• Demonstrate that white light is composed of colors.
• Explore how most objects reflect rather than emit light; the light they reflect gives them their color.
• Learn that each color in the spectrum is associated with a wavelength.
• Investigate how light is the only thing we really see.

• electromagnetic wave
• frequency
• reflection
• retina
• sight
• spectrum
• wavelength
• white light

• Class One: flashlight, straight-sided clear drinking glass, construction paper, white paper, thin cardboard, protractor, crayons, pencil
• Class Two: red apple, colored cellophane (red, blue, yellow, green), banana, flashlight
• Class Three: No special materials required.

The unit is divided into three class periods. Classes One and Two can be combined. Allow approximately 20 minutes for each online Science activity.

1) Ask students, "What color is light?" Write their answers on the chalkboard.

2) Darken the room slightly and shine a flashlight. Ask students if they want to change their answer to the question, "What color is light?"

3) Perform the following demonstration:

• Fill a straight-sided clear glass with water.
• Take a piece of construction paper and cut a 1/2" slit in the middle. Tape the paper to the side of the glass.
• Place a sheet of white paper on a table close to a window. Set the glass on it with the construction paper toward the window so that the sunlight shines through the slit. If it is a cloudy day, shine a flashlight through the slit instead.

Results: The water refracts the light and a spectrum is displayed on the paper. Ask students if they want to change their answer to the question "What color is light?"

4) Have students perform the following activity. For older students, you can prepare several colored discs ahead of time and let them do only the last step.

• Cut a 6" circle from a piece of thin cardboard.
• Use a protractor to divide the circle into 7 equal wedges (51.5° each). Color each segment in order according to the colors of the spectrum: red, orange, yellow, green, blue, indigo, violet.
• Punch a hole in the center of the circle. Place a pencil in the hole.
• Spin the circle on the sharpened end of the pencil as quickly as possible.

Results: The colors merge as the disc spins, and the disc appears pale gray. The students will probably not be able to achieve an effect of a white disc, because the colors are not pure enough.

5) Summarize for students: White light, such as sunlight or light from a car headlight, is really a mixture of all colors of light.

1) Hold up a large red apple and ask students, "What color is this apple?" Then ask them to suggest ways of using light to change the color of the apple.

• Darken the room and place a piece of red cellophane over the end of a flashlight.
• Shine the light on the apple. Have students note the color of the apple.
• Then put a piece of blue cellophane over the end of the flashlight, shine it on the apple, and have students note the color. Ask if they can explain why the apple now looks black.
• Repeat with yellow and green cellophane. Then ask students to predict what will happen if you repeat the entire demonstration with a yellow banana.
• Then do the banana demonstration.

2) Introduce students to the visible spectrum of electromagnetic waves:

Show them how wavelength is measured from wave peak to peak:

Explain that objects usually absorb light of some frequencies and reflect others. The apple in #1 absorbed most frequencies and reflected the red light, and therefore it appeared red. When only blue light (the flashlight with the blue cellophane filter) was shone on the apple, there were no red waves to be reflected, and therefore the apple "lost its color."

Ask students, "Why does your shirt look different under a fluorescent light than under a regular light bulb?"

1) Ask students, "Why can't we see in the dark?" If their suggested answers are varied enough, have the students discuss the different responses.

2) Have students stare at the ceiling light for about 20 seconds and then close their eyes. Ask them to note what they "see" over the next 10 seconds. Ask if anyone can offer an explanation.

Explain that light in the visible spectrum stimulates the eye's retina. If there is no light, the eye does not see because nothing stimulates it to send a signal to the brain. After staring at a light, the retina is so stimulated that it continues to send "after-images" even when the light is no longer entering the eye. The only thing that we see, actually, is light, either being emitted from a light source or reflected from an object.

3) Present the basic structure of the eye to students:

The human eye is a complicated organ with many parts that serve different functions.

The iris is the doughnut-shaped part of the eye that defines our eye color (blue, brown, green, etc.). The iris surrounds the pupil, which is the hole that is seen as the dark center of the eye. The iris expands and contracts, making the pupil larger or smaller, to allow more or less light into the eye. The iris is positioned between the dome-shaped cornea and the lens.

The eyeball is made of a tough white wall and is filled with liquids called humors. The liquid between the cornea and lens, which is watery, is called the aqueous humor. The rest of the eyeball is filled with a clear, jelly-like substance called the vitreous humor. The humors help the eye maintain its shape without affecting how it functions to enable vision.

Inside the eye, at the back, is the retina. Images are formed on the retina by the lens and turned into electrical impulses that are transmitted to the brain through the optic nerve. When you look at an object, you are seeing light rays that have been reflected from that object and have entered your eye.