PURPOSE:
To expose the student to new equipment and lab procedures. And to determine the focal length of a thin convex lens.

INTRODUCTION:
The focal length of a thin lens can be determined if the distance from the center of the lens to an object AND the distance from the center of the lens 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 lens to object and d_i = distance from lens to image. The d_i is determined when the image is seen clearly focused on a viewing screen and the distance from the lens to the screen is measured. If the object (normally a light bulb), lens and viewing screen are mounted on a meter stick, distances can be determined easily. Depending on the distance from object to lens, 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.

Three sliding devices will be used in this lab to determine the focal length of a lens. From left to right, 1) light source which will be our object, 2) thin lens and 3) viewing screen that will show the projected image. Since the screen is mounted facing to the left, 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 light slider over to the 10 cm mark on the meter stick. Drag the lens slider over to the 50 cm mark. The distance from the light (object) to the lens is now 40 cm. Turn on the light by clicking on the red button at its base.

4) Slowly drag the screen to the left until an inverted image of the light appears on the screen. Note: since the screen faces left, a "face on" viewing screen is displayed above. Record the distance from the lens to the screen under the 40 cm listing.

5) Relocate the light to the 15 cm mark and adjust the screen until an image is visible. Record the distance from lens to screen. Repeat this same procedure placing the light at 20 cm and 25 cm, recording each time.

6) Place the light on the 10 cm mark. Move the screen until the image appears. Move the light to the 20 cm mark and adjust the screen once again. What happened to the size of the image as the light moved closer to the lens? Record your answer under observation #1.

7) Using the equation above, calculate the focal length of the lens at the point where the light sits on the 20 cm mark.

8) Place the light so that it is twice the focal length from the lens. Focus the image on the screen. What is the relationship of the distance now from the lens to the image as compared to the focal length? Record your answer under observation #2. Click on the "Show ray diagram" button. Where does the light ray coming from the top of the lens and the light ray passing through the center of the lens meet?

9) Move the light to the left 5 cm and refocus the image. Click on the ray diagram button. Where do these two light rays meet?

10) Move the light so that it is the focal length from the lens. Focus the image on the screen. What did you observe? Record your answer under observation #3. Click on the ray diagram. What do you notice about the two light rays? Record your answer under observation #4. If you rearrange the focal length equation to solve for d_i it becomes:

1/d_i = 1/d_o - 1/f

What happens in this equation if f and d_o are equal? Record your answer under observation #5.

11) 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.