Reflection and Refraction
I.
Initial Setup
1.
Obtain the following equipment from your instructor:
1– Pasco Light Box With Power Transformer
1– Pasco Optical Bench
1– Pasco Optics Table With Acrylic Semi-Disc
1– Pasco Mirror and Lens Set
2.
Bolt the Light Box and Optics Table to the Optical Bench with the Silt containing
the Green/Red filter on the Light Box facing the Optics Table as shown below. It
is best to put the Optics Table as close as possible to the Light Box. A separation
of approximately 15-25 cm should be adequate.
Light
Box
RG Filter
Optics
Table
Optical Bench
3.
Adjust the Slit on the Light Box to produce a Single Light Ray from the Center of
the Light Box.
4.
Align the Optics Table such that the Light Ray enters and leaves the Optics Table
at 0° as shown below. This may require you to make fine adjustment of the Slit on
the Light Box, tilt the Light Box, or rotate the Optics Table.
5.
Place the Acrylic Semi-Disc with the flat side perpendicular and facing the light
ray as shown below. The flat side should line up with the 90° line on the Optics
Table. Your light ray should remain straight if your Acrylic Semi-Disc is
perpendicular to the light ray. You may need to do some fine adjustment to get the
disc and light ray properly placed.
II.
Preliminary Investigation (Air to Acrylic)
1.
Rotate the Optics Table so that the Light Ray enters the Acrylic Semi-Disc at an
angle of 30° with respect to the Normal.
Q1:
How many light rays do you see?
D1:
On the drawing add any rays you observe along with their angles. All angles
are measured with respect to the normal.
30°
2.
Rotate the Optics Table so that the Light Ray enters the Acrylic Semi-Disc at an
angle of 45° with respect to the Normal.
Q2:
How many light rays do you see?
D2:
On the drawing add any rays you observe along with their angles. All angles
are measured with respect to the normal.
45°
3.
Rotate the Optics Table so that the Light Ray enters the Acrylic Semi-Disc at an
angle of 60° with respect to the Normal.
Q3:
How many light rays do you see?
D3:
On the drawing add any rays you observe along with their angles. All angles
are measured with respect to the normal.
60°
III.
A.
Information
When a water wave encounters a barrier (Ex. Galveston Sea Wall), it
may be reflected and/or transmitted as shown below.
Incident Wave
Reflected Wave
Transmitted Wave
B.
Our water wave doesn’t actually reside at a point in space but is
spread out. When we describe the motion of the wave, we are actually
describing a surface of constant phase (ex. The crest of the wave). The
velocity arrow in our diagram is perpendicular to our wave crest and
describes the motion of the water wave in actually the same way that a
light ray describes the motion of a light wave.
C.
As a water wave moves from deep water to shallow water, the wave
will change speed. This change in speed may cause the water wave to
deviate from its straight line path. The bending of the water wave is
called refraction.
IV.
First Analysis Of The Investigation
T1:
In your inquiry drawings, label all reflected or transmitted rays that you
observed.
Q1:
In your inquiry, did the light ray refract as it went from air to acrylic?
Q2:
What can you infer about difference or similarities in the propagation of
light through acrylic as compared to air based upon your inquiry results?
Q3:
In your inquiry, did the light ray refract as it went from acrylic back into
air?
T2:
Develop a statement about refraction that explains both your answers to Q1
& Q3?
V.
Soldier Model
1.
Three soldiers are marching at the same speed along dry ground toward a mine
field as shown below.
Mine Field
Soldiers
T1.
Draw the positions of the soldiers at 2 later times but before the soldiers enter the
mine field.
T2.
On your drawing above, draw the incident ray that describes the propagation of
your soldier wave.
T3.
Draw a line normal (perpendicular) to the mine field.
Q1.
What is the angle of incidence of your soldier wave?
2.
When the soldiers reach the mine field, they must slow down to avoid being
blown up. However, each individual soldier still is traveling at the same speed as
their companions.
T4.
Draw the positions of the soldiers at 2 different times after they have entered the
mine field.
T5.
On your drawing above, draw the transmitted ray that describes the propagation
of your soldier wave.
Q2.
Did the soldier ray undergo refraction?
Q3.
Does the soldier wave obey your statement in T2 of part IV? Explain!
3.
In the drawing below, the soldier wave is traveling toward the mine field at an
angle of incidence of 60°.
Mine Field
Normal
T6.
Repeat steps T1 and T2 for our new soldier wave.
Q4.
Will all of the soldiers reach the mine field at the same time?
5.
Repeat steps T4 and T5 to construct the ray for the transmitted soldier wave in
this problem.
Q5.
Did the soldier ray undergo refraction when entering the mine field?
Q6.
Does the soldier wave obey your statement in T2 of part IV? Explain!
Q7.
What happens to the soldier ray when it leaves the mine field? Explain!
Q8.
Did you observe a similar for the light ray leaving the acrylic and returning
into the air?
Q9.
Based upon the Soldier Model, what can you say about the propagation of
light in acrylic as compared to air?
Q10. Is it the mines or the distance between the mines that causes the soldiers to
slow down? Consider the speed of the soldiers walking through a mine field
with lots of mines and little distance between the mines compared to the
soldiers speed for a mine field with very few mines and greater distances
between the mines.
Q11. What inside of mater affects the light in the same way that the mines effect
the soldiers?
Q12. Use your answers to Q10 and Q11 to predict what would you expect about
the speed of light propagation in water as compared to acrylic?
VI.
Second Investigation (Air to Acrylic)
1.
Rotate the Optics Table so that the Light Ray enters the Acrylic Semi-Disc at an
incident angle of 5°.
T1.
Record the angle of reflection and angle of transmission along with the
uncertainties in your measurements in the Data Table below.
INCIDENT ANGLE
REFLECTED ANGLE
TRANSMITTED ANGLE
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
2.
Rotate Optics Table and repeat step T1 at 5° intervals until the entire Data Table
has been filled.
VII. Analysis
Q1:
Looking at your data, what relationship can you draw between the angle of
incidence and the angle of reflection?
Q2:
In your first inquiry, did you see a reflected ray when the angle of incidence
was zero?
T3.
Explain your answer to Q2 based upon the relationship that you developed in
Q1.
Q3.
Observe what happens to the incident and reflected rays as you vary the
angle of incidence for smaller and smaller values near zero. Is your
explanation in T3 consistent with these results?
T4.
Using your data and Excel, make a plot of the angle of transmission as a function
of the angle of incidence. (Staple Graph to Lab Handout)
Q4.
Is the graph linear? If yes, then write the equation describing your graph.
T5.
Using your data and Excel, make a plot of the sine of the angle of transmission as
a function of the sine of the angle of incidence. (Staple Graph to Lab Handout)
Q5.
Is this graph linear? If yes, then write the equation describing your graph.
VIII. Information
1.
Light travels at a very fast but finite speed in a vacuum. This speed is
equal to 3x108 m/s and is denoted by the symbol c. It is one of the
fundamental constants of the universe and information about the
strength of electric and magnetic forces is contained within it. A small
change in this value would have dramatic consequences on the
universe as we know it!!
2.
Although light slows down in matter due to its interaction with
electric charges in the atom, its speed is still a really big number.
Since humans have trouble visualizing large numbers, we choose to
describe the propagation of light in matter by taking a ratio of the
speed of light in a vacuum to its speed in the material. This ratio is
called the index of refraction for the material, n.
n
3.
c
v
Snell’s Law describes the relationship between the angle of incidence
and the angle of transmission (refraction) for a wave passing from
medium 1 to medium 2.
n1 sin θ1   n 2 sin θ 2 
4.
The Law of Reflection describes the relationship between the angle of
incidence and the angle of reflection for a wave striking an interface.
θi  θ r
Ponder The Following:
How does your results compare with the law of Reflection and Snell’s Law?
IX.
Problems
T1.
In the figures below, you are given a group of parallel light rays incident on a
reflecting surface (Mirror). Using the results from your inquiry, a protractor and a
ruler, draw the surface normal and reflected ray for each incident ray.
Q1.
The location where incident parallel light rays are either made to cross or where it
appears that they emerge from is called the focal point. Do any of your mirror
systems have a focal point?
T2.
Adjust the slit on the light box so that multiple light rays are produced.
T3.
Replace the acrylic disk with the three sided mirror and compare the patterns
produced with your drawings.