July 14, 1996

PRELAB

Everyone has seen images formed by lenses and mirrors or objects behaving as lenses (e.g., a drop of water) or as mirrors (e.g., the front or back of a spoon). Any image has 4 properties that characterizes it. These image characteristics depend both on the properties of the lens or mirror and also where the imaged object is located relative to the lens/mirror. Identify these image characteristics (and understand what they mean) before coming to lab tomorrow.

APPARATUS

The apparatus that we will use consists of a fairly large cardboard box that is open on one side. On one small side of the box is a pinhole punched in a square of aluminum foil. On the opposite side of the box (and inside of it) is a pice of white cardboard, hereafter called the "screen." We will use this box as pictured in the following diagram.

PROCEDURE

1) Before actually holding up the box toward the ceiling (as shown in the above diagram), describe what you expect to see (if anything) on the screen when you do hold the box in place right under the ceiling fluorescent lights.

2) Now hold the box as suggested in the diagram. Depending on the size of your box, you may need to adjust the distance of the box from the light, how much stray light is allowed to enter the box, etc. How does what you see on the screen compare to what you expected?

3) Draw a full-page diagram that accounts for what you see on the screen.

(check with instructor at this point)

4) Does the image on the screen have the same orientation as the lights themselves? Come

up with a convincing way to decide; describe your method.

The next goal is to investigate what determines the 4 image characteristics.

5) a) What determines the size of the image? What can you do that will result in an

enlarged image? Brainstorm first before trying anything experimentally.

b) Draw a diagram (for each method) that clearly shows why the image became larger.

c) Use you diagram to determine the mathematical dependence of the image size on the

parameter(s) that you changed.

6) a) We now explore what determines the brightness of the image. You should be able to

come up with at least 3 things that can be changed to make the image brighter...

describe these changes and then test your hypotheses. (Actually, at this point, there

is one hypothesis I don't want you to test... if you don't know which one it is, please

ask before you start the tests.)

b) Explain why each of these 3 things (described above) results in a brighter image.

c) Can you determine the mathematical dependence of the image brightness on the

parameter(s) that you changed?

7) How would produce 2 images of the ceiling lights? 3 images?

8) a) The last image property to be investigated is the clarity of the image. What can you do

to change the clarity of the image? (hint: you should know what to do to make the

image disappear altogether... the ultimate in an unclear image!)

b) Draw a diagram that shows why what you did produced an unclear image.

9) a) Determine accurately the ratio of the diameter of the sun to the earth-sun distance

with your pinhole camera. Start with a diagram that shows what you are going to

measure and how you are going to determine the ratio from your measurements.

b) Compare your experimental value with the published value. "Compare" always means

find the % difference.

c) What would you estimate the uncertainty in your two measurements in part 8a?

What uncertainty then results in your determination of the ratio?

CONCLUSION