## 20140128

### Online reading assignment: total internal reflection

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 total internal reflection.

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.
"If light is going from a medium with a higher index of refraction (lower speed) into a medium with a lower index of refraction (higher speed) then there is a possibility of total internal reflection if the incident angle is equal to or greater than the critical angle. If the incident angle is less than the critical angle Snell's law can be used to determine the transmission angle."

"I would say, especially in equation form, I am the most comfortable with Snell's Law. I would also say that I have a fairly decent grasp on the critical angle for light traveling between two media."

"I understand that diamonds sparkle because of the light reflecting internally many times before transmitting. And that fiber optics follow a similar principle in order to maximize the distance the light will travel."

"I understand when Snell's law does and doesn't apply in regard to comparing the incident angle and the critical angle."

"The critical angle equation is just an equation manipulation of Snell's law. I also get that for TIR light moves from a high to lower medium index of refraction."

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.
"Equations, equations, equations. For me, once I figure out where everything goes, and why it goes there, I should be fine."

"I'm having some trouble determining which type of reflection is applied in certain situations (i.e. specular vs. diffuse vs. TIR)."

"I'm confused on distinguishing total internal reflection and no transmission. I'm not sure if they are separate or if they are the same thing."

"I thought I understood critical angles, but now I'm not sure."

If the incident angle of a light ray is less than the critical angle, the light ray will be:
 reflected. ** [2] transmitted. ******** [8] (Both of the above choices.) ************************ [24] (Neither of the above choices.) [0] (Unsure/guessing/lost/help!) * [1]

If the incident angle of a light ray is equal to the critical angle, the light ray will be:
 reflected. ********** [10] transmitted. **** [4] (Both of the above choices.) *************** [15] (Neither of the above choices.) *** [3] (Unsure/guessing/lost/help!) *** [3]

If the incident angle of a light ray is greater than the critical angle, the light ray will be:
 reflected. **************************** [28] transmitted. ** [2] (Both of the above choices.) * [1] (Neither of the above choices.) *** [3] (Unsure/guessing/lost/help!) * [1]

Total internal reflection is possible when a light ray in a __________ medium hits a boundary with a __________ medium.
 faster; slower. ********* [9] slower; faster. ********************* [21] (Both of the above choices.) ** [2] (Unsure/guessing/lost/help!) *** [3]

When a rainbow is created, the "reflections" that occur in a raindrop are __________ reflections.
 specular. *********** [11] diffuse ******** [9] total internal. ********* [9] (Unsure/guessing/lost/help!) ****** [6]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"How you can really determine if a diamond is real or fake? I don't understand what the determining factors look like in real life and how easy to recognize they are." (I will bring an obviously fake diamond to class, along with an unknown real/fake diamond for you to compare with.)

"I need some help finding when to apply Snell's law." (If Snell's law can give you a numerical answer, then you are allowed to use it. If it can't give you a numerical answer, then you can't use it.)

"I like spending about half of lecture doing problems so we could get your direct help to clarify anything."

"Could you go through some of the examples from the blog? I'm having trouble with the applications of total internal reflection."

"I liked how the surveys were worded last semester with the questions asking what we found interesting and what we found confusing (as opposed to what we understand or don't understand)."

"Is this box optional?" (If you completed and commented on all the above questions, yes. If you did not, then you should put something substantive here.)