5 Light and shadows: Lightness and darkness in space
Provided by Kenan Fellows Program.
Students will learn that light travels in straight lines until reflected or scattered by objects in its path. They will discover how this fact leads to the existence of shadows. Students will explore the way an object’s size, shape, position, and orientation determines the shadow it creates and how it is affected by a particular light source. They will identify an object’s shadow as the region from which a light source is not visible because the object obstructs the line of sight.
They will also find that we “see” the world by collecting light that enters our eyes. This means that if no light enters our eyes from a particular direction we perceive that direction as dark. This is less obvious than it seems. For example, in the classroom, once the lights are turned on there is usually no area that appears dark because the room is filled with reflecting objects — the walls and everything else — so that light is impinging on our eyes from all directions. In the emptiness of space, light (from the Sun) may be present but, with the absence of objects to reflect it to our eyes, we see darkness.
After this activity, students should be able to:
- Understand that light travels in straight lines and can allow one object to hide another
- Explain how an object obscuring a light source from a particular location causes a shadow at that location
- Understand how an object’s shadow is determined by the object’s shape, size, and orientation
- Demonstrate how one object can create different shadows by changing its orientation
- Demonstrate how objects of different shapes can create the same shadow
- Understand that a shadow is a region in space and distinguish this from its projection on a “screen” to make it visible
- Realize that a shadow still exists in absence of the screen
- Explain why astronauts in space seem to be surrounded by darkness even when the Sun is shining so brightly that the astronauts and their equipment appear to glow
The teacher will need the following:
- 1 or 2 overhead projectors
- Lamp with a 250-watt bulb
Each pair of students will need the following:
- Sheet of white paper
- Sheet of black construction paper
- Two pencils of different lengths
- Styrofoam ball
- Optional: objects for shadow games such as geometrically-shaped blocks
Teacher background information
This lesson examines properties of light. You may wish to lead a general discussion on the nature and properties of light with your students. Following are some essential facts. Some of the information may seem to be part of a long lecture on the obvious. The point is to make explicit our implicit, intuitive understanding, so that we can clearly apply this information in less familiar, less intuitive situations.
A form of energy
Light is a form of energy. It is often generated by heating matter — converting heat energy to light. See the following examples.
- Incandescent light bulbs: Thin wire is heated by an electric current until it glows.
- Flames: Gases heated by the burning fire glow in various colors.
- Stars: Interiors are heated by thermonuclear reactions, which in turn heats the gas on the surface, producing the glow we see as starlight or, in the case of our nearest star, sunlight.
There are other ways to generate light, such as by exciting electrons within gases. This is how fluorescent lamps work, and is the source of some of the light created by lightning.
The travels of light
Once generated, light is emitted from a source, typically, in all directions. The energy in a light beam travels in a straight line at a fixed speed so long as it is in vacuum (as in space). When traveling through most gases, such as those found in our air, light propagates in the same way as in a vacuum — in a straight line at the speed of light (about 186,000 miles per second).
When it encounters some forms of matter, light can be reflected, absorbed, or scattered. Most objects reflect light from their surface in all directions, no matter what direction the light originates from. However, some objects, like polished glass or the calm surface of water, have special reflective properties that allow an image to form. In most cases, only part of the light is reflected, while some is absorbed by the object.
The idea that light travels in straight lines conflicts with our everyday experience on Earth because we are surrounded by reflecting and scattering objects. Indoors, walls reflect light so that light produced by a source will in fact be impinging on us from all directions. Outdoors, the daytime sky is not dark because impurities in the atmosphere scatter light. This makes it difficult to understand why, for example, the Moon glows so brightly in the nighttime sky, when it appears to be just a rocky ball in the daytime sky.
Color and light
Certain colors of light are more readily reflected than other colors. White light (containing essentially all colors) falling on an object causes light of a particular color to be reflected. There is some oversimplification in this description but it captures the basic idea. Objects that reflect most of the light falling on them appear bright; objects that absorb most of the light and reflect little appear dark. An object that reflects no light at all would appear pitch black.
Our eyes collect the light that falls upon them. They distinguish colors, and essentially report to the brain a “map” of the color and intensity of light that they receive from each direction. This is the information the brain uses to construct a picture of the world. For example, when you are standing in a classroom illuminated by lights and looking at a blue book, white light from the lights hits the book and is reflected in all directions. This is why the book is visible from any direction. Your eyes see blue light from a particular direction, and the brain interprets the shading and color as a book. In directions from which little light is coming, we perceive darkness or the color black.
An object that absorbs or reflects light can hide another object from our view if placed in such a way that any straight line from our eyes to the target object is broken by the hiding object.
An object placed between our eyes and a light source can likewise prevent us from seeing the light source. This means there is a region behind any object where light from the source cannot reach. Another object placed there will not be reached by light from the source and will appear dark. This region is what we call the object’s shadow. Often, we use the word “shadow” to describe what happens when a screen (wall, ground, etc.) is placed behind objects. There is then a part of the screen that, lying in the object’s shadow, is dark because light is not reaching it, and we call this part a shadow.
The shape and size of the shadow produced on a screen, and indeed of the shadow region itself, are determined by shape, size, and orientation of the object producing the shadow. For a small (or distant) light source, the edge of the shadow is found by extending straight lines from the light source to the edges of the object. The area covered by the shadow grows as one proceeds farther behind the object. For a small source, if one holds the object at a constant distance from the source, the size of the shadow formed on a screen is proportional to the distance between the screen and the light source. In other words, moving the screen so that its distance from the light doubles, without moving the object, will cause the shadow on the screen to double in size.
By rotating the object, the edge presented to the light source will change, changing the shape of the shadow on a screen. Also, since the shadow depends only on the edge presented to the light source, objects of different shapes can produce identical shadow shapes. Understanding the way shadows are created and attempting to recognize shapes by their shadows are all good ways to reinforce students’ understanding of the way light propagates.
We cannot take students to space but will examine the way things appear when placed in a bright light against a dark background by using the overhead projector’s beam, as well as a 250-watt bulb. Unfortunately, while an overhead projector works fine for shadow production, if objects are placed on the surface, the beam — after passing through the lens — is not so good for producing shadow effects because it is collimated (parallel beams of light rather than beams originating in a small point-like source). Shadows will not grow with distance behind the object in the same way.
Students most likely have experimented with their own shadows in kindergarten or first grade although few understand the principles behind how shadows are formed and that shadows are not just the darkly-shaded space on the ground. Entering this activity with little understanding is fine as the students will discover and begin to piece together some of the basic principles and properties of light and dark.
- Set the lamp without a shade and fitted with 250-watt light bulb in the center of the room. This activity, as well as several others in this unit, works best if the classroom is dark, with the only light coming from the 250-watt bulb. If possible, close the window shades and try to block skylights or unshaded windows with construction paper or some similar solution.
- Distribute the materials to student pairs.
- If desired, set up the overhead projector so the beam projects across an open area of the classroom, where students will be able to put objects in its path. A screen is not needed.
- Ask students to look at the above photograph. Introduce the activity with the following challenge.
Is the astronaut in the light or in the dark? How does he see what he is doing? If he looked up, could he see the Sun? If not, where do you think he should look to see it? What is the bluish circle behind him? What is the black thing filling the rest of the frame?
Allow students to brainstorm and record ideas in their science notebooks.
- Lead a discussion of what we mean by “seeing,” and how this relates to light. To see something, we need light from it to enter our eyes. What if something gets in the way? Why can’t we see it then? Bring up the difference between a light source, which produces light, and is visible even in otherwise total darkness (a firefly at night, for example) and an object we see because it reflects light produced by other sources and is invisible in total darkness. There are two ways to make a non-luminous object invisible: hide it, so that the light it reflects can’t reach our eyes, or place it in total darkness so there is no light for it to reflect. Ask, “Which of these would work with a luminous object?”
- Darken the classroom and turn on the 250-watt light bulb. Ask students, working in pairs, to use a pencil to create shadows of various shapes and sizes on the white paper. They should solve the following puzzles, and write their conclusions in their science notebooks.
- With the pencil and paper in a fixed position, find the orientation of the pencil for which the shadow produced on the paper is longest, and the orientation for which it is shortest.
- With the pencil held to produce the longest shadow, move the paper, first placing it right behind the pencil, and progressively moving it farther away. What happens to the shadow as the paper is moved farther away?
- With pencil held to produce the longest shadow and the paper in a fixed location, try to tilt the paper and see if you can make the shadow longer.
If desired, provide students with objects of various sizes and shapes so they can experiment with the shadows of different objects. You can also have students take two pencils of different lengths and find a way to make them produce identical shadows; this can be played as a game, with one partner holding the pencils and challenging the other, looking only at the shadows, to guess which is which.
- Turn on the classroom lights and turn off the 250-watt bulb. Prepare students for a discussion to summarize their findings. In your discussion, cover the following questions:
- What determines the size and shape of the shadow?
- What is the shadow?
- What happens when the paper “screen” is not there?
Explain that the shadow covers all the points from which the pencil would prevent us from seeing the light source by obstructing our line of sight. Whether the paper is there or not, these points are darker than points the light can reach.
- Continue the class discussion by covering the next set of questions:
- If we place an object in the shadow, will we be able to see it?
- Were you able to see parts of the paper that were in shadow? How?
- If no light reaches a sheet of paper, can we see it? For example, can we see the paper in a completely dark cave?
- If the pencil prevented light from the light bulb from reaching the paper, how were we still able to see it?
Light is reaching the paper, even where the pencil hides the light, because light is reflected off the classroom walls, leaks in through windows, etc. If there were no walls, if the light bulb were the only source of light, the part of the paper in the shadow would be not just darker, but completely dark and invisible!
In the second set of activities, students will use the black construction paper as a screen and background. In space, there are no walls, and essentially nothing to scatter or reflect light. We cannot produce this situation in the classroom, but since the black paper reflects little light, it will make things look more like they would in space than would the white paper. We can make the contrasts even stronger by using a brighter light source such as our overhead projector. Darken the classroom and turn on the 250-watt bulb. As you lead the class through these investigations, determine which of the questions in each step you want your students to answer in their science notebooks and which questions you want to discuss as a class.
- Place the pencil in front of the black paper so that it is illuminated by the bulb. Replace the black paper with the white paper. Can you see the pencil’s shadow on the black paper? Does it look different from the shadow on the white paper?
- Put the black paper back. Now use your hand to produce a shadow. Place the Styrofoam ball so that it is hidden by your hand. Explain what’s happening to hide the ball. Put the Styrofoam ball between your hand and the paper, so that part of it is in the shadow of the hand. What does the shadow on the ball tell you about the shape of your hand? How does the part of the ball in the shadow look different from the part of the ball outside the shadow? How would this change if you really were doing the experiment in space?
- Again place the ball half way in the shadow of your hand. Move your hand to make a different-sized shadow. Move it to make a different-shaped shadow. How many different shadow shapes could you create with your hand? Place the ball in between the light and your hand. What does the shape of the ball’s shadow on your hand tell you about the shape of the ball?
- If you have the time, turn off the 250-watt bulb and allow students to repeat the experiment using the beam from the overhead projector as a light source. The brighter, more-focused light should make the contrast stronger. If possible try to borrow an additional overhead projector and split the class into two groups.
Use the students’ drawings and writings in their science notebooks as a pre-assessment. This pre-assessment is based on their prior knowledge of light, shadow, and the relationship between light and shadow.
In the activity assessment portion of the lesson, students will answer the questions listed below. For your convenience, these six questions are also included in the following student worksheet. (Question 7 on the worksheet is part of the post-activity assessment.)
Optional: If you have the time, turn off the 250-watt bulb and allow students to repeat the experiment using the beam from the overhead projector as a light source.
- Assessment activity
- Activity assessment
- Open as PDF (12 KB, 2 pages; also available as Microsoft Word document)
- How does turning the pencil in a different direction change the size of its shadow on the paper? Draw and explain how this works in your science notebook.
- What happens to the shadow on the paper when it is moved farther behind the pencil? Draw and explain how this happens in your science notebook. Include in your drawings the light source, the pencil, the paper, and the paths of some light rays from the lamp to the paper.
- What happens behind the pencil when no white paper there? Is there a shadow when there is no screen? How would you find it?
- Replace the black paper with the white paper. How does the pencil’s appearance change? Can you see the pencil’s shadow on the black paper? Does it look different from the shadow on the white paper?
- What does the shadow on the ball tell you about the shape of your hand? How does the part of the ball in the shadow look different from the part of the ball outside the shadow? How would this change if you really were doing the experiment in space?
- How many different shadow shapes could you create with your hand? What does the shape of the ball’s shadow on your hand tell you about the shape of the ball?
For the post-activity assessment have students answer the seventh question from the worksheet:
- Look again at the picture from space. Is the astronaut in light? In darkness? Can he see what he is doing? Why is the sky behind him dark? Looking closely, can you find which direction the Sun would be in in this picture?
Discuss with students that in the picture, there is clearly a part of Seller’s helmet that is in a shadow. Why is it not completely dark, then, since he is in space? In other words, where is the light coming from by which we see the top of his helmet? (It could be the Space Station near him, or else the more distant but larger Earth itself.)
Have students look at the part of Earth visible behind him. Would people living there be able to see the Sun? Would they be in light? In darkness?
Photos of astronauts’ shadows on the Moon
Additional photos of astronauts on the Moon will test your students’ ability to determine where the Sun is located based on the shadow’s size, shape, and direction. Also you can discuss why the sky is black if the Sun is shining.
Many useful photos can be found at the NASA Image Galleries website. See the following examples.
- astronaut walking on moon
- astronaut waving American flag on moon
- footprint on moon
- astronaut conducting field work on moon
- astronaut carrying equipment on moon
Geometric shapes activity
Allow your class to classify various geometric solids (rectangles, triangles, prisms, spheres, cones, and cylinders). Discuss what the shapes’ shadows will look like. Ask, “What will the shadows look like if the objects are resting on paper? What if they were in space with no screen?” They will notice that some objects can make two differently shaped shadows. Discuss how they can prove the object’s shape. Lastly, have them examine a sphere and its shadow. Discuss and compare the sphere with a quarter’s shadow, which is also circular but flat.
More shadow play
Ask, “Can you make two objects that are very different sizes appear to be the same size?” Allow your students to experiment with different coins and other similar objects and see how and why they can make their shadows appear to be identical. Students will learn more about the connection between a shadow’s size and distance from the light source.
- When setting up the bright light in the center of the room, it is important to find a way to connect the power cord so that students can move about the classroom without tripping over it and knocking the light over. The light will be placed in this central location fairly often in this unit, so this is a problem worth solving thoroughly.
- Warn students not to look directly into the light beam. Unlike the Sun, this source will not cause permanent eye damage, but it will temporarily blind them since it is so bright, and students who can’t see where they are going are not a good thing.
- a form of energy, produced by heating or otherwise exciting matter
In the absence of matter, light propagates in straight lines at a fixed velocity of about 186,000 miles per second (or 670 million mph). Light is understood as an electromagnetic wave; in quantum mechanics it also has a dual description as a beam of mass-less, charge-less particles called photons.
- when light hits a collection of matter, some of the energy “bounces off” the edge of the object as a reflected beam
Most objects reflect light in all directions pretty equally, no matter what the direction of the incoming beam. Many reflect light of a particular color more than light of other colors. Some materials, such as metal or glass, can be polished to cause the reflected beam to leave the surface at an angle. This allows mirrors to create images.
- a second possible outcome when a beam of light hits an object is absorption, in which some of the energy of the light beam is retained in the object, heating it up
The reflected beam is then less intense than the incoming beam. An object that absorbs most of the light hitting it will appear dark.
- the collective, random reflection of light from multiple, small objects, as in a cloud of dust or a plume of smoke, is called scattering
In general, a beam of light incident on such a collection will be scattered so that there is a diffuse glow in all directions.
- the region in space where light beams from some light source cannot reach due to the interfering presence of an object
In general, when there are multiple light sources (or a large source each small part of which can be considered a source) an object will create multiple partial shadows in regions where the light from some part of the light sources cannot reach. The shadow will be darker in regions obscured from more light sources. If there is one small (or distant) light source, shadows with respect to this source will be very dark.
- North Carolina Essential Standards
- Science (2010)
- 3.E.1 Recognize the major components and patterns observed in the earth/moon/sun system. 3.E.1.1 Recognize that the earth is part of a system called the solar system that includes the sun (a star), planets, and many moons and the earth is the third planet...
- Science (2010)
North Carolina curriculum alignment
- Goal 3: Geometry - The learner will recognize and use basic geometric properties of two- and three-dimensional figures.
- Objective 3.01: Use appropriate vocabulary to compare, describe, and classify two- and three-dimensional figures.
- Goal 3: The learner will make observations and use appropriate technology to build an understanding of the earth/moon/sun system.
- Objective 3.01: Observe that light travels in a straight line until it strikes an object and is reflected and/or absorbed.