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

• Physical Science, Transfer of Energy, Grades 5 to 8, p. 155, Item #6: "The sun is a major source of energy for changes on the earth's surface. The sun loses energy by emitting light. A tiny fraction of that light reaches the earth, transferring energy from the sun to the earth. The sun's energy arrives as light with a range of wavelengths, consisting of visible light, infrared, and ultraviolet radiation."
  
  
• Physical Science, Interactions of Energy and Matter, Grades 9 to 12, pg. 180, Item #1: "Waves, including sound and seismic waves, waves on water, and light waves, have energy and can transfer energy when they interact with matter."
  
  
• Physical Science, Interactions of Matter and Energy, Grades 9 to 12, pg. 180, Item #2: "Electromagnetic waves result when a charged object is accelerated or decelerated. Electromagnetic waves include radio waves (the longest wavelength), microwaves, infrared radiation (radiant heat), visible light, ultraviolet radiation, x-rays, and gamma rays. The energy of electromagnetic waves is carried in packets whose magnitude is inversely proportional to the wavelength."
  

Benchmarks for Science Literacy, Project 2061, AAAS

• The Physical Setting, Motion, Grades 6 to 8, pg. 90, Item #1: "Light from the sun is made up of a mixture of many different colors of light, even though to the eye the light looks almost white. Other things that give off or reflect light have a different mix of colors."
  
  
• The Physical Setting, Motion, Grades 6 to 8, pg. 90, Item #5: "Human eyes respond to only a narrow range of wavelengths of electromagnetic radiation - visible light. Differences of wavelength within that range are perceived as differences in color."
  
  
• The Physical Setting, Motion, Grades 9 to 12, pg. 92, Item #6: "Waves can superpose on one another, bend around corners, reflect off surfaces, be absorbed by materials they enter, and change direction when entering a new material. All these effects vary with wavelength. The energy of waves (like any form of energy) can be changed into other forms of energy."
  

Grade Level/Time

• Grade level: 6 to 9
  
  
• Time:
  
  
  • Background discussion: 20 minutes
    
    
  • Drill demonstration: 20 minutes
    
    
  • Recommended for students to try out the wave generation with hand power: 20 minutes
    
    
  • Post-demonstration discussion: 15 minutes
    

Materials

• 12 to 15 feet of 1/8" nylon cord
  
  
• 1 foot of 1/8" nylon cord
  
  
• Electric drill and chuck key
  
  
• 1 20-penny bent nail
  
  
• #2 barrel swivels (found in the fishing section of sporting goods)
  
  
• Slinky (optional)
  
  

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

• Have students individually generate waves manually while you observe. Ask them to produce a long-wavelength wave. Then ask them to produce a short-wavelength wave. Ask them to explain the difference in how they produced each wave and what that means in terms of the energy in the wave. Which wave would be most like visible light? Which wave would be more like UV radiation?
  

Modifications for Alternative Learners

• The visual nature of this demonstration lends itself to diverse learners. Accepting oral rather than written explanations of the lab questions may help some English Language Limited students.