Online reading assignment: total internal reflection

Physics 205B, 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 total internal reflection.

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 did not know that if you shine light on a diamond that no light will escape the back of the diamond. And rhinestones are actually coated in metal powder to force the reflected light back upwards."

"Light reflecting off the surface of air--I had never really thought of it that way when looking toward the sky from the bottom of a pool."

"That there's a major advantage to using prisms instead of mirrors for many applications. I was always fascinated with how light waves making up images can be bounced off sets of mirrors and not lose the image--for example, binoculars and a periscope!"

"Fiber optic cables used for long distance broadband data transmission. I remember reading somewhere that if someone wanted to cause trouble, he could try cutting the cables that run from North America to Europe under the Atlantic Ocean."

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

"I found it a little bit difficult to keep the rules for total internal reflection straight."

"The three possible cases if light starts in a higher refractive index material--I still get all the different angles mixed up in my head! Total internal reflection and the critical angle would be nice to see drawn out."

This may be a silly thing to be confused about, but even though I read the text; I am still not sure how messages are sent through fiber optics. I always thought that the 'remote cameras on wires' were just regular, small, digital cameras (with the sensor near the end) with wire for sending electrical signals back to a screen. Is the actual wire a fiber optic cable that transmits light from the end of the wire to a sensor at the receiving end?"

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

"For a ray of light incident on a stack of three substances, why does the second substance not matter? That doesn't make sense to me." (All three substances do matter when tracing the path of light, but if you only need to find the final transmitted angle in the third substance, then the second substance's n₂·sin(θ₂) term can be substituted out of the two sets of equations:

n₁·sin(θ₁) = n₂·sin(θ₂),
n₂·sin(θ₂) = n₃·sin(θ₃),

leading to n₁·sin(θ₁) = n₃·sin(θ₃), which can be solved for θ₃.)

Will you never be lecturing in class anymore on the slides?" (Not quite; I'll still discuss difficult concepts and do worked-out examples in class. That said, introducing new terminology, and looking at 'cool' applications of physics will be covered during homework rather than in class.")

"Can you describe the phenomenon of a rainbow? It's very interesting but the textbook doesn't describe it quite enough. What is the cause for the light ray to reflect twice inside a raindrop?" (Total internal reflection, where the light ray is greater than the critical angle as it tries to exit the raindrop.)

"A diamond is supposedly the hardest substance naturally create, with all the tight-knit carbon atoms, how does it refract light so well?" (None of the electrons that bond the carbon atoms together have energy level spacings that would absorb visible light photons (thus making it transparent instead of opaque), but they will readily scatter those photons, impeding their passage through the material.)