Investigations

Lesson 5

Title: Reflection, Absorption and Emission of Electromagnetic Energy.

Overview & Outcomes:

Investigations Lesson 4 illustrated the distinction between good (stratospheric) ozone and bad (tropospheric) ozone and that ozone gas has more than one role in the earth's atmosphere. This lesson is intended to link the electromagnetic energy concepts discussed in Investigations Lessons 2 with the upcoming lessons dealing with the greenhouse effect, Investigations Lessons 6 and 8 in particular. It serves as a crucial bridge between the electromagnetic spectrum and those greenhouse effect lessons. An understanding of the ultraviolet, visible and infrared portions of the electromagnetic spectrum and the concepts of reflection, absorption, transmission and emission of electromagnetic energy are essential elements needed for comprehension of the "greenhouse effect" and the "enhanced greenhouse effect." In Investigations Lesson 6, students' attention is focused on the phenomena known as the greenhouse effect and why the term "greenhouse effect" is actually a misnomer.

This lesson helps learners:

define the terms reflection, absorption, transmission, and emission as they relate to light or solar radiant energy.
relate the concepts of reflection and absorption of radiant energy to the color and the temperature of an object.
apply the concepts of reflection and absorption of visible and infrared energy to explain temperature differences in everyday examples.
describe the role of reflection, absorption, transmission, and emission of radiant energy in creating the greenhouse effect.

The concept map shows model relationships among concepts this lesson seeks to develop. Concepts introduced in this lesson are bolded on the concept map and concepts from other lessons are in plain text (not bolded).

Background Notes for Teacher:

Content.

Reflection -- Reflection is the phenomenon that allows certain substances to send back incoming radiation. Certain smooth surfaces are partially reflective, e.g., the surface of a lake, or certain glasses and mirrors.

Absorption-Certain frequencies are neither reflected by, nor pass through certain substances. Instead, the radiation is assimilated by the substance, and usually transformed from one form of energy into another (e.g., heat). For example, a green piece of glass is green because it absorbs all visible frequencies but green, which is transmitted through the glass. A red apple is red because it absorbs all visible colors but red, which is reflected back allowing us to see it. A white surface is white because it reflects all visible light. A black surface is black because it absorbs all visible light. Black surfaces are better absorbers of incoming radiation, which is turned into heat.

Transmission -- Is the movement of radiation through certain substances. For example, visible light is transmitted through (moves through) clear glass and the earth's atmosphere. Clear glass and the atmosphere are transparent to visible light.

Emission -- Emission is the act of giving off or sending out. Burning coals emit light. All objects emit (or radiate) some energy. Emittance occurs when electrons are discharged from a surface or electromagnetic waves are radiated from a body. Emittance is defined as the energy radiated by the surface of a body per second per unit area.

As previously mentioned in the electromagnetic spectrum lesson the term energy rather than radiation is used to try to avoid unnecessary confusion on the part of younger students. The concepts presented in this lesson are often very challenging, especially for younger students. Your careful selection of vocabulary, concrete examples and meaningful analogies will greatly facilitate the understanding of these concepts for your students. In the instructional procedures you will find some suggestions for illustrating the concepts of reflection, absorption, transmission and emission of visible light.

Using visible light is certainly the most useful way to help students comprehend the concepts in this chapter. However, it is ultimately the dynamics of infrared waves (or energy) that will emerge as the focal point for developing an understanding of the greenhouse effect. We cannot see the infrared part of the electromagnetic spectrum, but we can sense it as warmth or heat. You should be cautious of your use of the term heat in this lesson. You do not want your students to confuse the sensation of heat from infrared light energy with that from thermal energy such as steam or from heat that might be produced in an exothermic chemical reaction. Students may be familiar with the concept of infrared energy from cartoons or movies such as Predator.

Solar radiant energy contains all parts of the electromagnetic spectrum. From this only the visible light and a small percentage of the ultraviolet and infrared waves reach the earth's atmosphere. Of this predominately visible light energy, approximately 25% is absorbed by the atmosphere, 25% is reflected by the atmosphere, and 5% is reflected from the earth's surface. The remaining 45% is transmitted through the atmosphere and absorbed by the surface of the earth. It is this portion of the incoming short wave visible electromagnetic energy from the sun that contributes to global warming. (The percentages given for this phenomenon vary slightly from source to source. The figures cited here are taken from S. H. Schneider, Scientific American, September, 1989, page 72).

The solar radiation that reaches the earth's atmosphere and surface is eventually re-radiated or reflected back to space. Therefore, the amount of incoming solar radiant energy is balanced by an equal amount of outgoing energy. It is this concept of the Earth's Atmosphere Energy Budget that will help use understand the greenhouse effect and global warming. In order to better understand the Energy Budget, we will trace 100 units of solar radiant energy as it reaches the Earth's atmosphere. This energy reaches the Earth predominantly as incoming short wave radiation and may be reflected, absorbed and transformed or re-radiated.

Solar radiant energy reaching the Earth's atmosphere may be reflected by clouds or by the earth's surface. The reflected quality of a surface is its "albedo," which is expressed as a percentage of energy reflected. For example, darker colored objects have a lower albedo than lighter colored objects. Fresh snow has an albedo of 80-90. That is 80-95% of the energy striking a snowy surface will be reflected. Blacktop or asphalt has an albedo of 5-10 (again percent), which means most of the energy received by the asphalt will be absorbed, while only 5-10% is reflected.

Approximately 25 of the 100 incoming units are reflected as short wave radiation by clouds and particles in the atmosphere, and 5 units are reflected by the Earth's surface. This gives the Earth an average albedo of about 30 (%). The remaining 70 units are absorbed, 45 by the Earth's surface and 25 by the atmosphere. Following absorption or assimilation, the short wave radiation is converted into longer wave infrared radiation and re-radiated into space.

When the molecules on the surface of the earth absorb this solar energy they begin to vibrate. This short wave visible light is absorbed and re-emitted by the vibrating molecules in the form of longer wave infrared electromagnetic energy. These longer, lower energy infrared waves that are emitted from the earth's surface are not easily transmitted back through the earth's atmosphere. Most of this infrared energy is absorbed by accumulated gases (carbon dioxide, methane, water vapor, etc.) in the atmosphere and then re-emitted toward the earth's surface. This absorption and emission process of infrared energy is sensed by living organisms as warmth or heat. This phenomenon is referred to as the "greenhouse effect." Correspondingly, the accumulated gases which cause the phenomenon to occur are called greenhouse gases. Both the greenhouse effect and greenhouse gases will be further investigated in subsequent lessons. At present it is important for students to comprehend the interplay of electromagnetic energy with the earth's atmosphere and surface.

Materials:

For the class.

2 empty soda cans, painted
Sand
2 thermometers
Flood light if not conducted on a hot, sunny day
Solar energy transparency
REFLECTION and ABSORPTION EXPERIMENT sheets

For each group.

flashlight
mirror
black, white, green, and brown construction paper, 1 sheet of each
translucent plastic samples of different colors

Preparation:

Prior to day of lesson.

The activity using the painted cans, sand, and thermometers are one way you might wish to demonstrate reflection and absorption of solar radiation. Small cans are best to use given the time frame of one class period. Empty soda cans seem well suited for this activity. You may want to add a little sand to the can to help weight it down. To paint the cans, use flat paint, which increases the surface area for absorption. Be sure to paint the entire outer surface of the can.
Make transparency and copy handouts.

Day of lesson.

This demonstration should be set up just prior to class. You may wish to have several stations of this demonstration to facilitate quick and easy investigation by the students or if you have enough materials, you could have students pair up and do this as an experiment rather than a demonstration. If placed in a window sill on a hot sunny day, within ten minutes you should notice that the black can is about ten degrees warmer than the white. Allow students to feel the cans. The black can is warmer because its surface has absorbed more of the solar radiation than the white. Since both cans were exposed to the same amount of radiant energy, the white can must therefore have reflected more of the sun's energy. If a hot sunny windowsill is not available, expose the cans to a flood lamp to simulate the sun's radiant energy. You may also want to experiment with various colors such as green and brown to represent different land surfaces. Just remember to use the same type of paint on each can, i.e. flat paint.

Instructional Procedures: (2 Days, 40 minutes each)

Day 1. (40 minutes)

Begin today's lesson with a review of the previous lesson on the electromagnetic spectrum, especially recalling the ultraviolet, visible, and infrared portions of the spectrum. (If you had students write a journal entry or story about how the electromagnetic spectrum influences their life, you might begin this review by having students share their writings with the class.) Also review the vocabulary-reflection, absorption, transmission and emission.
Divide students into small groups. Provide each group with a flashlight, mirror, construction paper, and translucent plastic. Have groups investigate the effects of shining the light on the various surfaces.
Have each group identify an example of reflection, absorption, transmission, and emission from the materials they have just examined, i.e.: the flashlight emits visible light, the mirror reflects the light, the plastic transmits light and the construction papers reflect and absorb to varying degrees. Including various colors of plastic provides an example of substances that let certain wavelengths of light through, but not others, which is analogous to the atmosphere.
Have each group make analogies with their materials to various aspects of the earth's surface and atmosphere.

Day 2. (40 minutes)

Review the concepts of reflection, absorption, transmission and emission by having students identify common items in the room that illustrate these concepts.
Engage students in a discussion of why you feel hotter when you wear a black T-shirt than when you wear a white T-shirt.
Discuss the results of the "can experiment." Ask students for implications or examples of this same phenomenon.
Have students break into think/share pairs and have them conceptualize how the same phenomenon might affect different surfaces on the earth, i.e. snow, soil, asphalt, water.
Tie in the results of the can experiment with the reflective and absorptive properties of the earth's atmosphere and surface. Use the transparency provided to summarize the discussion of the sun's radiant energy, EMS, reflection, transmission, absorption and emission as they relate to developing the concept of the greenhouse effect which will follow in subsequent lessons.
Plan time at the end of class to describe the at home laboratory assignment described below if you plan to use this activity.

Assessment/Portfolio Items:

Project. Have students create and carry out their own at home experiment to illustrate absorption and reflection of solar radiation. Examples might include testing the rate at which different colored M & M candies melt or crack when exposed to light, testing the rate at which the same amount of ice placed in two zip lock bags melt when placed under a black versus a white T-shirt and placed in the sun, etc. Students should describe the set-up and results of their experiment. You may chose to use the example data sheets provided and have students include this in their portfolio.

Test/quiz. At the conclusion of day 2 you may find this time appropriate to administer a short teacher-made quiz to assess student understanding of the vocabulary, concepts and examples presented in the electromagnetic spectrum and the reflection and absorption Investigations Lessons.

REFLECTION AND ABSORPTION EXPERIMENT

Name:____________________________________ Date:____________________

Problem:___________________________________________________________

Hypothesis:_________________________________________________________

__________________________________________________________________

Materials used:______________________________________________________

__________________________________________________________________

Describe how you set up and conducted your experiment:

__________________________________________________________________

Describe the results of your experiment:__________________________________

__________________________________________________________________

Conclusions:________________________________________________________

__________________________________________________________________

In the space below, provide a sketch, drawing or photograph of your experiment.

Describe a common everyday experience that is an example of a situation similar to what happened in your experiment.

___________________________________________________________________

__________________________________________________________________

Illustration.

Click Here to return to Table of Contents

Click Here to go to Investigations Lesson 6: The Greenhouse Effect

This unit was produced by the editors listed on the masthead.