Title: The Electromagnetic Spectrum: Visible and Invisible Light

Overview & Outcomes:

In Investigations Lesson 1, students examined the composition and structure of the atmosphere. The purpose of this lesson is to introduce students to the Electromagnetic Spectrum (EMS), and the concepts of electromagnetic waves, electromagnetic energy, and electromagnetic spectrum. These are difficult concepts for most students to understand. However, most students have heard of infrared and ultraviolet light. Establishing a conceptual link between electromagnetic waves humans can see (visible light) and those which we cannot see (e.g., infrared and ultraviolet light), will be important. Several ways in which different types of electromagnetic energy interacts with various substances will be developed in more detail in subsequent lessons. Investigations Lesson 3 links the atmospheric composition concepts dealt with in Investigations Lesson 1 and the electromagnetic energy concepts with depletion of the ozone layer.

This lesson helps learners:

• understand the concept of energy.
• develop a working understanding of the ELECTROMAGNETIC SPECTRUM.
• be able to provide real life examples of various types of electromagnetic energy.

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

Background Notes for the Teacher:

Content. Have you ever wondered what similarities might exist between a) X-rays, b) solar radiation, c) a radio signal, d) a microwave oven, or e) an infrared lamp? Although these might appear to be completely unrelated phenomena at first glance, they are all the same type of energy-ELECTROMAGNETIC ENERGY. Briefly, electromagnetic energy is a wavelike propagation of an electric and a magnetic field. Electromagnetic waves differ from sound waves or waves on the surface of water in that they do not need to travel in a substance; that is, they can travel through a vacuum. The term "electromagnetic radiation" is also commonly used to refer to the same types of waves. In this unit, we use the term electromagnetic energy consistently to avoid possible confusion with the term "radioactivity", which refers to reactions involving the atomic nucleus.

Although identical in nature, electromagnetic waves differ from each other in their length and energy levels. The length of a wave is the distance between two subsequent crests. Energy levels are inversely proportional to the wavelengths; that is, the shorter the wavelength, the higher the energy. The arrangement of electromagnetic waves according to their wavelengths or energy levels is known as the ELECTROMAGNETIC SPECTRUM (EMS), which you can find in the overhead transparency master provided. The range of electromagnetic waves is subdivided into different categories, depending upon level of energy. In decreasing order of energy, or increasing wavelength, they were named: gamma rays, x-rays, ultra-violet rays, visible light, infrared rays, microwaves, TV and radio waves.

Visible light represents the only portion of the entire EMS which we can perceive with our eyes. These waves are just the right length to stimulate special cells in the retina (cones and rods) which cause neural impulses to be sent to the brain. Electromagnetic waves with wavelengths either above or below the visible light cannot be perceived by human eyes. This phenomenon is analogous to the higher sound frequencies which are inaudible to the human ear, but which can be heard by animals like dogs and bats (again, sound waves are different in nature from electromagnetic waves). For this reason, electromagnetic waves, or energy, outside the visible portion are also known as 'invisible' light. Within the visible portion, different wavelengths stimulate different sensations of color in our eyes.

Although not perceived by the human eye, the invisible portions of the EMS have very familiar effects, such as:

1. Gamma-rays: highest in energy. Used in medical treatments to kill and destroy cancerous cells.

2. X-rays: used, for example, to check for dental and bone problems, and to detect metals in unopened packages. X-rays are able to pass through many materials that stop ordinary visible light. The density of a material affects absorption of X-rays. Because bone absorbs more X-rays than skin and muscle, X-ray photographs have an important medical use.

3. Ultraviolet (UV) rays: have enough energy to damage or kill living cells. Doctors and hospitals use them to kill bacteria and sterilize equipment. UV rays also enable our skin to produce vitamin D, but frequent overexposure can cause cataracts of the eye and skin cancer. (We'll return to this in Investigations Lesson 3: Ozone Layer Depletion.)

4. Infrared (IR) rays: can stimulate the sensation of warmth in our skin. Certain animals detect the IR energy emitted by warm-blooded prey. An incandescent light bulb emits all visible wavelengths (which add up to its white color) in addition to infrared wavelengths.

5. Microwaves: just the right wavelength to cause water molecules in food to vibrate so rapidly that they cause the food to get hot and cook, a phenomenon employed in microwave ovens. Their name makes them sound very small, but in fact their wavelength is larger than the ranges just mentioned above!

6. Radio and TV waves: are the longest wavelengths, with the least amount of energy.

Materials:

For the class.

1. Overhead of the ELECTROMAGNETIC SPECTRUM.
2. Colored chalk or transparency markers.
3. Materials that students can relate to, such as: an X-ray picture, a 'black-light' lamp (source of UV), sun screen with UV protection, a flashlight, a hot plate or burner, a heat lamp, a microwave oven, a radio with antenna.

For each group.

1. Prisms and light sources (if several prisms are not available, activity may be done as a whole class demonstration).

Preparation:

1. Gather all available materials.
2. Experiment with a prism and light source so that you can better guide students in obtaining an intense spectrum of visible light. You will probably need to project the spectrum onto a white sheet of paper. If you are using a whole class demonstration, ensure that you have a light source intense enough to produce the desired results. You will probably need to darken the room.
3. Prepare overhead.

Instructional Procedures: (1Day, 40 minutes)

1. Start by having the materials you gathered (X-ray picture, 'black- light', sunscreen, flashlight, hot plate, etc.) set up on your demonstration table.

2. Ask students to briefly write down something that they know about each individual artifact. Also have them write down what they think are some possible commonalties among them.

3. Sample students' responses. It is unlikely that the notion of electromagnetic waves or energy will come up at this stage.

4. Introduce the lesson topic by saying that today we will study a phenomenon that is somehow related to all those artifacts: ELECTROMAGNETIC ENERGY.

5. Demonstrate and assist students in obtaining a spectrum of light by having a white beam pass through a prism and shine onto a white surface. (Or proceed with whole class demonstration).

6. Discuss observations, leading students to suggest that white light is composed of different colors, ranging from violet to red: the 'colors of the rainbow'. Explain the notion that light is a type of energy that travels as waves called ELECTROMAGNETIC WAVES.

7. Briefly define and illustrate the concepts of wave, wavelength, and electromagnetic wave. Point out the inverse relationship between wavelength and energy-the shorter the wave, the higher the energy.

8. Explain that our eyes can differentiate wavelengths-or levels of energy-through the sensation of color: different colors are associated with different energy levels, or wavelengths. Energy increases towards the violet end (as the wavelength decreases), and decreases towards the red end of the spectrum (as the wavelength increases).

9. Using colored chalk or markers, draw on blackboard or overhead the visible spectrum.

10. Develop the idea that what we see does not comprise the entire continuum. You may wish to assist a student in feeling the infrared energy given off by a hot plate or an infrared lamp. Introduce the notion that the sensation of warmth is caused by INFRARED light, another form of electromagnetic energy. Human eyes cannot see it, but we feel it as heat. Point out that certain animals are more sensitive to infrared than humans. Refer students to the sources of infrared light that you have gathered over the table. Ask for more examples (e.g., certain restaurants use infrared lamps to keep the food warm; the heating lamp in the bathroom of certain homes; the sun; etc.).

11. Explain that visible light and infrared light are but two forms of the same type of ELECTROMAGNETIC ENERGY, visible and invisible. We can only see electromagnetic energy of certain wavelengths, while others have different, familiar effects. Analogously, human beings are "deaf" to certain sounds that dogs or bats can hear (Remind students that the analogy is limited because sound waves are different in nature from electromagnetic waves).

12. Likewise, introduce and develop the concept of ULTRAVIOLET light. Again, refer students to the examples on display, and ask for more examples (e.g., sun; welding metals; etc.)

13. Using your drawing, help them find the "logical" place for UV and IR, by explaining the Latin prefixes: ultra=beyond; infra=below.

14. In a similar fashion, introduce and elaborate on the other ranges of electromagnetic energy. Constantly refer students to the items on display. Try to be systematic so that it is easier for students internalize the notion of energy increasing in the same direction that the wavelength decreases. Use your drawing to illustrate the full range of energies.

15. Present the overhead ELECTROMAGNETIC SPECTRUM. Indicate that this is just a visual representation of the electromagnetic waves ordered by level of energy or wavelength. Reinforce the notion that the visible part of the spectrum, which lies somewhat in the middle of the continuum, represents a very narrow range of wavelengths when compared to all the possible ones.

16. Emphasize the fact that the interactions of electromagnetic waves with different substances are very complex and will be studied in forthcoming lessons (number 3 and 4).

17. At this point you might want to mention 'the microwave oven' as a rather perplexing example: although it emits longer wavelengths than either infrared or visible light-thus, with less energy-it causes food to heat up much quicker than a light bulb or hot burner would. That happens because microwave ovens emit just the right wavelength to cause water molecules in food to vibrate so rapidly that the food gets hot and cooks. In other words, the effect of electromagnetic waves on a substance is not only determined by the energy level of the wave, but also by the substance's special properties.

18. Scramble all the items which you had for display (if they were ordered according to the EMS). Ask students to, in their groups, discuss what are the commonalties and differences between the items that were discussed during class. You may wish to have groups list all the items with their corresponding type of electromagnetic wave, in ascending order of energy, or descending wavelength. Sample responses.

19. A final brief activity is very important to help insure that students have appropriate conceptions about the different terms used in describing the EMS or referring to its individual components. The teacher should put on the board the following terms: ultraviolet energy, ultraviolet light, ultraviolet radiation, ultraviolet rays, and ultraviolet waves. Subsequently, the teacher should ask the students: "Do these terms all have the same meaning (or refer to the same thing) or do they have different meanings? Clarify that they all mean the same thing: The terms "energy", "light", "radiation", "rays", and "waves" are used interchangeably in describing the electromagnetic spectrum or referring to any of its components. The teacher should repeat the above question for "infrared" (energy, rays, etc), just to make sure students are clear. The teacher should also explain that the term "radiation," when used in describing the electromagnetic spectrum or different components of the spectrum (e.g., "infrared radiation") does not mean "radioactive" or refer to "radioactivity."

Assessment/Portfolio Items:

Journal Entry

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