Lenses

 

Part I.  Focal length

 

            Take a converging lens and examine it.  Is it thicker or thinner in the center?  What do objects look like through it?  Could it be used as a magnifying glass?  Try the lens both close to objects and far from them.  Record your observations here.

 

 

 

 

 

 

 

            If a lens is used to form an image of something infinitely far away, the distance from the lens to the image is defined to be the focal length.  For now, infinitely far will have to mean, say, across the street.  Use the lens to form an image of something far away....something outside the building, preferably.  To do this, place the lens in a lens holder on the optical bench and move the screen on the bench until a sharp image is formed on it.  Then read off the focal length using the scale on the optical bench.

 

 

 

 

 

 

 

Focal length = ____________

 

Is the image right side up?__________

 

Is the image magnified?_________

 

 

Part II  Image-object relationships.

 

A.  In this part, use the light box as the object; the lens will form images of this box on the screen.

 

 

 

 

Place the object and the screen at opposite ends of the bench, and move the lens between them until a sharp image is formed.  You should note that there are two positions for the lens which give sharp images.  Record the distance from the lens to the object (do) and the distance from the lens to the image (di).    Measure also the height of the image.

 

Move the object in about 20 cm and repeat the above.  Do this again and fill in the first 3 columns of the table below.

 

do

di

hi

f

hi/ho

di/do

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

B.  The equation relating the image and object distances of a lens is

 

1/do + 1/di = 1/f.

 

Use this equation to calculate f from the 6 sets of data in the table above, and fill in that part of the table. 

 

 

 

 

 

 

 

 

Do you get consistent values for the focal length f? 

 

 

Do these results agree with the focal length you found in the first part of this experiment? 

 

 

Which of the measurements of f do you think is the most accurate?  Which is least reliable?

 

 

 

Finally, fill in the last two columns of the table.  Do you find that the two ratios are equal?

 

 

 

C.  Place the lens in the center of the optical bench.  Now start the object (the light box) at one end of the bench, and place the screen where it shows a sharp image.  Gradually move the object in c loser to the lens, and observe what happens to the image.  In particular, observe what happens when the object is about a focal length away from the lens.   Record your observations.

 

 

 

 

 

 

Can you form an image on the screen when the object is closer than a focal length from the lens?

 

 

 

 

 

Part III  Diverging lenses

 

If time allows, think about how you can find the focal length of a negative (diverging) lens.  Can you use it to produce an image?  What if you used both a negative and a positive lens?  Describe a procedure you could use, and then do use it to find the focal length of a diverging lens.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Questions

 

1.  What is a real image?  Did you observe any in this experiment?

 

 

 

2.  What is a virtual image?  Did you observe any?