Printer Friendly

Activity: Wave Interference

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

Materials

  • Fig. 4.12
  • Long wave tank
  • Two paddles that fit snugly in the width of the wave tank
  • Water
  • Six rulers
  • Masking tape
  • Metronome or sound recordings of 100 beats per minute and 120 beats per minute
  • Construction paper
  • Pencil
  • Dry-erase marker, felt-tip marker or grease pencil
  • Scissors
  • Towels

<p><strong>Fig. 4.12.</strong> Long wave tank with two paddles (This image is not to scale; the paddle, paddlestop, and ruler have been enlarged relative to the size of the tank.)</p><br />


Procedure

Safety Note: If your wave tank is sliding on the table, place towels underneath it. Immediately mop up all water spills to prevent slipping.

A. Practice generating standing waves

  1. Set up the long wave tank as shown in Fig. 4.12.
    1. Fill the wave tank about half full with water.
    2. Tape two rulers, one to each end of the wave tank, to serve as backstops. The backstops should be positioned to prevent the paddles from going past vertical.
    3. Tape two more rulers along the top edge of the wave tank (the yellow rulers in Fig. 4.12)
    4. Tape the last two rulers 5 cm in front of the backstops as the paddle-stops. The paddle-stops will help you control the amount of water pushed by limiting the distance the paddle can move.
       
  2. Set your paddles in the paddle groves and practice generating standing waves from both sides of the tank. Recall that standing waves do not advance; they appear to move up and down in place.
    1. Take turns creating waves. Observe as others create waves and let the wave maker know when a standing wave has been created.
    2. Determine the best method for creating a standing wave.
       
  3. Experiment with creating waves that interfere with each other by generating waves from both sides of the wave tank at the same time.
     

B. Make watermarked profiles of standing waves of different frequencies generated from different sides of the wave tank.

  1. Make a watermarked profile of a standing wave at a frequency of 120 beats (waves) per minute.
    1. Use the metronome or sound recording to generate waves at a consistent and accurate frequency of 120 waves per minute. The paddle should hit the front paddle-stop once per beat.
    2. Use your best method for forming a standing wave from Part A Step 2.
       
  2. Print a watermark profile of the standing waves.
    1. Tape construction paper together to make a long sheet two-thirds the length of the wave tank.
    2. Mark a section of your wave tank for the edge of your watermark profile print, so you can line up the edge of your construction paper in the same place for each of the following prints.
    3. Hold the paper lengthwise near the top corners just above the water level.
    4. To make a wave profile, quickly but carefully dip the paper in and out of the water. The watermarks on the paper will show the profile of the waves at the instant the paper was dipped into the water.
    5. Trace the profile of the waves with pencil before the paper dries.
    6. You may have to repeat this step until you get a good wave profile.
       
  3. Record the wave properties of your wave watermark profile.
    1. Measure the height of the wave and the wavelength.
    2. (Optional) Print another watermark profile if you are not satisfied with your first print.
    3. With scissors, cut out the wave profile.
       
  4. Repeat Steps 4–6, but produce a profile of a standing wave from the opposite end of the tank at a frequency of 100 beats (waves) per minute. Remember to line up the edge of your construction paper in the same place on the wave tank for your watermark profile.
     
  5. Produce a profile of the interference pattern formed when wave sets intersect.
    1. Simultaneously generate wave sets from each end of the tank, with the settings and frequency the same as in Steps 4 and 7 (120 and 100 waves per minute).
    2. Line up your construction paper with your mark, and make a watermarked profile, as described in Step 5.
    3. With scissors, cut out the wave profile.
       
  6. Compare the interference pattern profile with the left wave set profile and the right wave set profile. Record your observations.
    1. Look for waves on the interference pattern profile that are about the same size and shape as the waves in either the left or the right wave set profile.
    2. Slide the cut-out profiles of the wave sets from the left and right sides over the interference pattern profile to compare profiles.
    3. Compare wave height and wavelength of the left and right wave sets to the interference wave set.
    4. Note whether there is any regular pattern to the occurrence and height of waves in the interference pattern.
    5. Decide whether the component left and right wave set patterns can be distinguished in the more complex interference pattern.
Activity Questions: 
  1. What happens to the size of waves when crests from two wave sets come together? Make a sketch to help illustrate your answer.
     
  2. What happens to the size of waves when two troughs coincide? Make a sketch to help illustrate your answer.
     
  3. What happens to the size of waves when the crest of one wave and the trough of another wave coincide? Make a sketch to show your answer.
     
  4. How does this activity relate to waves in the ocean?
     
  5. Refer to Fig. 4.13 to answer the following questions.
    1. What might the wave pattern look like as all the wave sets shown in Fig. 4.13 converge on the central square? Draw the pattern in the center square.
    2. What do the wave sets look like after they pass each other? Draw each wave pattern in the correct square.

<p><strong>Fig. 4.13.</strong> Converging wave sets</p><br />

Table of Contents:

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.