More about Refraction
Today’s Goal: Making sense of the Law of Refraction; Investigating a prism
Materials
laser pointer
prism
thick glass piece
glass slide
light station
single slot mask and triple slot mask
beam-blocker card
viewing screen
various bottles & beam observation
sheets from previous activity
plastic cube filled with water
Discussion
> Here is a top view picture of a mirror and an object "x". From some places the object
can be seen in the mirror; from other places it cannot. By drawing lines representing the
paths taken by light, determine the region in which the object can be seen reflected in the
mirror.
> There is also an object "y" in the picture. Find the image of "y" in the mirror, and use
this to find the places from which "y" is visible reflected in the mirror.
> Use the picture below to explain how Trish can sometimes count the same fish twice
(this is a view from above, and the dark lines represent the glass walls of the fish tank).
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1. Set up the light station with the single slot mask. Adjust it to make a narrow, bright
beam across the white screen on the table top. Place the square container in the beam,
so that light goes in one side and comes out the opposite side. Observe how the beam is
affected when you turn the container.
Draw lines on the diagrams to record what you observe. Also draw (and label) reflected
beams.
2. Suppose there is an object under water. We will
see the object when light coming from the object
reaches our eye. In the drawing at right, the lines
coming from the object represent light. Predict what
happens to the light beams when they leave the water
by continuing the lines.
3. We tell where an object is by noting the direction of the light coming from it.
Where does the underwater object of Q2 seem to be, to observers above the container?
Indicate this on the diagram.
4. Explore the water-filled flat-sided bottle in the
light beam made by the light station.
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5. Use Beam Observation Page 1 to record what happens to a
light beam passing through the upright bottle from various
angles. The page has space for three observations. For each
space A, B and C:
 Move the paper into the beam so it is centered on the line
 Stand the bottle in the box
 Draw on the paper the path of the light beam when it
leaves the bottle.
6. Explain why the light beam changes path as you go from A to B to C, using the
result of your previous investigations.
7. Using the other two water bottles, oval and round,
explore each one in the beam, and explain how the light
beam changes path.
8. Using the three-slot mask on the light station, explore how each of
the three different water filled bottles affects the light beams.
 Using Beam Observation Page 2, stand each bottle in its place.
 For each bottle draw on the paper the three light beams – before
and after the bottle.
 Draw in how the light beam went from where it entered to where
it left the bottle. Explain why it went the way it did, using the
results of your previous investigations.
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9. Nine students are trying to predict
what will happen when a light beam
encounters a square plastic block.
Some light will be reflected when a
beam encounters a surface, and
sometimes part of the beam will go
through the surface. In the diagrams
some of these beams have been
omitted because they are faint. But not
all of these diagrams can be right,
since they all start the same way, and
light follows a definite path.
According to what you observed
today, which ones are possible?
10. A bottle of water makes a shadow, even though the plastic and the water are
transparent. Explain how this works. A sketch might be useful.
Check #1: Discuss about your findings with an instructor.
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11. Place the triangular prism so that it is standing vertically in the path of the beam
from the light station, so that the beam hits one face (not a corner). There are reflected
beams and refracted beams, and even beams that are both reflected and refracted.
Record your observations.
12. Let’s try to figure out how light behaves in a prism. Here is a close-up
(top view) of your prism. The incident light beam is already drawn in. Follow
the procedure below to figure out what light does in the prism. Useful tip:
Look at one surface at a time, and don’t panic! It may help to rotate the
paper so that the edge you are dealing with is horizontal.
At almost every surface there is both a reflected and a refracted beam - in other words,
the beam splits at every surface, just as you observed with the water cube and the glass
slide. (Take this one step at a time, carefully!)
a) Draw the beam that reflects off of surface A, away from the prism.
b) Draw the (dotted) normal line and then the transmitted beam through surface A, and
extend it to and end it at surface B.
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c) Draw the (dotted) normal line and then the transmitted beam through surface B, and
away from the prism.
d) A beam reflects off of surface B inside the prism - draw this beam and extend it to and
end it at surface C.
e) Draw the (dotted) normal line and then the transmitted beam through surface B, and
away from the prism.
f) There are other reflected and refracted beams that keep going, but they’re pretty dim
and weak, so we won’t draw them.
13. If you aim the laser pointer back in along one of the outgoing beams, it will come
out in several directions. But one of these hits the light station. Explain why.
14. As you turn the prism, one of the beams becomes
colored and then abruptly disappears, while another
becomes brighter. What paths are these beams taking as
they go through the prism?
Check #2: Show your prism diagram to an instructor, and show your instructor that light
going into a prism actually behaves the way you predicted. Then discuss Q13 and Q14.
15. In diagram A, below, show what will happen to a laser beam directed towards a
wedge made of some material. In diagram B, show what happens to three parallel laser
beams.
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16. In diagram B, the light beams cross somewhere (“the focal point”) after going
through the six-sided object. Suppose instead there were a light source at the focal
point, emitting light towards the six-sided object. What would happen to these light beams?
17. If we want every parallel beam to go to the same focal point, we need an object
with a curved surface (a lens). On this diagram, continue the lines to show how the light
beams come together at the focal point.
18. Another way to accomplish this is to use many wedge-shaped pieces (this is called a
Fresnel lens (the word is pronounced “fre-NELL”). Draw lines showing how all the
light beams come together at the focal point.
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19. Lighthouses are put on reefs and islands to warn ships. If an ordinary light source
(that sends light in all directions) is used, most of the light is wasted. Draw lines
showing the light beams.
20. The solution to the light house problem is to use a Fresnel lens to direct most of the
light towards the ships. Assume the light source is placed exactly at the focal point of
the lens, and draw some lines showing how the light is redirected
21. A fish is swimming in a calm clear pond, and a bird is flying over the pond.
Draw some lines that show how the fish can see the bird, and how the bird can see the
fish.
From the point of view of the fish, in what direction does the bird seem to be?
the path that light takes.
From the point of view of the bird, where does the fish seem to be?
that light takes.
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Draw in
Draw in the path
Each group should hand in one copy of this page
Group:
Names of group members present:
1. Veronica has invented a new kind of lighting fixture that has a light bulb in the
center of a square glass container filled with water. The resulting lighting is not very
uniform: some places outside the lighting fixture get lit better than others. Draw in
some light beams that shows where the light goes (please use a ruler to make straight
lines).
2. A laser beam is directed towards a prism, as shown. Three visible light beams
result:
A) light that is reflected from the front surface,
B) light that enters the prism, travels to a different surface of the prism, and leaves it;
C) light that enters the prism, travels to a different surface of the prism where it is
reflected, travels to yet another surface (perhaps the first one, again), and leaves there.
Please draw in the path taken by these three beams (labeling them A, B, C)..
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Check #2
The prism
Lines a and d are the result of reflections. Their directions are set by the “equal angles”
rule. Line b is closer to the perpendicular than the original beam, because light has
entered the new medium. Lines c and e are farther from perpendicular, because the light
is coming out of the medium.
If we change the direction of the incoming beam just a little bit, the outgoing beam c will
cease to exist. Instead, all of beam b will be reflected to make beam d, which means that
beam e gets a lot brighter. When beam c is present, it will have a bit of color effect,
because blue light bends slightly more than red.
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