Download Descriptions RROF_PERC04 - Rensselaer Polytechnic Institute

Student Descriptions of Refraction and Optical Fibers
1
Mateycik ,
1,2
Wagner ,
1
Rivera ,
3
Jennings
Fran
DJ
JJ
and Sybillyn
1Rensselaer Polytechnic Institute, 2Grove City College, 3Russell Sage College
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 4 = “Incomplete Refraction”
4A = Mention refraction but not TIR
Stage 2 = “Enclosure” – an opaque coating that keeps light
4B = Know that TIR depends upon indices
from escaping is sufficient to guide light.
of refraction but no details
Stage 3 = “Reflection.” 3A = tiny mirrors where fiber bends,
4C = Tie Snell’s Law to TIR but make error
3B = silvered coating spread on inside of fiber,
Stage 5 = “Correct” describing how total internal
3C = unspecified reflective surface,
reflection can be used to guide light
3D = “quandary” not mirrored but don’t know what
else it could be
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
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
?!
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.
1Materials
Correct
Conclusion
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.
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.
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.
found at http://www.rpi.edu/dept/phys/ScIT/
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.
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
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.
Illusions
Prisms
Refraction 
Dispersion
D Speed
Lenses
Diamond
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.
End
TIR
Concept
Error
Correct
TIR
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.
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.
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.
Formulae
Light
Bends (at
Interface)
S314: [Light traveling from water to air] should
bend towards a 90 degree angle (towards the
perpendicular)
i
Dead-End
Resource
TIR Snell
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
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.
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
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
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.