Physics 221 - Lab 6
Plane Mirrors, Reflection and Refraction
Plane Mirrors - Part I: How Tall?
Prediction
A plane mirror is affixed to the wall. What is the minimum length that the mirror can be for you
to see your entire body from head to toe (just as tall, ¾ as tall, ½ as tall etc…)?
At what height does a mirror of this length need to be mounted to allow you to see your entire
body from head to toe? Why is this?
Procedure
Materials: Plane mirror mounted to the wall, meter stick, dry erase marker.
1. Choose your two most extreme group members, the tallest and the
shortest.
A. Have the shortest member, known as the object, stand in front of
the plane mirror. Another group member will mark on the mirror
with a dry erase marker where the object sees the top of their
head. Then mark where the object sees their feet. Cover the
unused areas of the mirror to verify that they are not needed for
the object to see themselves from head to foot.
B. Now have the object test to see how distance from the mirror affects the necessary size of
mirror. The object should stand very close and very far from the mirror the entire time
checking to see how the original head and feet marks align with their head and feet
respectively.
C. Measure the distance between the head and the feet marks.
D. Repeat procedures A-C for your tallest member.
2. A. Draw a large diagram for each object with the mirror in the center of your paper. Label the
length of the needed mirror, the height of the object, and the exact height off of the floor
that the minimal length mirror must be placed.
C. Draw in rays, with ruler and protractor: 1. going from the object’s feet to the mirror and then
to their eyes; and 2. from the top of the object’s head to the mirror and to their eyes.
Include the normal for each ray.
D. Identify the incident and the reflected rays.
E. Using the results from your two extreme group members, determine what fraction of the
object’s height the mirror has to be to show them from head to toe.
F. Extend the rays that go from the object’s eyes to the mirror behind the mirror to where you
think the image is located.
Plane Mirrors - Part II: Line of Sight
Prediction
If a small mirror is sitting flat in the middle of a table, where do two people have to stand so that
person A can see the person B’s eyes in the mirror?
If person A stands so that they can see person B’s eyes in the mirror, does that mean that B can also
see A’s eyes in the mirror?
Procedure
Materials: very small mirror (no more than 2” square), meter stick, 3 people and a table.
1. A. Place a mirror in the middle of your table. One person in your group will be the “object” and the
other the “observer”. The object should find a position that they can comfortably maintain
without moving their head. Now the observer moves until they find a location from which they
can see the object’s eyes in the mirror.
B. While the object and observer remain in the
positions described in 1A. above, a 3rd person
will create a scale diagram of the two people’s
eye positions relative to the mirror. Use an
entire sheet of paper so that your diagram uses
the majority of the paper. Include both the
horizontal and vertical distance from each
person’s eyes to the mirror.
2. A. The object remains in the same position they were in for part 1 (measure again if necessary to
position the object’s eyes the same as in part 1). Now the observer finds a different, 2nd, position
that allows the observer to see the object’s eyes in the mirror.
B. Clearly mark this 2nd position for the observer in your drawing from part 1. Include both the
horizontal and vertical distance from the observer’s eyes to the mirror.
3. Now the observer will try to find a location where they can see the object’s eyes in the mirror but
the object cannot see the observer’s eyes in the mirror? Describe your findings.
4. A. The object remains in a comfortable position; however, this time with their arms crossed in front
of them. Now the observer should find a position where they can see the object’s hand in the
mirror.
B. A 3rd person will create a NEW scale diagram with the observer’s eyes, the object’s hand and the
mirror. C. Describe what the object sees in the mirror while the observer sees the object’s hand.
Analysis
1. A. Using a ruler draw a straight line from the mirror to the object’s eyes and from the mirror to the
observer’s eyes.
B. Identify the incident and the reflected rays.
C. Find the angle between the normal and each of these rays.
D. What is the relationship between these two angles?
2. Repeat 1. A - D of the analysis for the 2nd position.
3. Repeat for the object’s hand and the observer’s eyes found in part 4. above.
Questions
1. Explain why it is that when the observer can see the object’s eyes in the mirror, the object can also
see the observer’s eyes in the mirror without exception.
2. Consider the case when the observer can see the object’s hand in the mirror. Why is it not the case
that the object can also see the observer’s hand in the mirror?
3. Many people claim mirrors switch left to right? Does the light switch from left to right? Why do you
think this claim exists?
Reflection and Refraction
Materials: For the following series of questions you will be using the
Bending Light PhET simulation.
1. Investigate the simulation and try out all the controls on the
first tab.
2. A. Investigate the relationship between the reflected and the
refracted angles of the laser beam at the water’s surface. Create
a data table and record the incident angle, reflected angle and
the refracted angle. Collect data for the following four incident
angles: 20o, 40o, 60o, and 80o.
B. Verify that when light strikes a surface and reflects that the
angle of incidence = the angle of reflection. 1 = 2
C. Verify Snell’s Law: n1 sin 1 = n2 sin 2, where n1 and n2 are the
indexes of refraction for medium 1 and medium 2 respectively.
Describe your method for verifying Snell’s law and include any
calculations used to support your results.
3. A. Now reverse the materials - have the laser beam originate in
water and shine into the air. Create a data table and record the
incident angle, reflected angle and the refracted angle at 20o, 40o,
60o, and 80o.
B. Do the same relationships hold for the incident and reflected light as you found in 2. above? How
about for the incident and the refracted light?
PASCO Optics Kit
Part I: Reflection
Procedure
Materials: optics bench, light source, ray table and base, component holder, slit plate, slit mask, ray
optics mirror.
You will use the optics equipment to verify that when light strikes a surface and reflects that the angle of
incidence = the angle of reflection. 1 = 2
Setup: Place the light source at the left end of the optics bench. Next place the ray table on the right
end of the bench with the shorter side towards the left. Place the base on top of the ray table. Now
place the component holder between the ray table and light source. Attach the slit plate to the right
side of the holder and the slit mask to the holder's left side. Turn off the room lights and adjust the
components until a single ray of light is aligned with the arrow on the ray table labeled "NORMAL".
Carefully align the flat reflecting surface of the mirror with the line labeled "COMPONENT"; the apex of
the mirror should lie on the normal line to the right.
1. Being careful not to disturb the relative position of the mirror, rotate the table and observe the
incident and reflected rays. Starting with 0 degrees, increase the angle of incidence in steps of 10
degrees up to 90 degrees, and for each angle measure the corresponding angle of reflection. Record
your results in a table.
To within experimental error, do you feel that you've verified the law of reflection (explain)?
Part II: Refraction.
Procedure
Materials: optics bench, light source, ray table and base, component holder, slit plate, slit mask,
semi-circular cylindrical acrylic lens.
You will now use the optics equipment to verify Snell’s Law: n1 sin 1 = n2 sin 2, where n1 and n2 are the
indexes of refraction for medium 1 and medium 2 respectively.
Setup: Use the setup described in Part I above, except replace the mirror with the semi-circular
cylindrical acrylic lens. Again, the components are adjusted so that a single ray of light passes through to
the ray table. Align the flat surface of the cylindrical lens with the "COMPONENT" line and make sure
that the curved surface is on the side of the "COMPONENT" line opposite from the light source.
It is important to center the lens. In this condition, rays hitting the first (left) surface of the lens will
then hit the second surface of the lens perpendicularly and will not undergo a second refraction.
1. Turn off the room lights. Without disturbing the lens, rotate the table and note the refracted ray for
different angles of incidence. Starting with 0 degrees, increase the angle of incidence in steps of 10
degrees up to 90 degrees, and for each angle measure the corresponding angle of refraction. Record
your results in a table.
2. Plot sin 1 versus sin 2. Determine the index of refraction of the acrylic lens from your plot. Include
the plot in your lab report.
3. The index of refraction of acrylic is known to be 1.49. What is your percent error?
4. Do you feel that you have verified Snell's law? Why or why not?
Part III: Total Internal Reflection
There is a “critical angle” for light passing from a medium with a higher index of refraction to a medium
with a lower index of refraction. Light incident at the critical angle is refracted at 90 degrees. At angles
greater than the critical angle, there is no refracted ray and the light is completely reflected this is called
– Total Internal Reflection. At the critical angle c, Snell’s Law becomes: n1 sin c = n2 sin 90.
1. Place the cylindrical lens so that the flat surface of the lens lies along the "COMPONENT" line, but
now the curved surface should be on the side closer to the light source. Then rotate the optics table
to vary the angle of incidence for the light exiting the cylindrical lens into air.
2. Find the critical angle for the cylindrical lens. Then use the critical angle to calculate the index of
refraction for acrylic and compare your value to the accepted value using a percent difference.
Additional questions
1. Why does the light that travels through the acrylic cylindrical lens only bend once?
2. Why is it that to verify Snell’s Law using the PASCO optics equipment, this lab asked you to plot
multiple values of sin 1 versus sin 2 rather than simply having you calculate the index of refraction
from one or two data points?
3.
Do light rays that go from a material with lower index of refraction to one of higher index of
refraction bend toward or away from the normal?
4. Does the law of reflection apply to all surfaces? Explain.
5. What would it mean if a substance had an index of refraction less than one? Note: The index of
refraction of a medium is defined as the ratio of c/v;
n=
=