Spring 2005                                                                                          Name____________________

                                                                             

                                                         Physical Science Experiment

                                              Lenses: Making a telescope and microscope

 

Before coming to class answer the first 6 questions.  Note that some of these will be on the second page.

 

Objective:  To build a simple telescope and microscope and measure their magnifying power.

Definition:  Focal Distance – this is a measure of the “power” of a lens.  The focal point is that point where light from a very distance object is focused.  An easy way to measure the focal length of a lens is to focus an object that is far away on a piece of paper, then measure the distance from the paper to the lens. An example of a short and long focal length lens are shown in Figure 1.  (a) is the shorter focal length lens, (b) the longer.

                                                      Figure 1

 

 

 


                 (a)                                                       (b)

            To build both the telescope and microscope you will need two lenses, one with a short focal length, the other with a longer focal length.   To make a simple telescope the two lenses are separated by a distance equal to the sum of their focal lengths.  In the figure below, the distance “d” should be this sum.  In the case of the telescope, the lens closest to the eye is the short focal distance (the “fatter” lens), in the case of the microscope, the “thin” lens is closer  to  the eye.

eye

 

(a)

 
                                                 Figure 2 (telescope)

d

 

(b)

 
                                                                                                   

 


      1.  What is meant by the focal length of a lens?

 

 

2.  In figure 1, does lens (a) have a: □ longer or □ shorter focal length than lens (b)?

 

3.  How can we measure the focal length of a lens?

4.  How far apart should the lenses of a telescope be?

 

5. In the microscope which lens should be the closest to your eye?  □ fatter, □ thinner

 

6.  How do you turn your telescope into a microscope?

 

 

Procedure:

CAUTION: ALWAYS HANDLE LENSES BY THEIR EDGES, DO NOT PLACE YOUR FINGERS ON THEM AND DO NOT DROP THEM.

     Choose one fat lens and one thin lens.  Take them in lens holders on a meter stick to a blank paper on the south wall of the classroom and focus the image on the paper.  You must take care to stand to the side so the light from outdoors can reach the lens.  Measure the distance from the lens to the paper for each of the lenses.  Write the focal distances f1 = _____________  f2 = ________

     Is the picture you see right side up?

 

     If the sun is shining take your lenses, meter stick and paper outdoors.  Focus the sun on the paper and again measure the focal distances. f1 = _____________  f2 = ____________  If these are not approximately the same as you measured in the first step of the experiment, speak with the instructor.

  Return to the lab.  Each person in your group is to take each lens and hold it in front of this paper.  Move the lens closer to you eye, then closer to the paper describe what you see and tell the difference between the two lenses.

Fat lens:

 

Thin lens:

 

     Put the two lenses on the meter stick so that they make a telescope.  If you are not sure how to do this speak with the instructor.  Go to the window and focus your telescope on the most distant objects you can see.  Describe the image.  Is it equally clear everywhere?  Is it right side up?

Now place one of the “feet” on your meter stick that will enable it to stand by itself.  Place it on the teacher’s desk at the front of the room so that one of the papers on the back blackboard is in focus.  Have one of your partners hold a meter stick close to the farthest lens from your eye.  With one eye look through the telescope, with the other look at the meter stick.  After some practice (this is not easy for many people) you should be able to see the magnified picture of the paper on top of the meter stick.  Write the number of centimeters the paper takes up on the meter stick  telescope image = ___________________ .

Now stand straight up and look at the paper without looking through the telescope and hold the meter stick the same distance from your eye it was on the above measurement.  Write down the size of the image as seen with the naked eye ________________

To determine the magnification, divide the size of the magnified image by that seen with the naked eye.

       Magnification = _______________ / ________________ = ______________

Return to your lab table and place the thick lens 5 cm from the end of the meter stick and place this against the lab table below one of the pieces of graph paper.  Move the thin lens away from the thick lens until you see a greatly magnified image.  Have the instructor check to be certain that you are seeing the correct thing.  Place a meter stick beside the graph paper and estimate how big the smallest squares are.  Magnified size of smallest squares = ______________________

Measure the real size of the squares with the meter stick.  Real size = ____________________

Magnification = magnified image/real size = _______________ / _____________ = __________

As you look through the microscope, move the fat lens that is close to the graph paper back and forth, closer then farther from the paper.  Describe below what you see.  Each person in your group should do this independently and write your own answer below.