• Characterize materials as transparent, translucent, or opaque, depending on the light they reflect, transmit, or absorb.
• Observe that some materials bounce light, or reflect it.
• Examine how some materials "bend" — or refract — light as it enters the material.
• Discover that the refraction of light may disperse it into its component colors.
• Integrate the phenomenon of reflection, refraction, and dispersion to understand how rainbows are formed.

• absorption
• angle of incidence
• angle of reflection
• concave lens
• convex lens
• diffuse reflection
• dispersion
• electromagnetic wave spectrum
• focal point
• incident ray
• index of refraction
• lens
• normal
• opaque
• partial reflection
• reflected ray
• reflection
• refraction
• translucent
• transmission
• transparent

• Class One: 5 shoe boxes, clear cellophane, sheer fabric, medium-weight fabric, sheet of paper, posterboard, 5 flashlights, old X ray (if available)
• Class Two: small mirror, CD jewel box, stainless steel spoon, clear drinking glass, textbook
• Class Three: clear drinking glass, magnifying glass, small telescope or binoculars, small glass bowl, pencil, 4 shoe boxes, Exacto™ knife, clear glass bottle, wide-bottomed clear drinking glass, magnifying glass, clear glass bowl, flashlight
• Class Four: diamond ring or other diamond jewelry, clear drinking glass, bar of soap, foil, flashlight
• Class Five: No special materials required.

The unit is divided into five class periods. Classes One and Two, and Four and Five can be combined. Allow approximately 20 minutes for each online SimLibrary activity.

1) Set up a display for the students in a slightly darkened classroom.

• Take 5 empty shoe boxes and cut the bottom from each box.
• Tape across the bottom of each box one of the following materials: a piece of clear cellophane or plastic wrap, a piece of sheer fabric (e.g., curtain lining), a piece of medium-weight fabric (like a white pillowcase), a blank sheet of printer paper, a piece of poster board.
• Stand the boxes on edge on a table, and place a flashlight inside each box, shining outward through the material.

Have students record their observations and thoughts about the path of the light when it meets the given material.

2) Define transparent: the characteristic of transmitting light without distorting its path. Ask students which box(es) in the display has a transparent material. Then ask students to name 5 other materials that are transparent. Record their answers on the board.

3) Define translucent: the characteristic of transmitting light but distorting its path. Ask students which box(es) in the display has a translucent material. Then ask students to name 5 other materials that are translucent. Record their answers on the board.

4) Define opaque: the characteristic of not transmitting light. Ask students which box(es) in the display has an opaque material. Then ask students to name 5 other materials that are opaque. Record their answers on the board.

Have students define a shadow using the term "opaque" in the definition.

5) Summarize for the students: Light generally travels in a straight line until its path is disrupted in some way. Light is transmitted through transparent objects, such as a pane of glass or a lens, but it is bent, or refracted, in the process.

Light can also be reflected or absorbed by an object. The color we see when we look at an object is determined by which colors of light that object reflects or absorbs.

6) Challenge: Describing X rays

Show students where X rays fall on the electromagnetic wave spectrum:

Ask them to use the terms "transparent," "translucent," and "opaque" to describe how X rays photograph the human skeleton. If you have an old X ray, bring it to show them.

1) Set up a display for the students. Place the following items on a table under an overhead light: a mirror, a plastic CD jewel box, a stainless steel spoon, a clear drinking glass full of water, and a shiny page in a textbook.

Ask students to look at themselves in each of the items and to record how their images appear.

2) Define reflection: the bouncing of light off an object. Ask students which item(s) in the display reflects light. Ask them to offer explanations about why some of the images are sharper than others. Record their comments on the chalkboard.

3) Show students the path of reflected light:

Define the following terms:

Normal : a line that is perpendicular to the mirror at the spot the ray hits

Incident ray: a ray of light that falls directly on a surface

Angle of incidence: the angle between the incident ray and the normal

Reflected ray: a ray of light that has bounced off a surface

Angle of reflection: the angle between the normal and the reflected ray

4) Define diffuse reflection: random reflection of light from a rough surface, resulting in a fuzzy, scattered reflection. Ask students which items in the display (#1 above) have diffuse reflection. Ask them to list 5 additional objects with diffuse reflection.

5) Challenge: Partial reflection

Ask students to look through the classroom window. Have them describe the path of light through the window. Ask them to look through a window at home that evening after dark. Have them describe the differences in the behavior of the glass of the two windows. See if they can offer explanations.

1) Set up two displays for the students.

• Place on a table the following items: a clear glass of water, a magnifying glass, a small telescope or binoculars, and a small glass bowl propped up on its side. Have the students look at a pencil through each of the items, and describe how the pencil appears.
• Place a white cloth on the table. Take 4 empty shoe boxes. Use an Exacto™ knife to cut 5 narrow slits in one end of the box. Place a flashlight inside each box. Place one of the following in front of each box: a narrow clear-glass bottle filled with water, a wide-bottomed clear drinking glass filled with water, a magnifying glass propped up on its edge, and a small glass bowl propped up on its side (concave side facing the flashlight). In a slightly darkened room, students should be able to see the paths of light.

2) Define refraction: bending of light rays, or the passing of rays from one medium into another medium at an angle. Define lens: any transparent material that refracts light.

Define index of refraction: the ratio of the speed of light in a vacuum to the speed of light in a substance.

Explain that the index of refraction (n) has no dimensions. Light travels faster in a vacuum than through any other medium, so the index of refraction for all other substances is greater than 1.

3) Define focal point: the point on the centerline of a lens or mirror at which reflected or refracted rays converge.

Define convex lens: a lens that is thicker at the center than at the edges. An object that is placed outside the focal point results in a smaller, inverted image behind the lens. An object on the focal point forms no image. An object inside the focal point results in a larger, upright image that is in front of the lens.

4) Define concave lens: a lens that is thicker at the edges than at the center. The image of an object is always smaller, upright, and in front of the lens wherever the object is placed.

1) If you have or can borrow a diamond ring or other jewelry with a sizable diamond, bring it to class. Shine it under the light and have students note the effect. Ask them to offer an explanation for the colors they see.

2) Define dispersion: the separation of light into colors. Waves of different frequencies travel at different speeds, so they refract differently and bend by different amounts when traveling through a medium.

Students can find more information in Unit 2: Visible Light and Color, Class Two.

3) Do the following demonstration:

• Fill a clear glass with warm water.
• Shave into it 3-4 slivers of bar soap. Stir to dissolve the soap.
• Create a cone around the light-end of a flashlight. Open the end of the cone slightly.
• In a darkened classroom, shine the flashlight horizontally through the glass. You should see a blue beam of light transmitted from the glass.

Have students offer an explanation for the color of the light. (The wavelength of blue light most closely matches the size of the soap particles, so the blue light is scattered more than the other colors.) Based on the demonstration, ask students to conjecture why the sky looks blue. Point out that when astronauts in outer space look down on Earth from above its atmosphere, the sky looks black. Ask them to explain.

1) A natural phenomenon that combines the concepts of reflection, refraction, and dispersion is a rainbow. Here are the factors involved in creating a rainbow:

• White sunlight enters a raindrop. The light ray refracts at the raindrop's surface.
• The ray passes through the raindrop to the back surface. The light is reflected back.
• The reflected light passes through the front surface of the raindrop and is refracted again. This double-refraction causes considerable dispersion of the white light into its composite colors.
• The reflected ray must be less than 42° in order for a rainbow to be visible.