Physics 1 Experiment

Images in a Concave Mirror

Question:

Do ray diagrams really locate images produced by a concave mirror?

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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.

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Equipment:

concave mirror	straightedge	clay
paper	support for mirror	ticker tape
meter stick	tape	graph paper
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:

1. 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.)
2. 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.
3. 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:

1. 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.
2. 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.)
3. 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:

1. 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?)

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Results:

1. 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).
2. For each object/image pair, construct a scale ray diagram to locate the image.
3. Draw a ray diagram to illustrate your findings when the object was between the mirror and the focus.

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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!