Wave at Me
Teacher’s Notes
Wave at Me is a concept development activity that identifies transverse and longitudinal pulses and waves, can be used to define crest, trough, compression, and rarefaction and leads to a discussion of the terms amplitude, frequency, wavelength, period and interference. The primary emphasis of this lab is that the speed of the wave depends only on the medium through which the wave travels. Many students confuse wave speed with frequency, believing that if the wave vibrates faster it travels faster.
Students should recognize the need to measure distance and time to compare velocities of the various pulses or waves. Most students should be able to organize their own data table that includes the following information: description of the variable, distance pulse traveled, time for pulse to travel indicated distance and pulse velocity. Students should determine that longitudinal pulses travel faster than transverse pulses; as the stretch in the spring is increased the velocity of the pulse increases, the shape of the pulse has no effect on the velocity and for a given stretch as the frequency of vibration increases the wavelength decreases. Students will be able to make better observations if they work in groups of at least three students. Be sure to warn the students to not overstretch the springs or release either end of a stretched slinky since this causes considerable tangling of the coils.
If a pulse is reflected from a rigid support it will reflect along the slinky on the opposite side. However, if the pulse reflects from a junction attached to a rope or less rigid material than the slinky, the pulse will reflect on the same side of the slinky. This is analogous to light reflecting from the boundary-y from a less optically dense medium to one of greater optical density where there is a 180' phase shift. When light reflects from the boundary-y going from greater to lesser optically dense material there is no phase shift in the light wave. You can show by generating two pulses of different shapes that the pulses do not reflect but rather pass through each other. Where the pulses meet the lateral displacement (amplitude) is greater than the amplitude of either pulse. When generating standing waves it should be seen that the wavelength and frequency are
inversely proportional to each other. In fact, the product of the wavelength and frequency is equal to the velocity of the pulse.
Answers to questions within the lab:
What happens to the shape of the pulse as it travels down the spring?
Ans: The pulse maintains its shape but loses some of its amplitude.
How does the speed of a longitudinal wave (pinched coils) compare to the speed of a transverse wave (pulled to the side) for the same stretch?
Ans: Longitudinal waves travel faster than transverse waves because the medium vibrates in the same direction as the wave travels.
How does changing the amplitude affect the speed of the wave?
Ans: If the students use a cord or rope, changing the amplitude will not affect the speed. If they use a coil or slinky the speed will change because stretching the spring changes the medium.
How does changing the medium affect the speed of the pulse?
Ans: The speed of a wave increases when the spring or slinky is stretched tight.
What happens when the pulse arrives at the boundary between the two media?
Ans: Part of the wave is transmitted and part is reflected. The transmitted pulse is in phase and the amplitude is smaller. The reflected pulse will be smaller
Is it the same when sent from the other end?
Ans: A portion of the wave will still be transmitted and a portion will be reflected, the reflected portion will have a phase change.
How do the phases of the original and reflected pulses compare:
· when the reflection is from a rigid barrier (hold one end of the spring tightly)?
Ans: The reflected pulse will be out of phase from the original pulse.
· when the reflection is from a non rigid barrier (hold one end of the spring loosely with a string)?
Ans: The reflected pulse will be in phase with the original pulse.
What happens when they meet? Do the pulses appear to pass through each other or do they “bounce back” from the collision with another pulse?
Ans: The pulses pass through one another
How does the maximum displacement where the pulses meet compare with the displacement of each pulse?
Ans: If the pulses are on the same side the displacement is larger than either pulse alone. (Constructive interference) If the pulses are on opposite sides the displacement is smaller than either pulse alone. (Destructive Interference)
Data table
Wave segments
/Wavelength
(m) /Frequency
Waves/sec / Wavelength x frequency m/s1 / 8 / .7 / 5.6
2 / 4 / 1.4 / 5.6
3 / 2.7 / 2 / 5.4
4 / 2 / 2.8 / 5.6
5 / 1.6 / 3.6 / 5.8
How are wavelength and frequency in a given medium related?
Ans :As the frequency increases the wavelength decreases, but the wave speed remains the same. The relationship is an inverse relationship.
Jan Mader and Mary Winn
Diagram from clip art Hewitt Drew It ™”, Laserpoint, 1328 W. Palo Alto, Fresno, CA 93711