LIGHT
Investigation #3
Investigation 3: Light and Shadows
WHAT MAKES A SHADOW
It has been asserted that everything in the universe is either matter or energy. If so, what is a shadow, matter or energy? Certainly not matter, so it must be energy. Shadows are intimately related to light—light defines and delimits shadow. Shadows are the absence of light—the null condition of energy. Shadow and darkness are two conceptions of the same condition. Darkness is generally conceived of as a lightless space you are in—a dark night, a dark room, a dark alley. A shadow is a lightless space you see from outside. Shadows are associated with objects that block the light, preventing it from filling the space behind it.
A shadow is the dark area behind an object where light can’t reach. To make a shadow, you need a light source and an object that blocks the light from the source. The shadow is all the space behind the object where light doesn’t reach. If you cut a cross section through that space, you would have a two-dimensional silhouette of the object. Often it is this silhouette projected on a surface, such as a screen, a curtain, a wall, or the ground, that we call the shadow. But the shadow is the entire space behind the object, not just the flat projection.
Shadows may be created when any type of light shines. Shadows can be formed by sunlight. They are also created by artificial light, such as light from a lamp or a flashlight. For example, if you stand with your back to a lamp, your shadow will fall in front of you. This happens because your body blocks the path of light traveling out from the lamp. The same thing happens if you stand in the path of sunlight. The light source can be a lamp or a flashlight, or very hot objects like fire or the Sun. The light can also come from something that doesn’t give off light itself, but is reflecting bright light, like the Moon.
Shadows have two different levels of darkness because some light shines around the object, causing the shadow. The umbra is the darker area in the middle of the shadow. The penumbra is the lighter area at the edges. Where the light is completely blocked, around the middle of the object, a dark umbra is created. Where some of the light leaks around the object, a lighter penumbra is created.
The size of a shadow from an artificial light depends on the distance the object is from the light. If you stand very close to a wall lit by a lamp, your shadow will be small. If you stand farther away from the wall and very close to the light, your shadow grows larger.
Making Shadows: Part 1
Introduce Light
What light sources do we use in our classroom?
Introduce the exploration
- Hold up a flashlight and ask students what it is called. Review the different parts of the light (bulb and switch) and if you want, show them the batteries inside, providing the energy to power the light. Review the appropriate use of a flashlight in the classroom and go over any safety rules.
- Ask students what will happen if you shine the light on a student’s hand. Start with the flashlight close to the hand and then gradually move it away, so that a shadow is projected on the board. Tell students that they will work with a partner to see what happens when they shine light on their partner’s hand. Remind students to take turns using the flashlight.
Encourage students to share their observations by asking some questions.
What did you notice about the beam of light?
When you pointed the light close to the table, what did it look like? [Small circle of light.]
When you moved the light higher from the table, what did it look like? [The circle of light got bigger.]
What happened when a hand was in front of the light?
What was the hand doing to the light? [Blocking some of the light or all of the light.]
Identify the dark, hand-shaped area as a shadow.
Focus question: What makes a shadow?
Work with materials
- Hold up a plastic bag of objects and remind students that they used these objects with the drop chambers when they were exploring sound. Now they are going to use these objects and the bag to explore light and shadows. Working with their partner, they should determine how to make shadows using the flashlight and each of these objects.
- Distribute a bag of objects to each pair of students and let them explore. Circulate to the pairs and ask them to explain what they are finding out.
Ask students if they were able to make shadows with the objects. Have them share their observations. Have a set of objects and a flashlight available for students to demonstrate an observation.
Which objects made shadows?
How were the shadows different from each other?
Which objects did not make a shadow?
After students have shared their observations, ask them to generalize their observations.
What is a general rule for making a shadow? [You need a light source and an object to block the light.]
Vocabulary
block
dark
flashlight
light
light source
shadow
sunlight
Science Notebooks
Distribute a copy of notebook sheet 8, Making Shadows. Ask students to review the focus question and to answer the question in their notebooks. They might choose to write a sentence or two, or answer with one or more labeled illustrations.
What makes a shadow?
For students who need scaffolding, provide prompts such as, “What did you do to make a shadow? What has to happen?” or a sentence starter such as, “To make a shadow you _____.” For maximum support provide the sentence frame, “To make a shadow you shine a _____ on an object that _____ the light.”
There are three objects and three shadows on the notebook sheet. Ask students to draw a line from each object to its shadow.
Sun and Shadows: Part 2
Introduce the exploration
- Gather students at the rug with a flashlight in your hand. Ask them to tell you how to make a shadow. Follow their directions as you make a shadow on the board. Ask,
What two things do you need to make a shadow? [Light source and something to block the light.]
- We made shadows using the flashlight indoors. How might we make shadows outdoors?
Confirm that the Sun is a natural light source. Listen for what objects students suggest will block the sunlight. You might make a list on the board.
- Tell students that they will go outdoors and form a circle at the location that you have selected. You have a number of shadow challenges for them to meet. You will give them one challenge at a time. They will work as an individual or sometimes with others to meet the challenge. When they have finished one challenge, you will give them a second challenge.
Read the first challenge from teacher master 16, Sun and Shadow Challenges, so that students have an idea of what they will be doing.
Challenge 1. Make a wide shadow and a narrow shadow.
- Take the class outdoors to the location that you have identified and have students form a large circle. Stand in the center of the circle and read the first challenge. Have students meet that challenge.
Continue to present the challenge verbally to the students and give them time to work with a challenge before offering another. Here is a complete list of the challenges.
1. Make a wide shadow and a narrow shadow.
2. Hide your shadow.
3. Find a small stick to put on the ground. Make a circle around it with your shadow.
4. Make a shadow with three arms. With four arms. With six arms.
5. Shake hands with your partner and look at the shadow. Now, figure out how to make a shadow look like you’re shaking hands without actually touching your partner’s hand.
6. Make your shadow jump.
7. Make a shadow on the wall and the ground at the same time.
8. Find an object that makes an interesting shadow. Turn the object in some way to make a different shadow.
9. Find a natural shadow on the playground.
10. Make up a shadow challenge for your partner.
What questions do you have about shadows?
What new challenge would you like to meet?
- Move to a location outdoors where there is shade. Ask students to describe how the shade is created. If there is enough room, ask students to stand in the shade. Tell them they are standing in the space that is the shadow.
Focus question: How can we use the Sun to create shadows?
Vocabulary
shade
Sun
Science Notebook
Distribute notebook sheet 9, Sun and Shadows, and review the focus question. Ask students to write an answer to the question. After they finish, they can glue the sheet in their notebooks.
How can we use the Sun to create shadows?
Light and Shadows: Part 3
Introduce the Exploration
Have the room lights adjusted for the demonstration. Hold up the flashlight, turn it on, and shine the beam on the screen. Ask students what they see on the screen and listen to their responses.
Show students the three objects you have for the demonstration—the wood, the paper, and the plastic bag. Explain that you are going to use each object to block the beam of light by placing the object against the flashlight lens. Their job is to carefully observe what happens.
Start with the wood. Ask,
What do you think will happen if I block the light with this piece of wood? What will you see on the screen?
Why do you think so?
Conduct the demonstration and listen to what students say. Confirm that the wood blocked all the light. Move the wood away from the lens, so that the wood casts a shadow. Ask students what they see on the screen now.
Move the wood back and forth so students can see the effect of this material in the path of the light.
Hold up the plastic bag. Ask,
What do you think will happen if I block the light with this piece of plastic? What will you see on the screen?
Why do you think so?
Conduct the demonstration and listen to what students say. Confirm that the light went through the plastic. Move the plastic away from the lens. Ask students what they see on the screen now. They will be able to see a slight shadow along the edges of the bag and by the zipper
Hold up the piece of paper. Ask,
What do you think will happen if I block the light with this piece of paper? What will you see on the screen?
Why do you think so?
Conduct the demonstration and listen to what students say. Confirm that the paper blocked some of the light, so that the light can’t be seen on the screen. But some light went through the material because you can see light on the back side of the paper.
Move the paper away from the lens. Ask students what they see on the screen now. They will be able to see a shadow of the paper.
Focus question: What happens when different materials block light?
Vocabulary
Transparent: hands making circle around eyes
Translucent: hands in front of eyes with fingers spread apart
Opaque: hands in front of eyes
No light travels through the piece of wood. Materials that do not allow light through are called opaque. (Sounds like cake.)
This piece of plastic is clear, and light travels right through it. Materials that are clear and let light through are called transparent.
This piece of paper is not clear, and only a little light travels through. Materials that allow some light through are called translucent.
Introduce the objects and sorting mat
- Show students a bag of objects and a sorting mat. Explain they will work with a partner to sort the objects before they test them. They should talk and decide if they think each object is opaque, translucent, or transparent. Then they can test them with their flashlight. Have each pair of students get their materials and return to their tables to investigate. Visit the groups as they work, reminding students to sort the objects before they test them. Look for groups that need assistance in getting started with testing a material and go through one item with them.
Have each pair of students get their materials and return to their tables to investigate. Visit the groups as they work, reminding students to sort the objects before they test them. Look for groups that need assistance in getting started with testing a material and go through one item with them.
Here are some things that students will observe:
The metal washer is opaque and blocks the light except for the hole in the center. Students can test narrow materials over the hole, such as the craft stick and the clothespin. Students will find that the craft stick is translucent. The thicker wood of the clothespin is opaque.
Translucent objects include the plastic chip, craft stick, paper cup, plastic spoon, and waxed paper.
Opaque objects include the cardboard, metal spoon, and aluminum foil.
The only transparent object is the plastic bag (except for the printing and the label on the bag).
What is the effect of putting an opaque object in front of a light source?
What is the effect of putting a transparent object in front of a light source?
After the video
- Tell students that they will see a short video that will explore the interaction of materials and light. Here is a summary of the 2-minute video, chapter 4 of All about Light.
Transparent objects allow light to pass completely through them.
Opaque objects do not allow any light to pass through them.
Opaque objects have shadows.
Translucent objects block some light, and allow some light to pass through.
After viewing, have students discuss with their partner what they recognized in the video and anything new they learned. Ask for volunteers to share one of the new vocabulary words about light and an example from the video. To deepen student understanding, lead a discussion about what is happening with the shadow of the girl on the curtain. [The girl is blocking the light and making a shadow because she’s opaque, but you can see her image on the other side of the curtain because it is translucent.]
2.Tell students they get to label objects in the room that are opaque, translucent, or transparent. Tell them,
Each pair of students will get three large self-stick notes. You need to first write a word on each label (opaque, translucent, transparent). Then find an object in the room that fits each word and stick the label on it. If you aren’t sure, you can test the object with your flashlight
Give each pair of students three labels and have them return to their desks to prepare the labels. Give them a few minutes to label objects in the room.
Science Notebook
What happens when different materials block light?
Distribute a copy of notebook sheet 10, Light and Materials to each student. Have students record with “yes” or “no” in the “Did it let any light through?” column. If they test objects not listed on the sheet, they should write the name of the materials and object in the first column. Have students write about what they observed with one of the materials in their notebook.
Summary Video
Here is a summary of the 13-minute video.
Chapter 1: What Is Light? (2 minutes 25 seconds)
You must have light to make a shadow; sight is the only sense you can use to observe light.
Children tell what light is, where it comes from, and how it helps us.
Light is made up of billions of tiny photons; photons bounce off objects into your eyes; your brain and eyes work together, allowing you to see an image; photons travel in straight lines.
Chapter 2: Formation and Characteristics of Shadows (2 minutes 25 seconds)
A shadow is created whenever an object blocks light.
The size of a shadow depends on the distance the object is from the light; the shadow gets smaller the farther it is from the source of light.
A sundial can be used to tell the time of day; explanation of how a sundial works.
Chapter 4: How to Make Shadow Puppets (5 minutes 19 seconds)
Shadow puppeteer Sonny Fontana demonstrates and teaches the host how to make shadow puppets.
Chapter 7: Conclusion and Summary (2 minutes 46 seconds)