PHYSICS 125 LAB 7: SPHERICAL MIRRORS
Goal: This lab will investigate image formation from spherical mirrors.
Background reading: Review section 23.2. You may want to draw the ray diagrams requested
in this Lab Report before coming to class.
Equipment needed:
Optics Kit with cylindrical concave and convex mirrors
Light box configured to produce 3 or 4 rays of light
PASCO linear optical bench with “object” source (light box and wall plug)
White viewing screen, mounted half-screen, mounted dual-surface (convex-concave) mirrors
White index cards (3”x5” and/or 4”x6”), paper, protractor, 4 rulers, flashlight, and table lamp
Experimental Procedure:
Mirrors – ray tracing using light box and cylindrical mirrors
There is an odd-shaped triangular piece which is actually three mirrors in one piece. Use the
light box to shine rays on the concave mirror and attempt to trace the rays. Determine the focal
point and the focal length of the concave mirror. Then repeat with the convex mirror. Just to
recall what you saw last week, look at the way that rays reflect from the plane mirror.
Concave Mirror
Construct the ray diagrams for the various cases shown in the first part of the Lab Report. In the
first case, notice how an object at the focus causes the outgoing rays to be parallel, hence the
image is at “infinity” since they don’t converge for a finite distance. The reverse of this situation
is for an object at infinity, and the rays converge at the focus. To see a demonstration of this,
take the concave spherical mirror (on the black plastic mount) over to the uncovered window in
the lab and notice the tall dormitory at a fairly large distance (compared to distances in the lab).
Holding the concave mirror facing the dormitory in one hand, move an index card in your other
hand to find the real image of the distant object. With the help of a lab partner and a ruler,
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determine the image distance for this image. Using the spherical mirror formula, calculate the
expected image distance for an object located at + infinity, and thereby explain the image you
see. Allow each lab partner to measure the image distance f for this concave mirror, average the
values, and record the results in the Lab Report.
Concave Mirror with do > R
Review the ray diagram for this case. Set up the situation on the optical bench, with the “object
source” at one end of the bench. The wall transformer should be plugged into the outlets on the
sides of the bench, after connecting the power cord to the “object source”. The concave mirror
should be located at a distance of do = (2f plus several centimeters) from the object. Notice that
it is easier to move the mirror than the source, which should remain near the end of the bench.
You can measure the object distance using the scale built in to the bench. The mirror holder has
a small pointer that indicates the location of the mirror’s vertex. It may be more convenient to
hold the mirror by one hand and use an index card in the other hand to locate the image. For this
case and the subsequent cases, record your results in the data table.
Concave Mirror with do = R
In this case, it may be necessary to hold the mirror about 3 cm above its usual position, and hold
an index card directly above the object while moving the mirror by hand to find the image.
Concave Mirror with f < do < R
In this case, use the same tactic as in the previous case, but now the index card will be held
above and somewhat behind the object. The mirror will have to be held above the bench.
Concave Mirror with do < f
The image is virtual and magnified and you will have to look into the mirror to see it.
Convex Mirror
Experimentally verify that the image in a convex mirror is a virtual image. Notice that the image
is always behind the mirror and you can’t get a real image with the index card.
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PHYSICS 125 LAB REPORT:
Name___________________________________
SPHERICAL MIRRORS
Concave spherical mirror: ray diagrams
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Concave spherical mirror
Radius of curvature of spherical mirror
R = _______________
Calculation of di for object at infinity:
Experimental focal length
______________
______________
______________
Average
______________
Data Table 1
Determine the image distance and magnification for four cases:
Experimental values:
do
di
M
do > R
do = R
f < do < R
do < f
Calculations (show work)
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Calculated values:
Percent
di
difference
Type of
image
Convex spherical mirror: ray diagrams
Conclusion about image in convex mirror:
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f
C
f
C
f
C
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