Astronomy Wave Lab
The goal of this exercise is to gain a basic understanding of the properties of waves. All types of waves share certain characteristic behaviors. Therefore the characteristics that you observe in this lab are valid for light waves, sound waves, radio waves, etc.
Open the program Waves on a String from PhET. Make sure it is the HTML5 version (control sliders will be at the bottom of the screen, not the top). The program allows you to control and measure a simulated wave in a ÒstringÓ of ÒparticlesÓ represented by red and green balls connected by a string. There are a variety of controls and options so read the directions carefully.
For all of the following exercises click on the ÒNo EndÓ option so that the waves appear to go out of a window at the right edge of the screen.
Source and Medium
A wave is a disturbance that propagates through a medium. Whatever is being disturbed is called the medium of the wave. All waves require a source. The source is what initiates the disturbance that travels through the medium.
o Experiment with these options: Manual, Oscillate, and Pulse. Notice that when in Oscillate or Pulse mode there are additional controls at the top of the screen for Amplitude and Frequency or Pulse Width. These are all of the controls for the source of the simulated wave. Experiment with all of these options and notice the results.
o Experiment with the controls for Damping and Tension. These sliders control the simulated characteristics of the ÒstringÓ and therefore these are the controls for the medium of the simulated wave.
Measuring the Wave
In the simulation there are tools for measuring distance and time. You will be asked to measure various properties of the waves that you create.
o Set the source to oscillate and then experiment with the Pause/Play button. Pausing the simulation allows you to Òfreeze timeÓ, which can be very helpful when making a measurement. You can also choose to run in Slow Motion.
o With the simulation paused, try the Step button. Notice this allows you to go Òslow motionÓ through a simulation.
o Click on the Rulers box. Notice that two rulers appear: a horizontal and a vertical. You can click and drag on these rulers to measure anything you want. Try it!
o Click on the Timer box. Notice that a virtual stopwatch appears. Try starting and stopping the stopwatch.
o Click on the Reference Line box. This line can be dragged around the screen and can be helpful in trying to judge the extent of the waveÕs disturbance of the string.
o With the stopwatch running, try pausing the animation or using the slow motion option. Notice that this pauses and slows the stopwatch as well as the wave – this can be very helpful and allows you to precisely time events on the screen.
Wave Parameters
There are five basic characteristics or parameters that make a wave unique: frequency, period, amplitude, wavelength and speed. Using the program you will observe and measure eight different simulated waves. Based on these results you will summarize your findings by answering 4 questions.
The directions below give specific details how to use the program to complete the table. Read the directions closely and compare your results to those shown in the first row of the table.
Set the source to oscillate and use an amplitude of 1.00 cm and a frequency of 2.00 Hz. Set the damping to zero and the tension on 10 (high). Continue to use the No End option that shows waves going Òout the windowÓ.
o Pause the animation and use the rulers to measure the distance from the dashed centerline of the wave to the peak of a crest or trough. The reference line may be handy for this. Amplitude is defined as the maximum level of disturbance, measured from equilibrium.
o With the animation still paused, measure the horizontal distance from one crest to the next crest. Also try measuring from one trough to the next trough. This is the wavelength. Wavelength is defined as the distance for one complete cycle (measured along the line of travel).
o Now use the timer to measure the time for one of the green balls to go down and back up to its original position. This is the period. Period is defined as the time for one complete cycle. (Note: remember you can pause the animation and step through time or use slow motion. This makes it easier to get precise timing of events onscreen.)
o Repeat the above for a different green ball or for any other ball in the ÒstringÓ. You should find the same result.
o Calculate
the frequency of the wave by taking the reciprocal of the period. 
Frequency is defined as the number of cycles per unit time and it is often
given in units of Hertz (Hz), which is equivalent to one cycle per second.
Is the frequency of the waveÕs particles the same as that of the source?
o Calculate
the speed of the wave by calculating distance divided by time. 
Use the ruler and the timer to measure the distance and time for one particular
crest of the wave moving across the screen from left to right. For
example you can pause the animation, place the ruler at a crest, and then step
forward to determine how far it moves in precisely one second of time.
o Calculate
the speed of the wave by calculating the product of frequency and wavelength.
Do you get the same result as found by dividing distance by time? You should!
(There is nothing to measure here because you have already found the frequency
and wavelength.)
Now use similar techniques to observe other waves and fill in the table.
Questions
1. Changing
the properties of the source of the wave can affect all of the wave parameters
except one. Which parameters are not affected by the source?
Hint: look at values in the data table. What happens if frequency
and/or amplitude change but tension does not?
2. Changing
the properties of the medium affects some of the wave parameters, but not
all. Which parameters are not affected by the medium? Hint:
look at values in the data table. What happens if tension changes but
frequency and amplitude do not?
3. Without
changing the medium, what is the effect of increasing the frequency of the
source of the wave? Describe the resulting changes in period and
wavelength if the frequency is increased. (e.g. if frequency
doubles what else changes and by how much?)
4. Without changing the source of the wave, what is the effect of increasing the tension in the medium? Describe the resulting changes in the wavelength and speed if the tension is increased. (e.g. when tension decreases which parameters decrease? increase? stay the same?)