vertically polarized light incident on it. Discuss why this set of two polarizers will pass through the same fraction of horizontally polarized and vertical polarized light. Explain your reasoning using the properties of light and polarization.

Solution and grading rubric:

• p:
  Discusses/demonstrates that the same amount of light will pass through the set of polarizers as:
  1. the horizontally polarized light is at an angle of 45° with respect to polarizer 1, resulting in cos²45° = (1/2) of this passing through, with a 45° polarization angle parallel to that of polarizer 1; and
  2. the vertical polarized light is also at an angle of 45° with respect to polarizer 1, resulting in cos²45° = (1/2) of this passing through, also with a 45° polarization angle parallel to that of polarizer 1; then
  3. since for both cases (initially horizontally polarized light or initially vertically polarized light) the light that has passed through polarizer 1 has the same polarization angle and intensity, after passing through polarizer 2 the amount of light (and polarization direction) will be same for both cases.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: midterm01cVdP
p: 26 students
r: 3 students
t: 4 students
v: 0 students
x: 0 students
y: 0 students
z: 0 student

A sample "p" response (from student):

Astronomy quiz question: telescope funding

Astronomy 210 Quiz 3, spring semester 2018
Cuesta College, San Luis Obispo, CA

Based on being able to detect its wavelength, and cost of location, which telescope should be funded?
(A) An x ray detector on a remote ocean platform (30' elevation).
(B) An far infrared telescope in the Mojave desert (5,000' elevation).
(C) A radio telescope on a high-altitude balloon (100,000' altitude).
(D) An ultraviolet detector in space (above 500,000' altitude).


Correct answer (highlight to unhide): (D)

X-rays and far-infrared cannot be detected at or not much above sea level (the bottom edge of the graph), due to absorption of these wavelengths by the atmosphere. A radio telescope would work on a high-altitude balloon, but would work just as well on the ground for much less cost. An ultraviolet telescope would work in space, and may work just as well on a high-altitude balloon as well. So while the radio telescope on a high-altitude ballon and the ultraviolet detector in space would both be able to detect their wavelengths, a vastly cheaper alternative for the radio telescope exists (putting it on the ground), compared to a slightly cheaper alternative for the ultraviolet detector, so the latter is most cost-effective than the former.

Section 30674
Exam code: quiz03nT7a
(A) : 1 student
(B) : 5 students
(C) : 1 student
(D) : 16 students

Success level: 72% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.83

Section 30676
Exam code: quiz03sB1g
(A) : 4 students
(B) : 13 students
(C) : 6 students
(D) : 17 students

Success level: 46% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.82

Online reading assignment: double-slit interference

Physics 205B, spring semester 2018
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.

"The general introduction to double-slit interference; more specifically the modified 'Train of Pain.'"

"We are able to find out if constructive or destructive interference occurs at a detector when two in-phase light waves are situated so that they are offset at an angle, which therefore also produces a difference in length. We can use an equation to find maxima/minima, angle, and length as they are related to each other."

"When comparing two waves that are both in phase, we're trying to figure out how much longer it takes for one wave to travel than the other. The path difference, how many wavelengths, etc."

"When considering path differences we can make the assumption that the detector is sufficiently far from the source of waves and that the two waves (d apart) travel along a parallel path. This makes it so that the location of the detector can be specified by a specific angle."

"Double-slit interference is basically a case in which waves from two side-by-side in phase sources interfere in both a constructive or destructive manner. In this lecture, source phases do not matter since we are looking at only in-phase sources, so our main focus of attention is looking at the path differences. The minima are where the paths interfere destructively, and the maxima are where the paths interfere constructively. There exists a trigonometric equation to determine how long a path is relative to another path as well."

"When comparing two waves that are both in phase, we're trying to figure out how much longer it takes for one wave to travel than the other. The path difference, how many wavelengths, etc."

"When we change the path of two waves, it affects the distance the waves have to travel to reach a destination. The equation delta ∆L = d·sinθ will tell if the interference is constructive (maxima) or deconstructive (minima)."

"The maxima and minima angles can give us constructive or destructive interference. This interference could be the product of waves having a path difference of a whole or half wavelength."

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.

"Nothing was really confusing so far. I think the only thing harder to understand for me was the diagrams and the angle itself how that is found."

"How the altered distance of the two parallel waves effects their in/out of phase properties."

"I need more help on applying the ∆L = d·sinθ equation."

"I seemed to have understood this alright. Some lecture time would help me though."

"I'm confused about when a path difference would be destructive. My assumption is that the waves are constructive when the angle is at 0 degrees and destructive when the angle is larger or smaller than zero because these cases require that one path be shorter than the other."

"The path difference is really confusing to me. I think because it is harder to 'picture' in my head as I think it out."

"I don't understand the principle of a diffraction grating."

"Could you go over the equations?"

"I really don't find this presentation confusing. But how do we know and test which angles will be destructive and constructive?"

"I was pretty lost on finding the maxima and minima. I understood how you can manipulate delta l to be able to plug in wavelengths and m and then solve for an angle theta, but I don't know how this angle really comes into play in the big picture of it all."

"I really don't understand how path length differences are calculated."

"I didn't find anything too confusing in this lesson."

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

"'Maxima' is constructive interference and 'minima' is destructive interference."

"When two in-phase sources in a double-slit interfere at maxima this is constructive interference, and when they interfere at a minima there is destructive interference."

Match the double-slit parameter with its symbol. (Only correct responses shown.)

Distance between slits: d [74%]
Any positive or negative whole number: m [83%]
Distance from slits to a projection screen: L [55%]
Wavelength of light passing through both slits: λ [97%]
Difference in paths for light passing through both slits: d·sinθ [62%]
Position along screen, as measured from the centerline: y [55%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [100%]
Path difference: integer number of wavelengths [97%]
Interference: constructive [97%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [61%]
Path difference: odd number of half wavelengths [94%]
Interference: destructive [94%]

Identify the characteristics of the sources, path difference, and interference type. (Only correct responses shown.)

Sources: in phase [90%]
Path difference: integer number of wavelengths [74%]
Interference: constructive [84%]

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

"Review the above three diagrams from this assignment?"

"I could never go wrong with some more practice with the types of problems above, hopefully we have some in class :) ?"

"What is y? I put 'position along the screen as measured from the centerline.' Not sure I see what we are talking about."

"I am sorry I could not think of anything funny to write or ask this time, I will do better next time I promise.