PHO 101 Photonics Concepts
Three Rivers Community College ONLINE

Home Lab #5

The Thin Lens Equation: Fractions!

Purpose: In this lab you will use the thin lens equation to measure the focal lengths of the two converging lenses in the OSA kit. You will check the focal lengths with the "quick" method, using an object "infinitely" far away.

Materials: small flashlight, two converging lenses from the OSA kit (labeled A and B), small piece of Aluminum foil, clothespins or other supports for components, card or heavy paper for screen, ruler or tape measure with metric (cm) markings

Background: To measure the focal lengths of the lenses, you will use the thin lens equation:          

(equation 1)

                       

 

 

Solving this equation for 1/f (by subtraction 1/do from both sides),

 (equation 2)

 

                                   

 

 

If an object is placed to one side of the lens (do) and the corresponding image distance is found by placing a screen at the position of the sharpest image (di), equation (2) can be used to find the focal length of the lens. Remember that in this equation, the object distance is a negative number.

Procedure:

I. QUICK CHECK OF FOCAL LENGTH (Object very far away)

In a semi-darkened room, stand away from the window and hold lens "A" vertically and focus the light coming through the window onto a sheet of paper also held vertically. Move the paper back and forth (toward and away from the lens) until the image of the world outside becomes clear. The focal length is the distance between the lens and the paper "screen".

Repeat for lens "B".

I. USING THE THIN LENS EQUATION

The thin lens equation method allows you to control the object distance. The set up for this lab is shown below, photographed from above:

 

 

 

 

 

The relative positions of object, lens and image. The distance from the end of the flashlight to the lens is do. The distance from the lens to the focused image on the "screen" is di.

 

1. To prepare the object, cut a small narrow triangle of aluminum foil and tape it to the center of the face of the flashlight. The triangle serves as the object (its orientation is readily observed.) Be sure it is small enough so that light can shine around it on all sides.

 

The object, lens and screen are supported by clip-type clothespins. It is important that the object, lens and screen be at the same height from the table top, as well as in a straight line along the ruler. That is, the optical axis, the line from the center of the object through the center of the mirror to the center of the screen, should be level and straight. (CD jewel cases make good bases for components!)

2. Begin with lens A. (Lens B is much more difficult!) Set the object so that it is 30 cm from the lens. Measure this distance (from the triangle on face of flashlight to center of lens support) as accurately as possible. Locate the position of the image on a screen placed on the other side of the lens. Measure the distance from the lens to the image location as accurately as possible. Record the image distance on the data table (below). Note whether the image is upright or inverted, and in the column labeled "size", whether the image is larger or smaller than the object.

3. Repeat the procedure of step 2 for lens A with the object placed 25 cm and 20 cm from the lens.

4. Replace lens A with lens B. Find the image distances for lens B when the object is placed 15 cm, 10 cm and 5 cm from the lens. Again, record the orientation of the image and whether it is larger or smaller than the object. This lens will take some patience because it has a VERY short focal length. Do the best you can.

 

Calculations

For each of the six measurements, calculate the focal length of the lenses from equation (2). Find the average focal length for lens A and the average for lens B. If any one measurement is very different from the others, you may want to repeat it.

REPORT

Fill out the data/results table completely. Show a sample calculation for finding the focal length, using your data. Answer the following questions:

1. What did you notice about the orientation of the images?

2. Were these images real or virtual?

3. Suppose you have an object and lens aligned so that a clear image is projected onto a screen. If you cover the top half of the lens with a piece of paper, what happens to the image? (This question was answered incorrectly by the majority of college physics students who were surveyed! You can try the experiment and find out for yourself!)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DATA/OBSERVATIONS/RESULTS

PART I: Quick Check

LENS A focal length___________________

LENS B focal length___________________

PART II: Thin lens equation

LENS A Data

object distance

image distance

orientation

size

calculated focal length

-30 cm

 

 

 

 

-25 cm

 

 

 

 

-20 cm

 

 

 

 

average focal length =_____________________

LENS B Data

object distance

image distance

orientation

size

calculated focal length

-15 cm

 

 

 

 

-10 cm

 

 

 

 

-5 cm

 

 

 

 

average focal length =_____________________

 

© J Donnelly 2001