Name (printed) _______________________________
ACTIVITY
A FIRST LOOK AT REFRACTION
OBJECTIVE
To observe optical refraction and theorize the mechanism for
the observation.
The refracted
image of the stem
shown in this
photograph (taken
by Lindsay Yellen,
Class of 2008) is in
a remarkably
different position
than the actual
stem. How is this
similar to what you
see in this activity?
PROCEDURE
1.
Start with no water in the cup and orient your line of
sight so that the penny is just out of view.
2.
While maintaining your position, slowly pour water
into the cup. You should notice the penny come into
view.
QUESTIONS
1.
What changes did you notice in the image of the
penny after the water was added?
2.
In the space below, draw the path you believe the light took from the penny to your eye.
After adding water
Before adding water
1
LAB
OPTICAL REFRACTION (PART 1)
INTRODUCTION
As it turns out, there is a very predictable pattern
that light follows when it moves from transparent
substance to transparent substance. You know a bit
about the bending from the diagrams on the previous
page. Your job in this lab is to try to figure out the
specific pattern – that is to figure out what direction a
ray of light will take as it enters a transparent
substance if it approaches at a specific angle. To do
that you’ll begin by mapping out the paths that light
takes as it passes from air to water at various angles
of incidence. A pattern will begin to emerge. Look
carefully for this pattern.
Normal
In Figure 1 below, three pins map out the path
that light takes as it passes through the air and then
into water (held in a semi-circular plastic dish).
Notice that the light does not follow a straight path.
(The light does not bend after moving from the water
to the air because it hits this boundary at 90°.) In
Figure 2 Amelia illustrates how to find this path. She
places two pins in front of the plastic tray to mark the
path of the incident ray. Then she looks through the
rear, semi-circular part of the tray and places the third
pin so that it lines up with the first two pins when
they are viewed beneath the surface of the water (see
Figure 3).
Angle of
incidence
Figure 2: She places the third pin so that it lines up
with the first two pins when viewed beneath the
surface of the water.
Angle of
refraction
Figure 1: Top view of set up
Figure 3: Note all three pins are aligned when viewed along a path
under the surface of the water.
2
PURPOSE
•
To qualitatively observe optical refraction.
•
To quantitatively measure optical refraction.
•
To discover patterns of refractive behavior unique to a particular transparent substance.
PROCEDURE
1.
Fill the plastic container half full with water and put a piece of cardboard under the graph paper on the
following page.
2.
Place the container on the graph paper so that the straight edge is along the horizontal line at the middle of the
page and centered. The curved surface should be facing you. Place a pin at the center point of the page. It is
very important to check this placement frequently throughout the lab (Figure 1).
3.
Place a second pin on the first incident ray a few centimeters from the container’s flat side.
4.
Looking through the water in the box from the curved side, turn the cardboard until the two pins are in line.
5.
Place a third pin on this line of sight. When you look through the water (be certain your line of sight is
under the surface of the water), the three pins will appear to be in a straight line (Figure 3).
6.
Use the placement of the third pin to measure the refracted angle. Measure and record this to the nearest 0.1°.
7.
Repeat this for all incident rays.
8.
Make calculations necessary to complete the data table. For all calculations, record to two places past the
decimal.
DATA
Record all data and calculations to three significant figures
Angle
number
Incident
angle, Ði
1
20.0°
2
25.0°
3
30.0°
4
35.0°
5
40.0°
6
45.0°
7
50.0°
Refracted
angle, Ðr
(Ði)
2
Ði ×Ðr
Ðr
3
sinÐi
sin Ðr
cosÐi × ( tanÐr )
2
tanÐi
cosÐr
4
5
QUESTIONS/CALCULATIONS
1.
Which column seems to have the most interesting pattern? What is that pattern?
2.
If you could do this lab with NO experimental error, what would you expect to see in the column that has
the most interesting pattern? Be specific.
3.
If you did this lab with another transparent substance, instead of water, what would you expect to see in the
column that has the most interesting pattern? Be specific. (Hint: What would be the same and what would
be different?
GET CHECKED BEFORE MOVING ON
OPTICAL REFRACTION (PART 2)
PROCEDURE
1.
Get a new plastic container from me and add Wesson oil to it.
2.
Use the technique you just learned in order to determine the quantitative degree of refraction for Wesson
oil. Use a procedure similar to the one you used in Part 1. However, you can choose however many
measurements you think are necessary. Consider your responses above when deciding which calculations
should be done.
3.
Make a neat, well-labeled table for your data in the space below.
4.
Make and show your calculation for the number that represents the way light refracts in Wesson oil.
5.
Return the oil to the Wesson oil container.
6
GET CHECKED BEFORE MOVING ON
ACTIVITY
OPTICAL REFRACTION
The photo to the right (taken by Victoria Palmer, Class of 2011) shows a glass
with five different liquids layered on top of each other (alcohol, oil, water, dish soap,
and honey). Their different indices of refraction cause the inserted stick to appear
broken. You should be able to show this phenomenon using Snell’s Law in the
diagram below. Use a protractor to measure the incident angle shown below. Then
calculate the path the light would take as it passes through each of the following
layers of material and then back out into the air. Show the path carefully, label all
angles, and show all of your calculations to the left of the diagram.
Important: Don't round numbers to be used in further calculations
Normal
Calculations
i
Air
Diamond
Water
Flint glass
Ethyl alcohol
Substance
Air
Water
Ethanol
Fused quartz
Crown glass
Flint glass
Diamond
Index of
Refraction
1.00
1.33
1.36
1.46
1.52
1.63
2.417
Air
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GET CHECKED BEFORE MOVING ON
REFRACTION
QUESTIONS AND PROBLEMS
1.
The refracted light leaving this vase produces two images of the
spoon inside. Assume that in the line drawing that you are
looking from the top of the vase down. Refract the rays drawn in
the diagram to show how the two images are produced. (You
don’t need to do calculations here. Simply bend the refracted
rays in the proper direction toward or away from the normal
shown.) Trace the refracted rays back into the vase as dashed
lines in order to show the two images produced.
Top view
2.
In the Refracted Images demonstration, a volunteer viewed an object through a set of prism-shaped glasses.
The volunteer wasn’t able to grab the object because, although he believed he knew where the object was,
he was actually looking at a refracted image of the object. In the diagram below, there is an object, a ray of
light coming from the object and one of the prism-shaped eyepieces. Show the path the light takes as it
moves through the prism and then into the eye and then show how that leads to the formation of the
refracted image.
Object
Light Ray
8
prism
GET CHECKED BEFORE MOVING ON
For credit, you must provide a clear and complete explanation for each of the remaining questions.
3. _____ A swimmer (in a lake whose surface is perfectly calm) looks upward through the water at the setting sun.
His view of the sun will make it appear...
a. lower than it really is
c. the same as viewed above water
b. higher than it really is
d. smaller than it really is
Provide a drawing for your explanation
4. _____ If you wish to spear a fish with a regular spear, you should compensate for refraction between the air and
water and throw your spear
a. directly at the sighted fish.
b. above the sighted fish.
c. below the sighted fish.
Provide a drawing for your explanation
5. _____ The figure below shows the path of a portion of a ray
of light as it passes through three different materials.
The figure is drawn to scale. What can be concluded
concerning the refractive indices of these three
materials?
a. n1 < n2 < n3
c. n3 < n1 < n2
b. n3 < n2 < n1
d. n2 < n1 < n3
n1
n2
n3
6. _____ A ray of light, initially traveling in a vacuum, is incident on the surface
of a flat transparent material as shown in the diagram below. Part of
the light is reflected at the surface and part is refracted. The angle
between the reflected ray and the refracted ray is
a. less than 40°
d. between 100° and 140°
b. between 40° and 50°
e. more than 140°
c. between 50° and 100°
7. _____ The speed of light in crown glass is 2 x 108 m/s. The index of refraction of crown glass is:
a. 1.0
b. 1.5
c. 2.0
d. 3.0
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8. _____ The speed of light in a transparent gel is 2.20 x 10 m/s. If the angle of incidence of a beam of light
directed at the surface of the gel is 30° what is the angle of refraction?
a. 1.36°
b. 21.5°
c. 40.9°
d. 47.2°
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