Standing Wave Demonstration

In this exercise, students will explore wavelengths in several ways. This demonstration is intended to help students better understand the electromagnetic spectrum.

Background

To understand how ozone is generated and how it functions in the earth's atmosphere, it is important to know something about the energy that comes from the sun. When scientists carefully analyze the sun's energy, they find that only part of the energy comes to us in the form of light we can see. Much of the energy comes in forms that we can't detect directly with our eyes. The full spread of this energy is called the electromagnetic spectrum.

Electromagnetic energy is sometimes described as traveling in waves and sometimes as traveling in packets of energy referred to as photons. Each concept is useful to scientists in explaining the behavior of light energy. This activity is a demonstration of energy as waves. Progressing from short wavelengths to long wavelengths, scientists have identified gamma rays, x-rays, ultraviolet light, visible light (between 400 and 700 nanometers), infrared (heat), microwaves, and radio waves. Short wavelengths have more energy per photon than long wavelengths.

Most of the radiant energy from the sun is concentrated in the visible and near-visible parts of the spectrum. The narrow band of visible light, between 400 and 700 nanometers (nm), represents 43% of the total radiant energy emitted. Wavelengths shorter than the visible account for 7 to 8% of the total, but are extremely important because of their high energy per photon. Ultraviolet wavelengths are divided into three sections: wavelengths between 200 and 280 nm are UV-C, between 280 and 320 nm are UV-B, and between 320 and 400 nm are UV-A.

Ozone absorbs all UV-C, much of the UV-B, and a little of the UV-A. The levels of UV reaching the earth's surface varies with time of day, day of the year, latitude, weather conditions, and the ozone aloft. Even though only a small amount of the sun's total radiation lies in the band of UV wavelengths, these very short wavelengths are damaging because of their high energy. The remaining 49 to 50% of the radiant energy emitted from the sun is spread over the wavelengths longer than those of visible light. These lie in the near-infrared range from 700 to 1000 nm; the thermal infrared, between 5 and 20 microns, and the far infrared regions.

In this demonstration, wavelengths are generated by an electric drill supplying energy to a cord. Energy moves from the drill along the cord to its end, which is held by a student. The cord itself merely moves up and down and does not move forward toward the student. As the wave moves forward along the cord, it carries energy with it. This demonstrates how electromagnetic energy travels from the sun to the earth.

Learning Goals

  1. Students will be able to explain that energy travels from the sun to the earth by means of electromagnetic waves.

  2. Students will understand that the shorter the wavelength, the higher the energy per photon and will be able to explain why shorter wavelengths of electromagnetic energy carry more energy than longer wavelengths.

  3. Students will be able to demonstrate how wavelength is measured.

Alignment to National Standards

National Science Education Standards

Benchmarks for Science Literacy, Project 2061, AAAS


Grade Level/Time




Materials

Procedure

  1. Before presenting this demonstration, provide some background information on the electromagnetic spectrum.

  2. Prior to the demonstration, you will need to bend a 20-penny nail as shown. You will have to use a vise to accomplish this.

  3. Attach a swivel to each end of the nylon cord.

  4. Tie the 1-foot piece of cord to one of the swivel holders. This is the piece of cord that a student will hold during the demonstration.

  5. Slide the bent nail through the eye of the other swivel.

  6. The nail end should be put into the drill bit fitting and tightened securely with the chuck key.



  7. To ensure the safety of your students, it is imperative that the cord not break during the demonstration. Be sure to test it before you present it to your students.



  8. Ask a student to hold one end of the cord.

  9. Plug in the drill and the demonstration begins. The less tension you apply, the more waves will appear. You can also vary the speed and reverse the direction of the drill to get different wave effects. Experiment and have fun!

Note: If you do not want the complication of using a drill, you can simply have students use their arms and hands to generate the wave using a length of rope. The more energy they put into the motion, the shorter the wavelength.

Another option is to use a Slinky to demonstrate energy and size of wavelengths.

Observations and Questions

  1. The length of the wave is measured as the distance from wave crest to wave crest. What happens to the length of the wave when the drill speeds up, i.e., when more energy is added? (The wavelength shortens.)

  2. What occurs to the wavelength when the drill is slowed? (The wavelength increases.)

  3. UV radiation is a relatively short wavelength. It is shorter than visible light. What is the energy of UV radiation relative to visible light? (It has higher energy.)

  4. Why do you think UV radiation is of such great concern? (Because it has so much energy, ultraviolet light in large doses can be damaging.)

  5. We hear so much about ultraviolet radiation these days. What about even shorter wavelengths, such as x-rays and gamma rays, that also come from the sun? Do they cause damage too? (Yes.)

Assessment Ideas

Modifications for Alternative Learners

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