Physics Experiment

Reflection in a Plane Mirror (Part 2)

Parallax Method

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Hypothesis:

An eland drinking at the Namutoni water hole, Republic of South Africa

If an object is placed in front of a plane (flat) mirror, its image will be located the same distance behind the mirror, so that the plane of the mirror is the perpendicular bisector of the line between the object and its image.

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Discussion:

In order to be considered correct, a scientific idea doesn't just have to survive one test, it has to survive many tests. This experiment asks the same question as Experiment 1, but attempts to answer it using a different technique, called the parallax method.

Here's how parallax works. Hold up the index fingers of your two hands so that one is directly in front of the other as you "sight" along them. Notice that if you keep your fingers still and move your head only, your fingers appear to move. This phenomenon is called PARALLAX - apparent relative motion of two objects caused by motion of the observer. You have observed parallax many times. For instance, have you noticed that as you drive down the street, a tree in front of a house appears to move across the front of the house as you drive by? Both the tree and the house were stationary, of course; their apparent motion was caused by your actual motion.

Now notice that as you move your fingers farther apart, the "parallax motion" is larger when you move your head by the same amount, and when you bring your fingers closer together, the apparent motion is smaller when you move your head.

We will make use of the fact that parallax causes apparent motion when the observer moves, to locate the image of a nail in a plane mirror. The idea is that if the image of a nail and a real nail are not in the same place, parallax will cause them to appear to move relative to each other when you move your head. When the real nail and the image are in the same place, however, there will be no parallax - and no apparent motion.

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Equipment:

plane mirror	paper	graph paper
2 large nails	tape
support for mirror	straightedge

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Procedure:

1. Draw a "mirror line" on your paper as you did in experiment 1.
2. Support the mirror so that it will stand vertically. Place the mirror on the paper so that its FRONT EDGE (where the reflection occurs) is on the mirror line.
3. Stand one nail on its head in front of the mirror so that you can see its image (at least the image of half of it) in the mirror. Mark its position "O1".
4. Hold the other nail behind the mirror as you look in the mirror. Notice that you can see the image of the head of a nail in the mirror, and the point of a real nail above the mirror.
5. Now move your head (while holding the nail behind the mirror still). If the nail you see formed by the image and the real nail breaks apart when you move your head, this is the effect of parallax. Adjust the real nail so that the nail formed by the image and the real nail stays together NO MATTER WHERE YOU MOVE YOUR HEAD. At this position the real nail and the image of the nail are at the same place. Label this location of the real nail "I1" (for Image #1).
6. You can get a check on the precision of this technique by taking away the nail behind the mirror (Don't move the object nail!) and locating the image again as in step 4-5 to see how closely you come to the same spot as before.
7. Now that you have located this image, you can move the object nail (in front of the mirror) to a new location. Label this position "O2". Find the image of the nail as in steps 4-6.
8. How many images should you locate? Well, you can't locate too many. Locate at least 10, if you have time. As before, try to place the object at a wide range of distances from the mirror.
9. Draw a line between each object and its corresponding image. Carefully measure the distance (in centimeters) from each object and image to the mirror line. You do not need to measure the angle at the mirror again in this experiment. Again, you may need to estimate ("eyeball") the point that you want to call the image position. Be sure to mark this position on your paper. Record your data in the data table provided.

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Results:

Analyze your data just as you did in experiment 1, that is construct a graph of object distance vs. image distance. Be sure you:

• make as large a graph as you can on the graph paper
• label the axes (with units of measurement)
• draw the BEST SMOOTH CURVE through the data points
• title the graph

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Conclusions:

Write a paragraph summarizing the results of your experiment, just as you did in experiment 1. In particular, the experiment was designed to answer a question (remember?) - so what's the answer? How confident are you in your answer (Go back and look at how closely your 2 image locations are actually one point.) 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 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!