Reflection and Refraction
1. Work together in CP153 from 2pm-3pm Thursday 03/07/2013
(a) Curved mirrors
2. Work together in CP153 from 2pm-3pm Thursday 03/21/2013
(a) Lenses
(b) Total internal reflection
(c) Glass fiber
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We do these projects table by table. Every body writes detailed notes of what you
have observed.
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Physics 160-002
Spring 2013
Instructions for Reflection and Refraction
(Mirrors and Lenses)
Text References are to Physics by Inquiry, Volume 2 (pp.539-597)
CHECKs:
4.1
4.6
4.9
6.1B
1.2
LENSES
1.3
1.5
1.6
1.8
TOTALINTERNAL REFLECTION
FINAL CHECK
General Notes:
1) For many of these experiments, you will be using special light boxes, in which you can insert
slits of different types to get beams of light. To allow storage, the light boxes are easily dismantled
and folded. Please carry them carefully, i.e. supporting them from beneath. Also, remember that
mirrors are fragile, so handle them carefully.
2) For many of these experiments, you will need to mark the paths of light rays. Cover your bench
with butcher paper, so that you can mark the paths of rays, etc. directly on the paper. (Replace the
paper as needed.)
I. Reflection (pp.539–545)
1) Do Experiment 1.1 (p. 539). Recall that an angle is a description of the
intersection of two lines.
a) For 1.1A, use a single-slit insert and a three-slit insert, covering up one slit
with opaque tape. To change the width of the slit, you can make the slit
narrower by covering up part of the slit.
b) The book’s figure for 1.1B(1) shows two mirrors arranged one behind the other. In 1.1B(1),
only use one mirror. When you use two mirrors in later experiments, do not arrange them this way,
but rather arrange them so that the light hits one mirror first and then goes to the other.
c) In 1.1B(4) and subsequent experiments, mark the path down the center of the beam.
2) Do Experiment 1.2. For part C: Only develop a rule for lines that you draw along the center of
the beam. Ignore the suggestion to look also at the edges of the beam. For the CHECK at the end
of 1.2, present only the statement of the rule itself, carefully defining any angles you use in the
rule.
3) Skip Part A of Experiment Experiment 1.3, but do the rest of this experiment (pg.541-542).
Remove the butcher paper from your table for this experiment, since it can interfere with your
observations. Instead of using a flashlight with a mask, use a Minimaglight adjusted by twisting
the cap so that the light beam is narrow.
a) For part B, where it says to use “red” construction paper, you can use any color construction
paper. For part C, use a different color construction paper.
b) After doing part F, shine the Minimaglight on a shiny book. What rule or rules would you
use to describe surfaces such as these?
c) The CHECK at the end of 1.3 covers all experiments and observations made in 1.3.
4) Omit Exercise 1.4.
5) Do Experiment 1.5. Predict what you will observe in each part of the experiment before doing
it. (Ask your instructor for help in setting up this experiment up if you don’t understand the
instructions.)
a) In Experiment 1.5A, use a Minimaglite as a point source. With the cap of the minimaglite
removed entirely, lay the minimaglite down on the paper. Arrange the mirror so that it faces your
side of the table at an angle instead of facing away from you as indicated in the drawing in the
book. Use metal clips to hold up the index card. We do not need to have the lights out for this
experiment. You can draw the light path using what you already know about the light rays
coming from the point source, the light beam that goes through the slit and the Rule of Reflection.
b) In 1.5B add a little clay to the head of the nail so that it will stand up easily.
c) In the last sentence in 1.5D, replace “bulb” with “nail”. Mark the RANGE—the “region in
which you must place your eye in order to see the image of the [nail]”—on your paper.
d) Add a CHECK at the end of Exercise 1.5, summarizing 1.5 Include in your discussion the
following points: i)in what directions one must look to see an image, ii) why the directions are the
same for the nail and the light, iii) why we use the card with the slit for these experiments, iv) why
is there a range of directions in which you can see the image, and what defines the range.
6) Do Exercise 1.6, and add a
CHECK summarizing 1.6.
7) We will all do Experiment 1.7 together. If you get to this point before others are ready for 1.7,
start working on Exercise 1.8. For Exercise 1.8 we have copies of the diagram for you to work
with. You need to draw carefully and precisely here; DO NOT just make a rough sketch ! The 1.8
CHECK will be judged in part on the quality of your drawing(s). Predict A and B before placing
the mirror in C.
II. Refraction (pp.562–567, 575)
6) Do Experiments 4.1A (p. 562), using a single slit with your light box. Also, instead of using
beakers, use on square plastic container. The water level in the container should be higher than
the top of the slit in the light box. Start with the container with water. If you cannot see the beam
of light in the water, sprinkle a tiny bit of coffee creamer in the water to make the beam path
visible in the water. (The creamer acts like dust particles in air to scatter the light to your eye.
However, often the water coming out of our tap is dirty enough that the creamer isn’t needed!) To
get a sense of the outgoing beam one can look at the light as it reflects off the butcher paper (or
you can put your head down to where the beam is going out, looking back along the beam to see
the image of the filament of the light bulb). Concentrate on the beam as it enters the water, is
transmitted through it, and then leaves the water. Don’t be concerned with light beams which
reflect off the different surfaces.
a) Do 4.1B, but omit the last paragraph, asking you to change the liquid in the container. We will
do this later as a demonstration for the whole class.
b) Before doing 4.1C, do the following experiment: Set up the square container of water so that you
observe a large deflection of the light beam, and draw an outline of the container and light light
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beamson the butcher paper. Then replace the container with an empty container of the same size
and observe (and draw) on the butcher paper the direction of the transmitted beam. Then answer
4.1C and 4.1D.
c) Place a CHECK at the end of Experiment 4.1; include a discussion of how a ray of light
changes direction when entering water and when leaving water: when does it turn toward the
surface, when does it turn away from the surface, and when does it not change direction?
(emember, we are considering angles between 0o and 90o.)
7) Skip Experiment 4.2. and do Exercise 4.3, using all your diagrams from 4.1.
8) Do Experiment 4.4. To see the effects of thickness, lift the glass into and out of the beam by
raising and lowering it. You should see the direction of the beam change and observe the effects of
changing the thickness of the glass.
8) Do Exercises 4.5 & 4.6A, making careful sketches, but skip all curved-surface parts. There is a
CHECK at the end of 4.6.
9) Do Experiment 4.7. The general question is: if you look at the path of your rays in each case, but
if there were no arrows drawn on your diagram, could you tell in which direction the light was
traveling? Perform experiments that show that the path of the light ray is reversible for each of
the 3 cases: for each case, first measure the path of a beam. Then move the light source so that the
new incoming beam overlaps the previous outgoing beam, and measure the location of the new
outgoing beam. Does it lie on top of the original incoming beam, i.e. does the ray reverse its
original path?
a) Add the following exercise: Use the reversibility of light rays to show that a light ray cannot
come into a surface at an angle less than 90 degrees from the surface and bend to an angle of
exactly 90 degrees in the second material. Move the 4.7 CHECK to the end of 4.9A.
10) Do Exercise 4.8; then do experiments to check your predictions about the first two cases
considered (i.e. use one container of water, with the light ray entering at different spots).
11) Do Experiment 4.9A. Do a CHECK at the end of 4.9 that discusses your conclusions from4.7,
4.8, and 4.9.
12) Go to page 575 and do Experiment 6.1, using a styrofoam cup.
a) Add the following experiment and exercise (call it 6.1b). Tape a nail to the inside of your
rectangular water container, and fill it with water so that the nail is partly in the water and partly
out (see the figure on bottom of p.575). Observe the nail from all four sides of the container.
When viewing it through the water, does the part that is submerged look closer or further away
that the part that is not submerged. Draw a ray diagram that explains your result. Add a CHECK
that discusses your results and diagrams for both this part and Experiment 6.1
III. Image formation by lenses (pp.580–581, 597)
13) Read the text on the top of p.580, do Experiment 7.2, and read the text after it.
14) Go to page 597 and do Experiment 10.1. There is a CHECK after this experiment in which you
summarize and compare the behavior of convex and concave lenses.
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IV. Total Internal Reflection
Equipment: Semicircular plastic dish, laser pointer, protractor
15) Place the semicircular dish on a piece of butcher paper and fill it with water (and a little creamer
if you need some). Shine the laser on the flat side of the dish, perpendicular to the surface. (Does
the light ray bend when it enters the water?) Start with the light ray hitting the center of the flat
side, and slowly move it to the edge, observing the ray after it hits the curved side of the dish.
(Hint: it may be helpful to slightly tilt the laser downward so that you can see it on the paper.) In
general, some of the light will be reflected at the curved surface and some will be refracted (i.e.
as it leaves the dish). Observe both of these rays.
a) Qualitatively observe and sketch what happens at different locations of the incident ray.
b) Does the refracted beam disappear when the incident ray is near the edge of the dish? In this case,
all the light is reflected; this is called Total Internal Reflection. Identify the spot where the
refracted beam first disappears, and then move the light very slightly back toward the center to
where you can just barely see the refracted beam. Trace the incident beam, the refracted beam, and
the curved surface on the butcher paper – what is the angle between the refracted beam and the
surface?
c) Now again move the laser toward the edge, so that you have Total Internal Reflection and, for
one location, draw the incident beam and curved surface. Is the angle between them greater or less
than the previous case (b)? If there was a refracted beam, where would it be?
d) Summarize what you think are the conditions for Total Internal Reflection? Could it ever
happen for light passing from air to water?
16) Now turn the dish around so that the laser light is incident on the curved side. For different
locations and angles of the incident beam, observe the location of the ray as it enters the water and
as it is reflected and refracted at the curved surface. Sketch a few cases. Do you ever observe
Total Internal Reflection? Are your observations consistent with your conclusion in 15c? If not,
modify your summary to include all the cases.
CHECK: Discuss your observations and summary with an instructor.
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Review
Do a final review CHECK on what we know about how light travels:
1. First, describe the case when light does not encounter anything as it moves through the
air or through space or through any uniform transparent medium. A transparent
medium is one in which light travels without change in direction and is one through
which one can see clearly, for example air or the glass in a window.
2. Next, describe what happens when light hits things like construction paper, smoke or
chalk eraser dust. Sometimes the phenomena, where the light goes off in many or all
directions, is called scattering of light. This behavior is also observed in translucent
materials, which allow light to pass through but block seeing clearly through them, for
example frosted glass.
3. Next, describe what happens when light hits a mirror.
4. When we see an image of object in a mirror, light rays from that object must travel to
the mirror and reflect from the mirror in such way that they enter the eye. Describe in
words and draw how the light rays get to our eye. Describe what happens when we see
an image in a mirror. In what direction would we look to see the image? Where is
image located?
5. What is an opaque material?
6. Describe what happens as light goes from one transparent medium (e.g. air) to another
(e.g. water). (Consider the case when it enters straight on and when it enters at an
angle.)
7. Describe what happens as light enters at an angle through one side of a solid chunk or
thin-walled container of a transparent liquid, goes through the material and then exits
the other side WHEN THE OTHER SIDE is be parallel to the side where the light
entered.
8. When we see an image of object through a transparent medium, light rays from that
object must travel through the medium in such way that they enter the eye. Describe
what happens when we see an image through a transparent object such as a glass of
water. In what direction would we look to see the image? (Draw a sketch of how you
would see a fish in a rectangular water tank.)
9. In most cases, some of the light is absorbed when it passes through a material. What
might be the effect of the absorption of light?
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