PURPOSE:
To expose the student to new equipment and lab procedures. And to determine the focal length of a concave mirror.

INTRODUCTION:
The focal length of a concave mirror can be determined if the distance from the center of the mirror to an object AND the distance from the center of the mirror to a projected image is known. The equation used is:

1/f = 1/d_o + 1/d_i

where f = focal length, d_o = distance from mirror to object and d_i = distance from mirror to image. The d_i is determined when the image is seen clearly focused on a viewing screen and the distance from the mirror to the screen is measured. If the object (normally a light bulb), mirror and viewing screen are mounted on 2 meter sticks, distances can be determined easily. Depending on the distance from object to mirror, the image will have different projected sizes based on the magnification of the projection. This magnification can be determined by the equation:

m = - d_i / d_o

Upright projected images will have a positive sign of magnification where inverted images will have a negative sign. The center of curvature can also be determined by the equation:

C = 2f

where C = center of curvature and f = focal length of the mirror. C represents the radius of a circle that fits the curvature of our mirror.

Three sliding devices will be used in this lab to determine the focal length of a concave mirror. From left to right, 1) viewing screen that will show the projected image, 2) light source which will be our object, and 3) the concave mirror. Since the screen is mounted facing to the right, a "face on" diagram will be shown at the top of the lab window.

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If you get into trouble and perform some procedure that causes the lab to fail(lab equipment will no longer operate), you can press the "Reset" button and the simulation will return to the starting position.

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PROCEDURE:

1) You can adjust the background shading by clicking on the "Special" button to the right and selecting "Background". Click on the "Special" button and select "Print Blank Report" to obtain a web page that can be printed and used as a lab report. (the program will not be interrupted)

2) Click on the "Special" button and adjust the background until it is black. Viewing of this lab is best done in the dark.

3) Drag the mirror slider over to the 80 cm mark on the meter stick. Drag the light slider over to the 30 cm mark. The distance from the light (object) to the lens is now 50 cm. Turn on the light by clicking on the red button at its base.

4) Slowly drag the screen to the right until an inverted image of the light appears on the screen. Note: since the screen faces right, a "face on" viewing screen is displayed above. Record the distance from the mirror to the screen under the 50 cm listing.

5) Relocate the light to the 40 cm mark and adjust the screen until an image is visible. Record the distance from mirror to screen. Repeat this same procedure placing the light at 50 cm and 60 cm, recording each time. Note: certain focal lengths may not permit you to take the 20 cm distance reading(light at 60 cm). If this is true for your lab, record "not found" on your data sheet for the 20 cm entry.

6) Place the light on the 30 cm mark. Move the screen until the image appears. Move the light to the 50 cm mark and adjust the screen once again. What happened to the size of the image as the light moved closer to the mirror? Record your answer under observation #1.

7) Adjust the light and screen to a point where they both have the same distance from the mirror and the image is focused. Record this distance. Compare the size of the object(light) and image(screen). Record your observation under observation #2.

8) Using the equation above, calculate the focal length for your first four d_i readings. How does the average of the four focal lengths compare to the distance observed in procedure #7? Record your answer under observation #3.

9) Calculate the requested values asked for on the lab sheet and any given by your teacher. For help on these values click on the "Special" button and select "View Data & Hints". Select "File Report" to send a copy to be viewed by your teacher.