Experiment 18
THE CONVERGING LENS

PROCEDURE

1.     Open up the Lens Workbook, and fill in the header information.

2.    Set up the equipment as shown in Figure 18-1. Turn on the light source and slide the lens toward or away from the crossed arrow target to focus the image of the target onto the viewing screen. Answer Questions 1 through 3 on the Worksheet.

Fig 18-1: The Converging Lens Set Up

3.     Remove the light source and the crossed arrow target and then move the viewing screen backward or forward to get a sharp image of an object that is "far away". An object that is more than 7.5 meters (about 25 feet) should be sufficiently "far away" like a window or the doorway. The object distance is so large that we may consider it to be infinitely large. This means that the quantity (1/do) is very nearly equal to zero and so, to a very good approximation, di= f. With this arrangement, we can directly evaluate the focal length by measuring the image distance. Record the value in millimeters, in cells D42 and D44 on the Worksheet.

4.    Place the light source and the crossed arrow target on the optical rail again.

Set do to the values (in millimeters) listed below:

do (mm) = 500 , 450 , 400 , 350 , 300 , 250 , 200 , 150 , 100

At each setting, move the viewing screen to get a sharp image and measure and record the value of di in cells B51:B59. Also measure and record the height of the image, hi in cells D51:D59. The object height ho, is the height of the arrow on the crossed arrow target. Record this value in cells C51:C59.

5.    Smoothly slide the object in closer to the lens so that do is just larger than 80 mm. As you do so, take the viewing screen and try to keep the image in focus (this is best accomplished with 2 partners). Put your estimate for the image distance di, that corresponds to this object distance in cell D63.

6.    Move the object to a distance do = 50 mm from the lens. The image in this case is virtual, so the only way it can be viewed is by looking back through the lens toward where the object is located. View through the lens and estimate the magnification factor m, of the image. Record this value in cell D82 of the Worksheet.

7.    Obtain a print out of the Worksheet, and use it to develop your lab report.