Mirror Tracing 5x4c4s

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MIRROR TRACING Experimenters: Two experimenters are required for this lab. One experimenter to record errors and time and one subject. Experimenter and subject then change roles and repeat the experiment. Apparatus: One standard mirror tracing apparatus is required along with a pattern to trace, two pencils, data sheets, and a watch with a sweep second hand or a stop watch. Check out the mirror tracing apparatus from you instructor or construct your own. Look at the picture at the end of these lab instructions to see the basic configuration of a mirror tracing apparatus. Problem: To demonstrate a simple form of motor learning and eye-hand coordination in human subjects. Introduction: The mirror tracing apparatus is a standard psychology laboratory instrument which has been used in a variety of experimental studies. The subject's task is to trace, by looking in a mirror, a pattern which is on the board. A shield prevents the subject from looking directly at his/her hand or the pattern; yet, it does not interfere with hand or arm movements. One of the common patterns which has been used is a star-shaped figure, which is made up of two sets of lines approximately a quarter of an inch apart. The subject has to trace between the lines with a pencil without touching the sides of the star. Errors are tallied by counting the number of times the subject touches the lines. In addition, total time required to trace the complete pattern is recorded. A star pattern is included in this lab package. The pattern can be printed and photocopied. Procedure: The subject should take the pencil in his preferred hand and begin tracing the pattern on the board by looking into the mirror. The subject should start at the point on the star closest to his body and work in a clockwise direction. He/she should attempt to trace the star without touching the lines of the figure. The experimenter should tell the subject when to start tracing and should start timing the trial at that point. The trial is over when the subject returns to the starting point. A new pattern should be placed in the apparatus and a new trial started. Trials can be continued until the subject has apparently learned the task. A criterion for saying that the subject has learned the task is two errorless tracings in succession with each having a relatively low elapsed time. After the first subject has learned the task, the experimenter should take the role of subject and repeat the experiment. Many variations on this experiment are possible, such as, switching to the nonpreferred hand to observe bilateral transfer, massed vs. distributed practice, etc. After all data are collected, the data should be plotted resulting in two learning curves for each subject. Plot errors and time (latency) on the vertical axis and trials on the horizontal axis. Your complete lab package should be turned in including your graphs. One report can be turned in for both experimenters, but both should share equally in the work. Caution: Mirror tracing looks easy at first, but can be very frustrating for some participants. Use sensitivity in doing this experiment and assure participants that difficulty in performing this task does NOT imply that they are deficient in any way.

Mirror Tracing Apparatus Star tracing pattern is on the next page.

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Plane Mirror Reflection

Introduction The plane mirror is perhaps the most familiar of all reflecting surfaces. It usually consists of a flat or planar piece of glass on which a silver coating has been placed that reflects visible light radiation. The motion of this radiation, while wave-like in nature, can be simplified by drawing rays that represent the direction of wave propagation. Through the use of these rays, you can determine general features of reflection, including a simple relation between the angle of incidence and the angle of reflection for radiation that is reflected from a surface. By “angle of angle of incidence or reflection, ” we mean the angle between the respective ray (incident or reflected) and a vector that is normal to the surface of the mirror. For a plane mirror, this normal vector is simply perpendicular to the plane of the mirror. The “law” of reflection states that the angle of incidence i) for an incident ray is equal to the angle of reflection r). This law is illustrated below. Reflection from a Plane Mirror

I

i

r

R

i =  r This law of reflection has some interesting consequences. When an object is placed in front of the mirror, the image will appear to be situated at the same distance behind the mirror as the object is situated in front of the mirror. This image will appear to be situated at the same distance behind the mirror as the object is situated in front of the mirror. The image will be horizontally inverted (left becoming right, and vice versa), and the size of the imaged object will be the same as the actual object. To illustrate these ideas, consider a simple object consisting of only two points, A and B. To see that the object distance (D) is equal to the image distance (D') we first draw the point A, the rays emanating from A, and the rays reflecting from the mirror. Projecting the reflected rays behind the mirror and determining their point of intersection locates the “virtual image” of point A, or A'. (We say an image is “virtual” when it only seems like light is emanating from the image’s position.) We will then see that the distance between A and the mirror is the same as the distance between A' and the mirror. In addition, by adding point B to the diagram, we will note that the distance between A and B in the object space is equal to the distance between A' and B' in the image space.

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A'

A'

B'

A

B

D'

D A

In today’s experiment, you will try to the law of reflection by testing the relationship between incident and reflective angles, as well as measuring object and image distances and sizes. You will position pins to mark points along the imaginary rays of light. The pattern of pin positions will then trace out the paths of the light rays. Equipment: mirrors, styrofoam, pins, paper tape. Procedure 1.

Use masking tape or pins to secure a piece of white paper to the styrofoam board. Use the masking tape to secure the wood block to the paper. The mirror should be placed in the center of the paper.

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Using a straight pin as object A, place it through the paper at approximately 2" from the surface of the mirror and 3" to the left of the center of the mirror. Label the point of insertion as A on the paper.

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Looking horizontally into the mirror, locate the image of A. This will be the point of insertion of the pin located at A. Always line up the points of insertion, where the pins penetrate the paper. Move your eye so that you are a little to the right of A. Place a pin in the path of the incident ray from A. The pin will be placed in such a way that it hides point A. Place two more pins in the path of the reflected ray such that when viewed together into the mirror, all the pins appear in a straight line. You should have a total of 4 pins in the paper. These pins will determine the location and direction of the incident and reflected rays. Remove all the pins except the very first and label the points of insertion to correspond to ray A 1.

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Move your eye a little bit more to the right and repeat step 3, labeling the points as corresponding to ray A 2. Again, you should end up with 4 pins (the original plus the 3 you added) when you are finished. Now remove all 4 pins.

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Insert another pin as object B about 2" from the surface of the mirror and about 2" to the left of the center of the mirror. (This point should be about 1” to the left of insertion point A along a line parallel to the mirror surface.) Label the point of insertion as B.

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Repeat steps 3 and 4 for the new object, finding rays B1 and B2 for object B, this time placing the second pin to the left of point B.

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Make two marks to represent where the ends of the mirror were.

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Remove the block and mirror from the paper.

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Label at least one pair of incident and reflected angles. Analysis

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Use a ruler to construct and extend the reflected part of rays A 1 and A2 behind the mirror until they intersect. Label the point of intersection as A'.

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Repeat for reflected rays B1 and B2, labeling the point of intersection as B'.

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Use a ruler to construct the incident parts of rays A1, A2, B1, and B2, and extend them until they intersect with their respective reflected rays.

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Draw a line through the points where the respective incident and reflected rays intersect. This line is the "effective reflective surface."

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Use a drafting triangle to construct a line normal to the reflecting surface to points A, A', B, and B'. Question 1. Is the distance from A to the surface equal to the distance from A' to the surface? What about B and B'? Report the distances measured in centimeters..

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Use a protractor to determine i and r for all the incident and reflected rays. You should have four incident angles and four reflected angles. Question 2. Are they the same for their respective rays? If they are different, by how many degrees do they differ?

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Use a ruler to determine the distance between A and B, and A' and B'. Question 3. Are the object and image sizes equal? If they are different, how different are they? Express your answer quantitatively.

Conclusion 1. Question 4. Summarize your results for this experiment. 2. Question 5. Looking at your results for this experiment, what would be the image of the palm of a left hand in a plane mirror? 3. Consider the convex mirror, a mirror that is spherically curved like the back of a spoon. The reflection law still applies, but parallel rays striking different parts of the mirror have different incident angles, since the angle is measured from a perpendicular at the point of incidence. Question 6. Draw the case of an image reflected in a convex mirror. Is it larger or smaller than the original? Closer to the mirror or farther away? 4. Question 7. Now consider two plane mirrors placed at right angles to one another. What will the image of the palm of a left hand be in this mirror? You may want to draw a ray diagram to solve this or, you may want to solve this by doing a simple experiment. Error Analysis: Report what you believe are sources of error in this experiment. Why did we have to use the "effective" reflecting surface for the mirror?

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