Physics 1 Experiment
Images in a Concave Mirror
Question:
Do ray diagrams really locate images produced by a concave
mirror?
Discussion:
You have learned to use ray diagrams to locate images produced by
concave mirrors, and you have seen images produced by concave
mirrors. In this experiment, you will measure the focal length of a
concave mirror, locate images produced by real concave mirrors and
compare their position to the position predicted by a ray
diagram.
Equipment:
concave mirror
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straightedge
|
clay
|
paper
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support for mirror
|
ticker tape
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meter stick
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tape
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graph paper
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light source
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|
|
Procedure:
IMPORTANT:
* Do not look directly into the sun, or directly at
its reflection in the mirror.
If you use a candle as a light source, tie back long hair,
roll up any loose sleeves on your shirt. Keep your face and hands
away from the area above the candle flame.
A. Find the Focal Length of the Mirror:
- In order to construct a ray diagram for a concave mirror, you
must know the focal length of your mirror. Remember that all light
rays parallel to the axis of the mirror will pass through the
focus. A light source very far away from your mirror will produce
rays that are pretty-much parallel when they strike the mirror.
(If it is a sunny day, the sun provides an ideal light source for
locating the focus of your mirror. Check with your teacher before
trying this, however.)
- To locate the focus of your mirror using a small light source
(such as a candle), mount a piece of ticker tape about a meter
long on your lab table with tape. Use a small piece of clay to
support your mirror vertically near one end of the ticker tape.
Mark and label the mirror position on the ticker tape. Place the
light source as far as is practical - at least a meter or two -
from the mirror.
- Light rays from the source will be concentrated at a single,
bright spot (the focus) in front of your mirror. Use a small piece
of paper or index card to locate this spot. Now measure the focal
length of the mirror - the distance from the mirror to the focus.
Note: It is very important that the focus be located
accurately. Spend a little time, effort, and thought
here.
B. Locating Real Images:
- If you have not done so, mount a piece of ticker tape about a
meter long on your lab table with tape. Use a small piece of clay
to support your mirror vertically near one end of the ticker tape.
Mark and label the mirror position on the ticker tape.
- Place the light source in front of the mirror - farther from
the mirror than one focal length, but no more than 3 focal
lengths. Adjust the height of the light so that the light is near
the axis of the mirror. Use a small piece of paper or index card
to find the image of the light bulb. Mark and number the positions
of both the object and image on the ticker tape. (O1 for Object 1,
I1 for Image 1, etc.)
- Repeat the previous step until you have located at least 5 or
6 images. Try to locate your object light source at a wide range
of distances from the mirror - but outside the focus.
C. Virtual images:
- Place your light source between the mirror and the focus.
Where is the image now? Can you find it in front of the mirror
using your piece of paper or card? (It is not necessary to try to
quantify its location - but in general, where is it?)
Results:
- From your ticker tape, measure each object and image distance
and record these data in a data table. Be sure to label the
columns of the table and indicate units of measurements (which
should be centimeters - not inches).
- For each object/image pair, construct a scale ray diagram to
locate the image.
- Draw a ray diagram to illustrate your findings when the object
was between the mirror and the focus.
Conclusions:
Please write a short paragraph summarizing the results of your
experiment. In particular, the experiment was designed to answer a
question (remember?) - so what's the answer? How confident are you in
your answer? If your results conflict in some way with the "accepted"
(textbook) ideas - why do you think this happened? (Hint: do NOT
(NEVER, NEVER, NEVER!!!) use the terms "human error" or "experimental
error". If you made a mistake (human error), go back and fix it, if
possible. If some aspect of the equipment or technique used in the
experiment was not precise enough to enable you to answer the
question (experimental error) confidently, then specifically what
needs to be changed and how should it be changed.) In other words, I
want you to write a short paragraph, but I expect some thought to go
into it!
last update September 16, 1999 by JL
Stanbrough