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Pinhole Optics

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VANDERBILT STUDENT VOLUNTEERS FOR SCIENCE http://studentorgs.vanderbilt.edu/vsvs Pinhole Optics
Fall 2005

Purpose: To learn how a pinhole affects how we see things using a pinhole magnifier and pinhole camera.

I. Introduction

A pinhole is exactly what it sounds like, a small hole made with a pin. Experiments with pinholes are useful because the way light travels through the hole is very similar to the way light travels through the eye. The pupil, the small black area in the center of the eye, acts similarly to the pinhole. It is small compared to the big things in the world you are looking at. The retina, the back part of the eye where special cells sense light and send signals to the brain via nerves, acts similarly to the screen in the camera.

II. Pinhole Magnifier

Materials:
20 Pinhole magnifiers (index card with a small hole punched in aluminum foil inserts)
20 Pieces of newspaper (in an envelope)

A pinhole in an index card can act like a magnifying glass, helping your eye focus on an object that is very close to you. However, by limiting the amount of light that reaches your eye from the object, the pinhole also makes the object appear dimmer.

Pass out a pinhole magnifier (index cards with a small hole punched in aluminum foil inserts) and piece of newspaper to each student.

Use the pinhole magnifier to look at the text of the newspaper and notice that it appears larger.

Now try making a pinhole magnifier with your fingers like this:
[pic]

The pinhole magnifier works on a very simple principle: The closer you get to an object, the bigger it looks to you. This is because the closer you are to the object, the larger the image the object forms on your retina. Unfortunately, however, there is a limit to this. If you get too close to the object, your eye is not able to bend some of the light rays enough to obtain a focused image. As a result, the image becomes blurry or fuzzy. The pinhole magnifier sidesteps this problem by limiting the rays that come to you from each part of the object.

Unfortunately, there is a trade-off between the resolution, or sharpness, of the image and its brightness. A tiny pinhole produces a very sharp image, but since it decreases the number of rays that enter your eye, the pinhole makes the object look much dimmer.

By using a pinhole magnifier, nearsighted people who normally see things fuzzily at a distance will be able to see them clearly; likewise, farsighted people who normally see things fuzzily close up will be able to see them clearly.

Try it! If you or any of the students wear glasses, remove them before looking through the magnifier.

III. Pinhole Cameras
Materials
1 Sample pinhole camera
20 Pringles cans with lid cut into 2 pieces
20 Pieces of Aluminum foil ~1’ square (wrapped around paper towel roll)
8 Rolls of scotch tape (in a Ziploc bag)
20 Thumb tacks (in a jar)

• Organize students into 8 groups. • Give each group: a roll of tape • Give each student: a large and small piece of a Pringles can with the lid, a piece of aluminum foil that is approximately 1’ square, and a thumb tack. • The Pringles can has already been cut so there is one long piece, with the lid on top, and one small piece about 2” long. • Show students the sample pinhole camera to give them an idea of what they are creating.

Students then: • Use the thumb tack to make a hole in the center of the metal bottom. • Put the plastic lid on the cut end of the shorter piece. This will act as your screen. • Put the cut end of the longer piece on top of the screen. • The assembly should look like this. ( • Tape all of the pieces together. • To keep light out of the tube, wrap the foil all of the way around the tube, then tape the loose edge closed. If you have extra foil at the top, just tuck it neatly inside the tube.

Alternate Directions with Cardboard Tubes
Materials
1 Sample pinhole camera
20 Cardboard tubes (one slides into the other)
20 Pieces of Aluminum foil ~1’ square (wrapped around paper towel roll)
20 Pieces of Aluminum foil ~3” square
12 Rolls of scotch tape (in a Ziploc bag)
20 Thumb tacks (in a jar)

• Organize students into 8 groups. • Give each group: a roll of tape • Give each student: a set of cardboard tubes, a piece of aluminum foil that is approximately 1’ square, and a thumb tack. • Show students the sample pinhole camera to give them an idea of what they are creating.

Students then: • Place strips of scotch tape across the opening of the smaller tube. • Note: Use as little tape as possible so the smaller tube will easily slide into the larger tube. • Put a final piece of tape around the circumference of the opening to secure the strips of tape. Again, use as little tape as possible. • Slide the smaller tube into the larger tube. • Place the smaller piece of aluminum foil across the top opening of the larger tube. • Secure the aluminum foil into place with tape. • Poke a small hole in the center of the aluminum foil circle with the thumbtack.

• Now you can use your pinhole camera! You’ll need plenty of light though. So, if it’s a sunny day, take the students outside (approve this with the teacher first). The camera requires a lot of light and won’t work nearly as well inside. • Close one eye and hold the tube up to your other eye. You want the inside of the tube to be as dark as possible, so cup your hands around the opening of the tube. • What do you notice about the images you see? (They are upside down.) Why? Only a few of the light rays reflecting off each point are traveling in a direction that will let them pass through the hole. Light travels in straight lines. In particular, light travels in straight lines through the pinhole. So light that starts on the left of the pinhole passes through the hole and ends up on the right. [pic]

The human eye works the same way as the pinhole camera. The pinhole is comparable to your pupil and the screen is like the retina.

• Then, why doesn’t the world look upside down to your eye?

Your brain interprets the upside down image and flips it because you know that the sky is up and the ground is down.

• What do you think would happen if you looked through a pinhole camera for a very long time?

Scientists have actually done this experiment using special glasses. After a long time (several weeks) you will no longer see the image on the screen as upside down because your brain will stop flipping the image. However, when you remove the glasses the world does not look upside down to your naked eye. No one really knows why.

• Why does the pinhole magnifier image not look upside down?

Remember that there was no screen for the magnifier. Your retina is still acting as the screen so your brain flips the upside down image so that it is right side up.

Optional: Give take-home sheets to students.

References

http://www.exploratorium.edu/snacks/pinhole_magnifier.html http://www.exploratorium.edu/science_explorer/pringles_pinhole.html http://isaac.exploratorium.edu/~pauld/summer_institute/summer_day3eye_and_brain/pin_and__hole.html
Pinhole Optics Take Home
Pinhole Magnifier
Make a pinhole magnifier at home:
Cut a 1 inch square hole in the center of an index card. Tape a piece of aluminum foil over the hole. Use a thumbtack to create a small hole in the center. Experiment with different sized holes. Adjust the distance between the magnifier and your eye and the distance between the magnifier and the object. How does what you see change? Is the amount of magnification the same? Brightness? Sharpness?

Now experiment with using your fingers to make different sizes and shapes for the pinhole. What differences do you see? What might tell your parents if they are out to dinner and are having a difficult time reading the small print on a menu in a restaurant?
[pic]

Pinhole Camera
Make a pinhole camera at home: • Find an empty Pringle’s can and BE SURE TO SAVE THE LID. • Ask an adult to help you use a pair of sharp scissors to cut the can in half about 2 inches above the metal bottom. • Use a thumbtack to make a small hole in the center of the metal bottom. • Place the lid over the cut you just made on the sort end. • Place the cut on the long end over the lid and tape together. • Cover the entire can with aluminum foil and secure with tape. • Go outside on a sunny day. You are now ready to use your camera!

Why does the image appear upside down on the screen?

Only a few of the light rays reflecting off each point are traveling in a direction that will let them pass through the hole. Light travels in straight lines. In particular, light travels in straight lines through the pinhole. So light that starts at the top of the pinhole passes through the hole and ends up on the bottom.

Q Why does the pinhole magnifier (the card with a hole) image not look upside down?

A Remember that there was no screen for the magnifier. Your retina is still acting as the screen so your brain flips the upside down image so that it is right side up.

For more information, ask your parents for permission to go online and check out: http://www.exploratorium.edu/snacks/pinhole_magnifier.html http://www.exploratorium.edu/science_explorer/pringles_pinhole.html
Instructions for Creating Pinhole Camera

With wax paper:
1. Cut out a circle of wax paper slightly larger than the opening of the smaller paper tube. (about 1 cm larger in all directions)

2. Place the smaller paper tube vertically over the wax paper circle and center it.

3. Tape the wax paper to the tube at four points (90° between each point). Make sure to keep wax paper as taunt as possible. Note: Use as little tape as possible so the smaller tube will easily slide into the larger tube.

4. Cut four slits into the edges of the wax paper between each point that was taped. This will make folding over and securing the rest of the wax paper easier.

5. Tape the remaining edges of the wax paper to the tube and put a final strip of tape around the circumference of the opening to secure wax paper. *Remember to use as little tape as possible.*

6. Slide the smaller tube into the larger tube.

7. Cut out a piece of aluminum foil and place it across the top opening of the larger tube.

8. Secure the aluminum foil into place with tape.

9. Poke a small hole in the center of the aluminum foil circle with the thumbtack.

With scotch tape:
1. Place strips of scotch tape across the opening of the smaller tube. *Use as little tape as possible.*

2. Put a final piece of tape around the circumference of the opening to secure the strips of tape. Note: Use as little tape as possible so the smaller tube will easily slide into the larger tube.

3. Slide the smaller tube into the larger tube.

4. Cut out a piece of aluminum foil and place it across the top opening of the larger tube.

5. Secure the aluminum foil into place with tape.

6. Poke a small hole in the center of the aluminum foil circle with the thumbtack.

-----------------------
Camera
Assembly

Wrap the assembly with Al foil

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