Physics Experiment

Images in a Plane Mirror - Ray Tracing Method

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Question:

Elephants at night - at Kwa Maritane water hole, Republic of South Africa
An experiment is a carefully-planned set of observations designed to answer a question or test a hypothesis. For this experiment, the question is:

"If an object is placed in front of a plane (flat) mirror, where will the image of the object be located, and what does its location say about the way light reflects?"


Discussion:

When you look in a mirror, you see your image "in the mirror" - but where, precisely, is the image located? It may not have occurred to you that an image even has a location, but it must. In this experiment, you will attempt to locate it. Its location will probably surprise you!

Try this:

Using Line-of-Sight DiagramPick some reasonably distant object (across the room is far enough) and look directly at it. Now hold up a straightedge (the edge of a book will do) so that the straightedge is in line with the object and your eye. Notice that you may now move your head (leaving the straightedge still) and the straightedge still determines a line pointing directly at the distant object. In fact, this line is the path that the light ray must take to get from the distant object to your eye. If you move to some other location and repeat this procedure, you would establish a second line pointing directly at the distant object - and if the object is located on both lines, it must be located at the intersection of the 2 lines that you have created.

This is the method (called ray tracing) that you will use in this experiment to locate an image in a plane mirror. Instead of a distant object, you will be looking at the image of a pin in a mirror, and your "rays" will point at the image of the pin, not the pin itself.

Equipment:

plane mirror

cardboard

support for mirror

pin

tape

protractor

straightedge

paper

graph paper
Procedure:

  1. Place a blank piece of paper on the cardboard, and draw a straight line across the middle of the paper. Label this line "Mirror".
  2. Support the mirror so that it will stand vertically using a block and a rubber band. Place the mirror on the paper so that its FRONT EDGE (where the reflection occurs) is on the mirror line.
  3. Stick the pin into the paper in front of the mirror so that you can see its image in the mirror. Label this point "O1" (for object #1).
  4. Sight along the straightedge at the IMAGE of the object pin in the mirror. Draw a line along the straightedge to record its position and label it "I1" (for Image #1).

  5. DO NOT MOVE THE OBJECT PIN OR THE MIRROR! - Move your eye to another position in front of the mirror. Repeat step 4 to establish a second line pointing directly at the image. Be sure to label this line "I1" also.
  6. The two lines that you have established should intersect at the location of the image - but how precisely have we located this image? A physicist is always trying to verify her measurements and establish their precision. One easy way to see how accurate your measurements are is to establish a third line as a check. Go back to step 4 again to establish this third line.
  7. Now that this image has been located by three lines, you may move the object pin to another location. Label this position "O2". Now repeat steps 4 - 6 to locate the image of this pin. Label the image pins "I2". Be sure that you do not move the mirror from the mirror line!
  8. After locating 2 or 3 objects, your paper will probably be pretty-well covered with lines and labels. Remove this paper, redraw the mirror line on a clean sheet, and continue. How many images should you locate? Well, you can't locate too many. Locate at least 10, if you have time. Try to locate your objects at a wide range of distances from the mirror - some fairly close, and some as far away as convenient on your paper.
  9. Once the paper is removed from the cardboard and the mirror is out of the way, use a straightedge to carefully extend the lines until they intersect to locate the images. You may very well be surprised at the location of the image!
  10. 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, and the angle between the mirror line and the line connecting the image and object. In most cases, your 3 lines will not intersect in exactly the same place - in this case you will 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.

What to Measure Diagram

Results:

You probably see a relationship between object and image distances from your data table, but scientists have found that it is usually far more instructive to view their results in the form of a graph rather than tables of numbers.

  1. On the graph paper, the two axes have been drawn for you. Notice that the origin of the graph is near the lower-left corner of the page. Notice that the horizontal (across) axis is labeled "Object Distance", and the vertical (up and down) axis is labeled "Image Distance". It is conventional to place the quantity that you change (the object distance) on the horizontal axis, and the quantity that nature changed (the image distance) on the vertical axis. The scale for the axes is chosen so that you have the largest convenient graph on the page. This means that you want the graph to be as large as possible without resorting to some strange scaling (like 7.358 cm per square!). On this graph, each dark line represents 2.0 cm, and each small square represents 0.2 cm.
  2. Plot the data points on the graph. Now, draw the BEST SMOOTH CURVE through the data points (for this experiment, the best smooth curve is probably a straight line). DO NOT PLAY "CONNECT THE DOTS"!
  3. The graph has been titled "Object Distance vs. Image Distance for a Plane Mirror". All graphs should have a title.

Conclusions:

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? (Go back and look at how closely your 3 lines intersect at 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!

Next PageOptics IntroPhysics 1 at BHS[Help]
last update October 11, 2001 by JL Stanbrough