Getting an angle on light

Students will discover that the angle of the sun can mark time and is responsible for the color we see in the sky. Students will also understand that light can be reflected and refracted.

A lesson plan for grade 3 Mathematics and Science

By Janet Jones

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Related topics

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Learning outcomes

Students will:

• learn to tell time using the sun’s shadow
• learn about opaque objects
• learn what happens when light is refracted and why
• learn why the sky changes colors
• learn that light can be reflected using mirrors

Teacher planning

Time required for lesson

3 days

Materials/resources

• three-foot dowel
• masking tape
• large clear outside area
• two thirty-two-ounce clear plastic containers
• small square mirrors
• object that supports mirror at forty-five degree angle
• one sheet of white paper
• water
• spoon
• dry powdered milk
• dark room
• flashlights
• measuring spoons
• liquid measuring cup

Pre-activities

Students should have knowledge of the earth/sun/moon system. Students should review the Earth’s revolution around the sun before beginning.

Activities

Day One

1. Take students outside to a clear area of the school grounds that gets sunlight throughout the day. Place a dowel in the ground so that it is standing straight up. Observe the shadow of the dowel on the ground. Explain that the dowel is opaque which means no light can go through it. Have the students also observe their own shadows explaining that they are also opaque.
2. After giving them time for exploration of different shadows measure the dowels shadow with centimeters and mark it with masking tape. Explain that the class will observe the effect of the sun’s position and angle on the dowel’s shadow for the rest of the day. Making a chart in the room record the time and length of the dowels shadow.
3. Continue to go outside every hour and observe, mark, and measure the shadow. You may also want to write the time on the tape and discuss how sundials were once used to measure time. At the end of the day have students brainstorm all of the things they observed throughout the day about the shadow of the dowel, comparing measurements from the chart. Write a language experience report as a class or have students write individual reports on how the position and angle of the sun affected the shadow.

Day Two

1. Review the previous day’s lesson about the sun’s changing position. Explain that the sun’s angle is responsible for the different colors we observe in the sky at different times of day. Sunlight may look white, but it is actually made up of the seven different colors of the rainbow.
2. To show this position the small square mirror in a plastic container at a forty-five degree angle. Fill the container with water so the mirror is covered. Darken the room and turn on a flashlight. With one hand hold white paper twelve inches above the water facing the mirror. With your other hand, point the flashlight down into the water at the mirror. The light should reflect off the mirror onto the white paper. Move the flashlight until you see the reflection of the rainbow on the paper. Different colors of light travel at different speeds in the water and in glass.
3. When the white light moves from air to glass or from air to water at a slant or angle it bends or refracts changing the white light into a band of color.
4. At this point the children may color a picture of the rainbow they observed on the paper. Discuss the reflection of the light off the mirror. Give plenty of opportunity for the students to use flashlights to investigate the reflection of light off different objects in the room.
5. Next place both thirty-two-ounce containers on a large sheet of white paper. Fill each with three cups of water. Darken the room and turn on the flashlight. Shine the light into the top of each container. Have students observe what color they see looking through the side of the containers. Add ¹/₈ teaspoon of dry powdered milk to the water in one container. Stir it until dissolved. Have them observe how the appearance of the water changes.
6. Now hold the flashlight twelve inches above the water and shine the light into each container. The light you see in the plain water is clear and colorless while the light you see in the container with the water and milk looks blue. The light we see from the sun looks white. However, they have learned that the light is made up of a spectrum of colors. Water droplets, dust and dirt in the sky scatter the white light from the sun, making only the blue color visible to us when the light is coming down straight. Have students observe the sunset for homework.

Day Three

1. Review what the students have learned about the sun’s changing position and how light is refracted by the particles in the air. Review why the sky appears blue.
2. Ask the students how the sky looked when the sun was setting. Make a list of all the students’ observations on the board. Discuss the sun’s angle when it is setting. When the sun sets, it is shining light at the Earth at a different angle and through larger particles in the sky so we see a different set of colors in the sky, orange and red.
3. To demonstrate this, place the two containers on a sheet of white paper. Fill each container with three cups of water. Shine the flashlight into the side of the container at an angle. Ask the students what color they see on the white paper. Add ¹/₂ teaspoon of the dry powdered milk to the water in one container. Stir with a spoon to dissolve it. Shine the flashlight through the container from the side at an angle. What color do they see on the water?
4. Add another ¹/₂ teaspoon of powdered milk to the container with the milky water. Stir with a spoon to dissolve it. Shine the flashlight into the container from the side at a diagonal. What do they observe now? Look at both the color of the water and the color on the paper. The Water in the container with the milk will look blue, while the light coming out of the container will look red, like the sunset.

Assessment

Students should be able to answer the following questions:

• How does the changing angle of the sun change the shadow of the dowel throughout the day?
• Why does the dowel cast a shadow?
• What happens when light is refracted?
• What happened when the light was shined on the mirror?
• What color did you see when the light was shined on the mirror?
• What did adding the milk to the water do to the light?
• How did changing the angle of the light affect the color of the light coming through the water?
• What is in the atmosphere that causes the refraction of sunlight so that we see different colors in the sky?

Supplemental information

Comments

The experiments can be used as demonstrations for the whole class or in individual groups under teacher direction, or in a science center. I like the students doing all of the measuring themselves. I also like to give the students time to do the experiments themselves if I have done them as a whole class demonstration.