PPTPinhole Camera

Developer Notes

  • Exercises need improvement.
  • A pin-hole camera (camera obscura) was first ? It might be good to have a little history here. Maybe even on the first cameras.

Version / Date / Who / Revisions
01 / 2004/07/07 / sc / Initial version
02 / 2005/01/05 / dk / Removed these goals:
  1. Students should understand that light is a type of wave (electromagnetic wave).
  2. Students should understand that light waves require no medium to be transferred through.
  3. Students should understand that light has energy (it does work)

03 / 2005/04/13 / dk / Updated per Arny’s notes

Goals

  1. Students should understand that light is a wave and emitted in all directions.
  2. Students should understand that light rays travels in straight lines.
  3. Students should learn to draw light rays.

Concepts & Skills Introduced

Area / Concept
Physics / Light waves
Physics / Images

Standards Addressed

Time Required

Warm-up Question

Presentation

The main goals here are to reinforce that light radiates (is a field) and travels in straight lines (for everything we can see). Students should also learn to draw light rays. Students may recall the ray diagrams from the inverse-squared activity.

  • Tin cans work nicely for this activity. They aren’t flammable, they’re pretty tough for storage, they don’t rust, they’re cheap, they’re hand size, and they’re opaque. Use cans about 100 mm long. Cut one end out. Make two round holes near the center of the other end. Make one hole about 3 mm diameter, and the other about 1.5 mm diameter. Drilling the holes works well. The rounder, the better. Make another can with both ends cut out - you may need to eliminate internal reflections by blackening the inside with either flat black spray paint or black construction paper.
  • White plastic grocery bags work pretty well for image screens. They’re translucent and can be stretched pretty flat. Also cheap. Cut them to size and use rubber bands to hold them on the open end of the cans. Wax paper can also be used. The screen should be as smooth and flat as possible.
  • If you don’t have good images to view, candles work pretty well as a light source because light comes from every part of the flame, and the flame is not symmetrical. A frosted chandelier bulb might work well also, but I haven’t tried it. Candles are flames, which is a bit dangerous, but they don’t require wiring and electricity. Flashlights don’t have asymmetrical shapes.
  • A viewing tube helps a lot. Roll a piece of opaque paper into a tube (black construction paper works well) and rubber band it around the can. Images will appear much brighter.

Start by looking at some object in your hand (an apple?). This is a repeat from the stations. Everyone can see the object because light is reflecting from it in all directions. Draw a ray diagram of the object, with rays emanating from one point in all directions (to all students).

In the activity, it would be good to hand out the huge-hole cameras first, so students don’t get ahead of themselves. Go to the pin-hole cameras after analyzing the huge-hole cameras. Students should look around using the huge-hole cameras. They can look at the lights in the room, the windows, anything that is fairly bright, preferably something with detail in it. Buildings across the street work well. They should note that the screen is lit up all the way across. The image of the object is impossible to make out. Make them explain why using what they learned from looking at the apple. Light from every point of the object is traveling in every direction (more or less), so light from every part of the object is hitting every part of the screen, hence there is no discernible image. Draw a ray diagram using two points on some object, like a light, a house, or a window. One point should be at the top, one at the bottom, Rays from each point should go to (almost) all parts of the screen.

Now switch to the pin-hole cameras. Have the students look around again, at the lights, windows, whatever. They should cover one hole at a time and compare the images they see. They should be able to see that the images are upside down, that the image from the smaller hole is clearer, and the image from the larger hole is brighter.

Now they need to explain it using ray diagrams. Again, the ray diagram should use two points, at the top and bottom.

The points, again, are that light emanates in all directions from objects (unless they’re black, there’s no light on them, they’re transparent, they’re completely smooth, etc.), and that light travels in straight lines (as far as we can see). Light actually bends around objects, through objects, and in gravitational fields.

Assessment

Writing Prompts

Relevance

Answers to Exercises

  1. Why can everybody who’s looking at an orange see it? [Because light is emitted in all directions from every spot on the orange.]
  2. Does everyone who looks at an object see the same image? [No, everybody sees a different image. Even your two eyes see different images.]
  3. Why can everybody who’s looking at a candle see it, even if there are no lights on? [Because light is emitted in all directions from the flame.]
  4. If light didn’t travel in straight lines, would vision be as useful as it is? Explain using a ray diagram, but make the rays curve.[Light wouldn’t be as useful if rays curved because then objects wouldn’t be where they appeared to be.]

Answers to Challenge/ extension

Equipment

Pin-hole cameras are just a light-tight box with a small hole in one end and a viewing screen on the other end.

Viewing screen – A thin, translucent material makes a good viewing screen. Plastic grocery bags work well, as does wax paper. Use a rubber band to hold the screen over the open end of the camera.

Pin-hole camera – Cut one end out of a tin can. Make two small holes in the other end. The holes should be near the center of the tin can, about 1 cm apart. They should be two different sizes, about 1/16” and 1/8”. Drilling makes nice clean, round holes, but you could make them with an ice pick or nails, too. You could also make two separate cameras with one hole each. Put the screen on the open end.

Huge-hole camera – Cut both ends out of a tin can. Put the screen on one end.

Viewing tube –The image on the screen will be relatively dim. In order to make it more visible, make a viewing tube to block out ambient light. Roll a sheet of black construction paper over the grocery bag end and hold it in place with a rubber band.

Background

How does light travel from objects to our eyes? How can we all see the same thing? Are we really seeing the same thing?

Problem

Investigate the behavior of light using a pin-hole camera.

Materials

1Huge-hole camera

1Pin-hole camera

1Ruler

Procedure

Assemble the pin-hole cameras if needed, following the teacher’s instructions.

Look at some bright object, like windows, or lights, or objects outdoors, through the huge-hole camera. If the light is bright enough, look at a classmate. What do you see on the image screen? Try to explain it using your knowledge that light goes in all directions from every point on an object, and that light travels in straight lines.

Now look at the same object(s) using the pin-hole camera. Cover one of the two small holes in the end of the can, and look at the object, then cover the other hole. What do you see?

Summary

  1. What is the orientation of objects seen through a pin-hole camera? Are they upside-up or upside-down?
  2. Using a ruler, draw a ray diagram showing why objects appear in the orientation you see on the screen. In your drawing, show the object, the pin-hole, and the screen, something like this:
  3. Through which hole in the pin-hole camera do objects appear clearer?
  4. Draw two more ray diagrams with different size holes, showing why the objects differ in clarity.
  5. Explain
  6. Through which hole in the pin-hole camera do objects appear brighter? Explain.

Reading

Look at the size of the pupil of your eye or a friend’s eye. Compare that to the size of the holes in the pin-hole camera. Your pupil serves the same purpose as the holes in the can. Stand in front of a mirror and look at your pupil. Keep looking at the same spot, but turn the bathroom light off. Wait a few seconds and turn it back on again. You can see your pupil get smaller when the light goes on. At night, your pupils gets bigger to allow more light (energy) in so that you can see in the dark. During the day, your pupils get smaller so that things aren’t too bright for your eyes.

For many people, vision is better during the day than at night. That’s because their pupils are smaller, so the image formed in their eyes is sharper and clearer. Compare your night and day vision. If you wear glasses, take them off and try looking at something through a very small hole. You can make a small hole by putting the tips of your thumbs and two fingers together. The image should be clearer.

The image in the pin-hole camera was upside down on the screen. Does that mean that the image in your eyes is upside-down, too? Yes. The image formed in your eyes is physically upside-down, but your brain turns it over so that down for you is in the direction of your feet.

Light basically travels in a straight line, radiating out in almost all directions from every point on an object. You can use this simple knowledge to help you understand how light works.

Exercises

  1. Why can everybody who’s looking at an orange see it?
  • Does everyone who looks at an object see the same image?
  • Why can everybody who’s looking at a candle see it, even if there are no lights on?
  • If light didn’t travel in straight lines, would vision be as useful as it is? Explain using a ray diagram, but make the rays curve.

Challenge/ extension

  1. Investigate cameras to find out how they work. You may need an old mechanical camera, even if it doesn’t work.

a)Look at the relationship between aperture and depth-of-field.

b)Look at the relationship between aperture and shutter speed relative to light energy.

c)Why does aperture (f-stop) have a non-linear relationship to shutter speed?

Glossary

  • Light ray–a line showing the path light follows

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