PHYSICS 125 LAB 6: REFLECTION AND REFRACTION
Goal:
This lab will be an introduction to the reflection of light from plane mirrors and the refraction of
light through a glass slab. It is intended to reinforce the understanding of the law of reflection
and Snell’s Law of refraction.
Background reading:
Review sections 22.2 and 22.3.
Equipment needed:
Corkboard (about 18” square) and a stand to support it (such as the Force Table)
Plane mirror mounted on wooden block
Glass cube (about 5 cm on a side), trapezoidal prism (from Optics Kit), and half-round
Light box configured to produce 3 or 4 rays of light
Pins (push pins and long straight pins or T pins), blank paper, pencil, protractor, and ruler
Theory:
The basic theory needed in this experiment is the law of reflection, incident = reflected and
Snell’s Law of refraction:
nincident sinincident = nrefracted sinrefracted
Experimental Procedure:
Plane mirror
1. Place the corkboard on the stand so that it is easier to view along the surface of the corkboard.
Put a piece of paper on the corkboard. You may want to use pins on the corners of the paper to
hold it in place. Place the mirror on the stand so that its plane is perpendicular to the plane of the
paper, with the mirror near the center of the paper. Using a pencil, draw a line along the mirror
to indicate its location. Lay a long straight pin about 5 cm from the mirror, approximately
parallel to the mirror. This pin will be the object. Mark the locations of the ends of the object
pin on the paper. You may also want to stick colored pushpins into the paper at these two ends
to make them easier to see. The image of the object pin should be visible in the mirror.
2. Stick a reference pin in the board to one side of the object pin, but near the edge of the paper,
as shown in the figure. Now by looking past the reference pin toward the image of the object
pin, place another pin so that it is in a line with the reference pin and the head of the object pin.
Mark the location of these two pins with R and H to indicate “reference” and “head” so that a
line can be drawn to locate the image behind the mirror. Then move the “head” pin over so that
it lines up with the image of the tail of the object pin. Label this point T.
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IMAGE
MIRROR
PIN
OBJECT
H
R
3. Of course, it takes two lines intersecting to define a point location, so you will need to repeat
this procedure, starting with another reference point on the other side of the object pin. Once
you have these locations, you can remove the pins, and use a straight edge (or ruler) and pencil to
draw lines in order to locate the image of the object pin. Measure the length of the object pin
and the image and record in the Lab Report. Then, measure the distance of the object and the
image from the mirror location and record in the Lab Report.
Rotation of a mirror
ROT
ATED
MIRR
OR
MIRROR
C
B
E
D
A
F
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4. Starting with a new sheet of paper, place the mirror on the paper, with the mirror near the
center of the paper, and use a pencil to mark the position of the mirror. Also mark the
approximate center of the mirror. Then stick two pins into the paper to the left of the center, as
shown in the figure, so that they are in line with the center of the mirror, and label their locations
as A and B. Viewing the image of these pins, stick two more pins in line with the image and
label their locations C and D.
5. Now remove pins C and D, but leave pins A and B in place. Rotate the mirror by
approximately 15o and mark its new location with another line. The angle between these lines
will be called . Place new pins E and F so that they line up with the new image of pins A and B
and label the locations E and F.
6. To analyze this situation, remove the mirror and pins from the paper and draw lines defined
by points A and B, by points C and D, and points E and F. The angle between lines CD and EF
will be called . With a protractor, measure the angles  and  and record these in the Lab
Report. Double the value of  and compare this to  by calculating the percent difference
between 2 and . Make a conclusion about the relationship between the angle of rotation of a
mirror and the angle of deflection of the reflected ray.
Refraction in a glass slab
1 A
B
C
R
CUBE
2
C’ B’ A’
C’’
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7. Again, starting with a new sheet of paper, place the glass cube on the paper, near the center of
the paper, and use a pencil to mark the outline of the cube. With a protractor or the corner of a
sheet of paper, draw a line normal to one of the sides of the cube, and place a pin at the
intersection of this line and the face of the cube, labeling this point R as shown in the figure.
Measure an angle of 15o with respect to the line, and place a pin A at about 8 cm from the cube,
as shown.
8. By looking (sighting) through the glass cube from the other side, place a pin A’ close to the
face of the cube, as shown, so that it appears to be in a line with the pins R and A (as seen
through the glass cube).
9. Repeat this procedure with pins B and C that are at angles of 30o and 45o from the normal.
For the 45o angle case, add an additional pin C’’ at a distance from the cube so that the exiting
ray may be drawn.
10. Trace the various rays, measure the angle of incidence 1 and the angle of refraction 2 for
each of the rays, and record in the Lab Report. Use the law of refraction n1 sin(1) = n2 sin(2)
to compute the index of refraction of the glass. Compare the result with the range of values
typically found in glass ( n = 1.5 to 1.7 ).
Refraction through a thick plate with parallel surfaces.
11. Put a sheet of white paper on the table. You might want to tape it down to keep it from
sliding. Plug the power adapter cord into the black light box and then plug the adapter into an
outlet in the center of the bench (near the sink, not under the side of the table). Place the light
box on the paper so that you observe the rays traveling along the surface of the paper. You will
probably need to dim or turn off some of the lights in the room. Now place the trapezoidal piece
of plastic on the paper so that the rays pass through two parallel sides, missing the slanted
section. You should be able to see the rays that pass through the slab, and see the deviation that
you deduced from your experience with the pins and glass cube.
Tilt the slab to get a reasonable deviation and try to trace some of the rays with a ruler and
pencil. This is just for you to see this phenomenon in a different way – no calculation needed.
Total internal reflection – Trapezoidal prism
12. Put a fresh sheet of paper on the table. Position the trapezoid so that you get total internal
reflection from the slanted face. Trace the outline of the trapezoid and several rays to illustrate
this. Ask the instructor to verify that you found total internal reflection. (There are two ways to
do this with these slabs: how are they related?)
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Total internal reflection – Half round
We have a limited number of half-rounds (half a solid circle) which can be used to find the
critical angle for total internal reflection. Get one from the instructor. Use a single ray from the
light box to find the critical angle by shining the ray through the curved side, reflecting it
internally from the flat side, and rotating until the refracted ray is parallel to the flat side. This
gives the critical angle. Use a pencil to trace the outline of the half-round and the rays, and then
use a protractor to measure the critical angle. Calculate the index of refraction from your
measurement of the critical angle, using the formula n = 1.0 / (sin c) and compare with a
typical value for glass (n = 1.5).
incident ray
reflected ray
Total Internal Reflection
incident ray
reflected ray
refracted ray
Partial Internal Reflection
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PHYSICS 125 LAB REPORT:
Name_________________________________
REFLECTION AND REFRACTION
Plane mirror
Length of pin
_____________________
Length of image
dobject
_____________________
_____________________
dimage _____________________
Percent difference between length of pin and length of image ___________________
Percent difference between object distance dobject and image distance dimage _______________
Rotation of mirror
Angle of rotation  _____________
2 _____________
Angle of deflection of ray  _______________
Percent difference between  and 2 ______________
Refraction
incident
refracted
computed n
Ray ARA’
_______________
_______________
_______________
Ray BRB’
_______________
_______________
_______________
Ray CRC’
_______________
_______________
_______________
Show work on back
Average n
Total internal reflection in half-round
Critical angle
c = _______________
Index of refraction
n =
_______________
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_______________