Feb 19, 2016 0
Making upside-down images
Use Light to Turn Your World Upside-Down!
by Greg Voth
We can easily observe light with our eyes, and so it is one of the most familiar parts of the world around us. And yet, light often does amazing and unexpected things. Light travels in straight lines from the source to our eyes. This fact allows us to understand many of the cool things that light can do. In this lesson, we will observe how light creates mirages and shadows. And we will build a pinhole camera which makes things appear upside-down. We can understand the upside-down images by thinking about the straight line that the light took from the object to the screen.
Approx time: 1 hr 10 minutes
Things you'll need:
- Cardboard tubes 2x10in and 2 ½x 10in (brown mailing tubes work well)
- Translucent screen (wax paper, tracing paper or vellum)
- Black paper (construction paper works well)
- Nail, pushpin or sharp pencil for making the pinhole
- Scotch Tape
- Masking Tape
- Stickers to decorate the scope (Optional)
- Light source that is different at the top and bottom (We used a uniformly illuminated box with a gel taped to it in the shape of a tree with a large red trunk and green top.)
Duration 5 minutes
Ask the kids what they know about light. Ask them if they can name different types of light sources (candle, the sun, light bulbs, etc.). Ask them what they know about how light travels (Does it bend around corners? Does it go straight?) Show them an example by taking a flashlight and pointing it to the ground or the table or the wall and show them that the light beams travel straight from the flashlight. Put an object partially in the path of the light and ask them if the light bends around the object and comes back or if it just stops. Tell the kids that you are going to show them some cool things that they can understand by thinking about the straight paths that the light travels on.
Mirage Generator Demonstration
Duration: 5 minutes
A leader shows the ‘Mirage’ generator to the kids. Outside of the view of the kids, place a small object (pig, dinosaur, etc.) in the mirage generator and have the kids look at it. (You will likely need to carry it around among them. It takes a little practice to get the correct observation angle, so be sure that you can see it before you show it to the kids. Carrying it around can introduce them to the fact that science club is not just sitting listening).
Ask the kids if they see the object (pig, dinosaur, etc). Ask them to touch the object. When they find out that nothing is there, ask them how it works. Tell them when scientists see something new and interesting they start with a hypothesis: an idea about how it might work. Then they test the hypothesis. If the kids came up with any kind of guess, see if you can think of a quick way to test their hypothesis. If not, propose that something inside the container must be sending light out in such a way that it looks like there is an object on top. (A good test of this hypothesis is to use an index card to block the exit of the generator and show that the image disappears). Then open the generator and show them the object inside.
Duration: 5 minutes
One fun thing we all notice about light is that we have shadows. Ask them what you need in order to see a shadow. Did you ever notice that your shadow is sometimes very long and sometimes very short? Ask them when they think their shadows are long and when they are short.
(Hopefully you’ll get answers like its long in the morning or evening and short in the middle of the day, but you’ll probably also get answers like its long for tall people or other less connected answers.)
Use a flashlight and a model object to demonstrate how the shadow length changes as the position of the light source changes. Once you determine that time of day is what changes the position of the light source (the sun), you can then take your object, indicate one direction to be North and then model the sun’s movements from East to West over the course of a day and have them observe the shadows that you make.
Give each child a flashlight and let the child choose an object to use to make shadows. (Dinosaurs, balls, army men, shells, etc., make good 3D objects. Objects smaller than the size of the flashlight beam when the child holds it at arm’s length are good). Have them try to simulate the movement of the sun in the day time and let them describe the length of the shadows that they see. If there is enough time, have the kids choose a different object to see what kind of shadows it casts.
Introduction: 5 minutes
Show the children the large pinhole scope and how to use them. Describe the main components of the scope: pinhole, screen, tubes that allow the distance between the screen and the pinhole to be adjusted. Let each child come up and look at the light source through one of the pinhole scopes. Ask them how the image they see compares with the object the scope is pointing at.
Build your own pinhole camera/scope
Duration: 25 minutes
Tell the kids that their assignment is to figure out why the image seen in the scope is upside down. They can build their own pinhole camera and experiment with it. Start by making a screen on one end of the smaller tube. Tape a piece of vellum (translucent paper) so that it covers the end. Then make a pinhole in the end of the larger tube. (The leaders may need to help the kids do this.) Tape a piece of black paper over the end and poke a hole in it with a large pin or a small sharp pencil.
Try out the camera by pointing it at some bright scene. Observe how the image changes as the distance between the screen and pinhole is changed by moving the tubes in and out. If you look through the pinhole scope for 30 seconds or so, your eye adjusts to the dark and you can see the dim image better. (For older kids you can have them try a few different pinholes with different sizes and compare the images with different pinhole sizes: the image gets brighter and more blurry as the pinhole gets bigger. You can understand this because (1) the larger hole allows more light through and (2) a larger hole allows light from the same point in the scene to arrive at different points on the screen, resulting in a blurry image.
In this lesson plan, we focused on the science of light. We observed shadows and a mirage, and we created our own pinhole cameras! We also learned how we can be scientists who observe things and develop explanations of the things we observe. Make sure that the kids know how the pinhole cameras work, and encourage them to take it home and explain it to their parents or guardians.
Be sure you have tried out the mirage generator and the oatmeal container pinhole cameras before the lesson. One of the oatmeal container cameras has a lens. This does not change the basic functioning, but it makes a much brighter image. Use this one if students are having a hard time connecting what they see in the camera to the image they are looking at. The oatmeal camera with only a pinhole makes a similar image but it is much dimmer. Its advantage is that it doesn’t have any ‘magic’ components like a lens, so may help students be confident in their understanding of how it works.
Plan the logistics of the shadow activity carefully. It may take a bit of time for them to choose their model object and they may have strong preferences. Think about how you will hand out and collect the flashlights and the objects.
Technically we are building a pinhole imaging scope, not a camera since we do not have film to record the image—but building a camera just sounds cooler than building a scope, so we call it a camera. The pinhole is the simplest kind of camera lens and the screen plays the role of film. When taping the black construction paper to the outer tube and the tracing paper to the inner tube, place the paper over the hole and tape the four corners of the paper to the tube first using Scotch tape. Use masking tape to secure the paper all around the tube by carefully tucking the paper down and taping it a little at a time. You may need to help the kids do this.
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