Lesson: Reflectivity and Absorption

Prerequisites:

Objectives:

Demonstrate differences in reflectivity and it's effects on temperature.

Materials:

Introduction:

  1. Use the following questions or your own to have a discussion about reflective properties of different surfaces which the students have experienced: "In the summer, where are some places you have walked barefoot (list on chalkboard)? Which of the places listed on the board felt warmer on your feet?"

  2. Show the class the black paper, white paper, and mirror. Tell the class that you are going to shine a flashlight on these three different surfaces (black paper, white paper, mirror). Ask them: "Which surface will the light bounce off or reflect the most? Which surface will the light bounce off or reflect the least" Dim the lights and stand 2-3 feet away from a piece of white posterboard hung or taped to a wall. Tell the class to watch the posterboard as you shine the flashlight on the three different surfaces trying to reflect the light off the surface and on to the posterboard. Do the demonstration first with a mirror, then the white paper, and finally the black paper. Use questions like these to get the students interested in reflecting surfaces: "Which surface made the light bounce off or reflect the most? (mirror) Which surface did not reflect the light very well (black paper)? Did the white paper reflect more light or less light than the black paper (more)? For those surfaces which do not reflect the light well, where does the light go? Does it shine through the paper? (may need to hold some white paper behind the black paper to show that the light is not passing through the black paper)"

Body:

  1. On a sunny day (as close to summer as possible), take the students outside. Divide the class into 4-5 groups. Each class needs a piece of white paper, black paper, and a mirror. Have each group place each piece of paper and mirror in a sunny spot as close to each other as possible. Leave the objects in the sun for 15-20 minutes. During this time, discuss with the students what they think is happening to the black and white papers and the mirror. Possible discussion questions are: "What's happening to the paper and mirror right now? Is the sun hitting them? Is the same amount of sun hitting all of them? Is the sun bouncing off any of them? Are any of them reflecting the sunlight? Are they all reflecting the same amount? Do you think any of the surfaces are absorbing or soaking up the sunlight? If yes, are they all absorbing or soaking up the same amount of sunlight?

  2. Now have each group take their weather journals to their papers and mirror. Have the students touch each of the surfaces to determine which is the warmest and which is the coolest. Ask the students to write hot, warm, cold in the appropriate page of their weather journal under the pictures of the white paper, the black paper, and the mirror.

  3. Return to the classroom for a discussion of the experiment. Use the following possible quetions in the discussion to connect the ideas of reflectivity and light is heat. It is important that the students understand that the mirror (and white paper somewhat) reflects the sunlight and absorbs little. Therefore it is not warmed by much light. The black paper however, reflects little and absorbs a lot of light. Since light is heat, the black paper gets the hottest.

  4. Ask: "What did you notice the light does on the mirror? (bounces off, reflects) Do you think the sunlight outside bounced off the mirror? (yes) Did the mirror stay cool? (yes) Did the mirror absorb or soak up much of the sunlight? (no) Does the black paper reflect the sunlight? (no) Does the black paper get hot? (yes) Why did the black paper get hot? (absorbs or soaks up sunlight, light is heat) Does the white paper get hot? (no) etc.

  5. To demonstrate the idea of absorption an reflection, drop 4 tablespoons of water from a cup onto a block of wood. Ask: "What happened to the water? Did it soak in or bounce off the wood?" Drop 4 tablespoons of water from a cup onto a damp sponge. Ask: "What happened to the water? Did the water bounce off the sponge or was it soaked up by the sponge? Which of these two is more like the flashlight on the mirror? Why? Which of these two is more like the black paper? Why?"

Conclusion:

Using this lesson and the light is heat lesson, conclude that different surfaces absorb and reflect light and heat in different amounts. Remind students of the glasses of water in the light is heat lesson. The glass set in the sun became warm. Ask: "Why did the water become warm? What happens to the light which hits the water in the glass?"

Vocabulary Words:

Evaluation:

  1. See the picture of the mirror and the flashlight in the weather journal. Have the students draw the path the light takes after it leaves the flashlight. Have them draw in the rays of light as they hit the mirror and what happens after they hit the mirror.

  2. Ask: You are going to be going outside to play on a very hot and sunny day. You are worried that you might get too hot. You have a choice between wearing your clean white shirt or your dark shirt. Which do you think would be a good choice? Why?

Followup Lessons:

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