Student Descriptions of Refraction and Optical Fibers

1. Student Descriptions of Refraction and Optical Fibers Fran Mateycik1, DJ Wagner1,2, JJ Rivera1,and Sybillyn Jennings3 1Rensselaer Polytechnic Institute, 2Grove City College, 3Russell Sage College Dead-End Illusions Prisms Refraction  Dispersion D Speed Lenses Diamond Formulae Light Bends (at Interface) End TIR Concept TIR Snell Error Correct TIR This poster reports our research into how students describe and think about optical fibers and the physical phenomena of refraction and total internal reflection (TIR) basic to their operation. The study was conducted as part of the improvement and expansion of web-based materials for an innovative Rensselaer introductory physics course1 which examines the physics underlying information technology. As we developed the prototype module, we examined students' understanding of the phenomena of refraction, TIR, and optical fibers through the use of clinical interviews. As students discussed refraction and tried to explain how optical fibers work, several patterns emerged. Our analysis of these patterns drives our assessment of the effectiveness of the revised materials in addressing students' transfer of learning as well as the development of a multiple-choice diagnostic tool. This poster presents our categorizations of student responses. Descriptions of Optical Fibers organized into 5 “Stages” Descriptions of Refraction formed more a network than a progression Stage 1 = “Unique models” not involving light carrying signal. Stage 2 = “Enclosure” – an opaque coating that keeps light from escaping is sufficient to guide light. Stage 3 = “Reflection.” 3A = tiny mirrors where fiber bends, 3B = silvered coating spread on inside of fiber, 3C = unspecified reflective surface, 3D = “quandary” not mirrored but don’t know what else it could be Stage 4 = “Incomplete Refraction” 4A = Mention refraction but not TIR 4B = Know that TIR depends upon indices of refraction but no details 4C = Tie Snell’s Law to TIR but make error Stage 5 = “Correct” describing how total internal reflection can be used to guide light Correct Conclusion Resource S316: hmm, refraction of light, means that if light passes through different things, the speed of light changes, and causes an illusion, per say [sic]. … the illusion that is formed with the refraction of light, deals with depth perception, and how deep the object looks to you, and how deep it actually is. … the light is bended by a material of different density, and this changes its direction, so the object will look different. S302: diffraction is when light passes through a prism and is split into different wavelengths, and refraction is light being recombined into one wavelength. S5: Uh, I know we did a section on optics because I remember playing with a laser in class and that was kind of fun and I remember like convex and concave lenses umm… but I don't remember a lot of it because it was like 1988 I think, it's been a while. S312: An optical fiber is a strand of glass that transmits light data signals over large distances at very high speed(the speed of light) … it is a glass core, wrapped in a reflective glass layer (similar to a mirror), then covered by an outer sheathing, therefore when light travels in straight lines it bounces off the sides of the fiber and is forced to continue to the end S7:And, I, I’ve seen, I’ve seen, it almost looks like it’s a, it’s a plastic substance, I know, cause they use it for now, uh, that, that cable … for computers and things, and it almost looks like it’s a, it’s a plastic, consistent construction, but I don’t, I don’t know what they use; and it’s gotta be reflecting somehow. I don’t know. S317: [An optical fiber] is a type of wire connection that uses fibers to make it faster … Inside the wire there is a type of fiber substance around the wire that helps in the moving of electrons like in tv fiber optics … optics deals with sight so the optical fibers find a way to maximize the speed of electrons with refractors in the fibers to display for sight … im guessing the fibers are made of small mirrors that are attached to insulators in the wires … mirrors imbedded in a solid flexible wire is what im saying … electrons [travel down the wire] with different frequencies and wavelengths containing information … [the mirrors] help [electrons] to move quicker by deflections ... the more mirrors the more quicker which will give a more optimized effect but when electrons slow down thats when the sight becomes snowy or unclear ... the electrons produce a picture or information S305: [In refraction] light is effectively "bent." It enters the glass and then slows down while in the glass and then leaves the glass it resumes the previous angle it was traveling. ?! S313_post: okay, given the right angle the light hits the layer, it will [totally internally] reflect it with respect to the normal I: What determines whether it's at the right angle? S313_post: a formula determines that S307: Yeah, I've heard of [TIR] as one of the reasons diamonds appear so sparkly, because of the lights inability to escape from inside it. [The reason light can’t escape has] Something to do with the way the light was bent when it entered the diamond not having the correct angle to escape, I would imagine. S6: An optical fiber, I know I've seen it in a commercial once or something - they’re describing it and like whatever sound wave or whatever comes in it kind of breaks up into its particles and an optical fiber sort of filters those particles out to make their sight or sound or whatever they are trying to achieve. Optical meaning light so I am assuming that it's light particles being broken up and then kind of reassembled. With higher resolution perhaps these fibers help create better resolution. S314: [Light traveling from water to air] should bend towards a 90 degree angle (towards the perpendicular) P1: In total internal reflection, the predicted propagation direction for a ray of light as it passes through the surface and goes through to the other side of the surface … turns out not to be permitted … by Snell’s Law. i S311_post: An optical fiber is used to send data through wires by means for using total internal reflection … The wire is made of glass fibers. -- There is a cladding layer surface inside the wire which traps the light inside. …The density in the cladding layer is more dense than the density in the core, so light can not escape the core, instead it is reflected. … [Refraction is involved] because the angles at which the light waves ae [sic] reflected is determined through refraction. This is necessary to decrease interference. S8: Hm, well, I guess it could be some tube that the light is going to go in and if uh, you want the light to go in different directions inside of the tube you would use the mirrors on the inside of the tube … Somehow … screw , uh, put mirrors on the sides of the tube at a certain angle so when the light … enters from this side it would be reflected here and from here it would be reflected here (points at diagram) ... that’s what I can think of. S309: [An optical fiber] is a very thin piece of glass that is used to transmit information in the form of light -- it uses the phenomenon of total internal refraction to keep the light "in" the fiber … [TIR is] when light reaches a boundary where it has to change speed, the ray will refract -- total internal refraction occurs when the angle of refraction gets so large that the light will not pass through the boundary, but all of it will be reflected back into the material S305_post: The fiber has total internal reflection; the index of refraction inside the fiber is greater than that of outside the fiber. When light strikes the outside edge of the fiber (from the inside) it is relected [sic] back in. The angle at which the light strikes also plays a role in the matter (the shallower the angle, the less likely it is to escape - for any index of refraction). Also, the fiber usually has a cover (plastic or other form of insulation) to help keep light in and keep foreign light out. S310_post: if a beam of light is traveling from a dense material to a less dense material, the light will be reflected as long as the angle is greater then the critical angle I: What happens if the angle is less than the critical angle? S310_post: it will travel through the material and will be bend away from the normal I: So why does TIR occur? S310_post: hmm, im not really sure to be honest, I would guess that it has to do with the way the light is moving so that the reflected angle is greater then 90 degress -- actually i meant to say the angle of incidence instead of the reflected angle I: And what if I [increased the angle of incidence] more? S7: So that [the refracted ray is] almost parallel with [the surface]? I: mm-hmm. S7: oo-fah. It would still have to exit the glass, but damn, it almost looks like it would stay in the glass, huh? Would it travel through the glass at that point? I don’t know. Participants (See our other poster for more about the study ) Group A: S1-S10, P1-P2 Convenience Sample of REU students, In-service teachers, and physics faculty Group B: S301-S322 ScIT Students pre-instruction S3xx_post are post-instruction 1Materials found at http://www.rpi.edu/dept/phys/ScIT/ RPI work supported in part by NSF CCLI Program under grant DUE-0089399. Thanks to Leo Schowalter for access to ScIT students, and to the rest of the ScIT advisory committee: Karen Cummings, Toh-Ming Lu, Saroj Nayak, Jim Napolitano, Peter Persans, and Wayne Roberge.