Site Map

INTRODUCTION

    In this experiment, you will examine the optical properties of converging and diverging lenses. Your report will be graded on the care you take with your measurements.

I. Images of distant objects.

1. Observations:
   1. Hold a converging lens at arms length, and look at a distant object outside the window. Now, hold this lens very close to your hand or your text. Note the difference in the magnification and orientation of the images.

   2. Place the converging lens in the optical bench and move the plastic screen back and forth until you get a sharply focused (real) image of a distant object through the window.

   3. Look straight at the screen, then slowly slide it up out of the way to see the image without the screen. Sketch the appearance of the distant object in your report.

   4. Use a second converging lens with a shorter focal length to examine the image created by the first lens. Sketch the appearance of a distant object as viewed through your simple telescope.

2. Measurement:
   1. The screen is located at the focal point of the lens when you see a sharp image of a distant object. Measure the distance between the lens and the image, and note the focal length marked on the lens (see notation explanation below). Your measured focal length should agree with the value printed on the lens.

      Lens notation: The lenses you will be using are marked with a series number (the last three digits, e.g. 400 or 800). These series numbers were invented as a way of grouping the lenses in our collection by focal length. The corresponding focal length (in centimeters) is determined by taking the inverse of the series number, and multiplying by a factor of 104. So, for an 800 series lens: f = (1/800) • 104 = 0.00125 • 104 = 12.5 cm. Note that most of our lenses follow this notation scheme, but you may find some that you have to multiply the inverse by 103, or are marked incorrectly. Also note that the series number indicates the diopter strength of the lens by moving the decimal point two places to the left. The focal length (in meters) is the inverse of the diopter strength. For example, an 800 series lens has a strength of 8.00 diopters. Therefore, f = (1/8.00 diopters) = 0.125 m.

   2. Repeat this procedure to measure the focal length of two or three other lenses.

3. Diverging Lens:
   1. Repeat step 1a using a diverging lens. What's the difference? Sketch the appearance of the distant object, and compare it to that seen using the converging lens.

   2. Why can't you repeat step 1b or 1c with the diverging lens?

II. Comparison of image distances determined theoretically and by measurement.

1. Theoretical Location of the Final Image:
   1. Set up a table like the one at the end of these instructions. Initial calculations will be based on a "generic" lens with a focal length of 1.0 units. These calculations will then be applied to the lens you will measure on the optical bench. Quantities with a prime ( ' ) denote distances calculated for a generic lens; those without a prime denote distances calculated and measured using a real lens.

      Calculate i' for each o' value using the Thin Lens equation:

2. Measurement of the Final Image:
   1. You will now calculate the expected distances from the theoretical distances just calculated. Using the measured focal length of the 800-series lens, calculate the lens-object distance (o = o'f ) and the expected lens-image distance (i = i'f ) for each generic object distance given in your table.

   2. Now put the lens in the optical bench and measure the lens-image distance (i) for each case. Your instructor will show you how to use the virtual image detector to locate the virtual images.

   3. Find the % difference between the expected and measured values for i.

3. Calculation of focal length:
   1. Use Kaleidagraph to plot ¹/_o (the actual object distance on the optical bench) vs. ¹/_i (the measured image distance). Calculate the focal length of the lens from the parameters of the best-fit line. How does your measured value compare to that found in step 2a?

			f = __________ cm
Generic object distance, o'   (units)	Generic image distance, i'   (units)		Actual object distance, o = o'f   (cm)	Expected image distance, i = i'f   (cm)	Measured image distance, i     (cm)	% difference
1.4
2.0
3.0
4.0
0.5
0.7

Return to Physics 152

©	St. Lawrence University	Department of Physics

Revised: 25 Jun 2003

Canton, NY 13617