Summary: Students will explore the property of light essential to understanding the natural phenomena we study in this unit – light propagates in straight lines until reflected or scattered by objects in its path. They will also find that we “see” the world by collecting light that enters our eyes. The first fact leads to the existence of shadows. Students will understand this and probe the way in which an object's size, shape, position, and orientation determine the shadow it creates given a light source. They will characterize an object's shadow as the region from which a light source is not visible because the object obstructs the line of sight. The second fact 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: in the classroom, once lights are turned on no direction appears dark as we are surrounded by reflecting objects – the walls and everything else in the room – so that light is impinging on our eyes from all directions. In the emptiness of space, light (from the Sun) may be present, but absent objects to reflect it to our eyes we see darkness.
After you have introduced the Challenge/Motivation the students’ drawings and writings in their Science Notebook act as a pre-assessment. This pre-assessment is based on their prior knowledge of light, shadow and their relationship.
After this activity, students should be able to:
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 lamp to paper.
What happens behind the pencil when there is no white paper there? Is there a shadow when there is no screen? How would you find it?
Replace the black paper by 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 than 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?
If you have the time, turn off the 250W bulb and allow students to repeat the experiment using the beam from the overhead projector as a light source.
Look again at the picture from space from our Motivation/Challenge. Is Mr. Sellers 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 this picture?
In the picture, there is clearly a part of his helmet that is in the shadow of another part. 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? (Could be the Space Station near him, or else the more distant but larger Earth itself.)
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?
In this activity we deal with the properties of light. As was the case with gravity in “The Earth is Round,” for example, a discussion of the nature and properties of light in general is a very natural extension. If you wish to pursue this, you will need to prepare this. For our purposes here, the essential facts are these:
Group Size: Whole class and small group experiment teams
Each pair needs:
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 darken 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 darkness.
Prepare the 250W light bulb in the center of the room. This activity, as well as several others in this unit, work best if the classroom can be made quite dark, with the only light coming from the 250W bulb. If possible, close window shades and try to block skylights or unshaded windows with construction paper or some similar solution. Distribute the materials to student groups. 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.
Astronauts’ Shadows on Moon Photos:
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 is the sky black if the Sun is shining?
Many of the NASA’s Moon photos can be found at NASA’s NIX site:
Individual photos are located at
Geometric Shapes Activity:
Allow your class to classify various geometric solids (rectangular and triangular prisms, spheres, cones, and cylinders). Discuss what the shape’s shadows will look like. 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 different shaped shadows. Discuss how they can prove the object’s shape. Lastly, have them examine the sphere and its shadow by turning it. Discuss and prove the sphere’s differences to a quarter or coin’s shadow that is both flat and circular.
More Shadow Play:
Can you make two objects that are very different sizes appear to be the same size? Allow your students to experiment with different sized 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.
Interactive Websites with fun Light & Shadow Activities:
BBC’s Educational website offers activities and assessment for your students:
A wonderful site teaching about what we can learn about an object’s size and shape from its shadows is found at a site teaching about how CAT Scans work:
After your students have experimented with geometric solids let them have a go at using multiple light sources to further understand the relationship between an object & its shadow and the light source at
Exploring Light (Print-Non-Fiction). Ed Catherall. (Exploring Science Series). Wayland, East Sussex, England (NEL), 1989. 48 p. ISBN 1-85210-703-0 ($21.95 hdc.)
The Science Book of Light by Neil Ardley, Harcourt Brace Jovanovich, 1991. The book contains simple light experiments demonstrating the principles of light with colorful photographs.
Light: 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 massless, chargeless particles called photons.
Reflection: 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 to the normal equal to that of the incoming beam. This allows mirrors to create images. See the notes for Additional Activity 1, Mirror Mazes, for more information on this.
Absorb: 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 which absorbs most of the light hitting it will thus appear dark.
Transmit: Finally, some objects allow some of the light incident on them to continue through the object.
Scatter: 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.
Shadow: The region in space which light beams from some light source cannot reach due to the interfering presence of an object is the object's shadow with respect to that source. 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.