An example of a longitudinal wave is a sound wave. So for example when we listen to music, the speakers send out longitudinal waves through the air which then reach our ears so we can hear the playing music.
The second type of wave is a transverse wave. Transverse waves move up and down so when demonstrated with a slinky spring, your hand movement would obviously be moving up and down as shown below.
Examples of transverse waves are light waves and also water waves, which are the waves that we are investigating in this experiment. The vibrations in these waves are perpendicular to the direction of which the waves travel.
Transverse waves have very important features which, can be shown in the diagram below.
The amplitude of a wave motion is the maximum disturbance caused by waves. The wavelength is the distance between corresponding points on two successive disturbances. Finally the frequency is the number of waves produced in one second.
The relationship between wave speed frequency and wavelength is as follows.
Wave Speed (m/s) = Frequency (Hz) x Wavelength (m)
This equation shows that for a wave that travels at a constant speed, the frequency of waves is inversely proportional to the wavelength.
All waves including light waves, water waves and sound waves can be reflected, refracted and diffracted. Waves are reflected when a barrier is placed in their path. Incident waves are sent out which then hit the barrier, the waves then bounce off (reflect) the barrier at an angle and therefore give off reflected waves. Diffraction is simply when waves move through a gap, as the waves diffract they spread out from the edges. Refraction is a movement, which is relevant to all waves. Refraction occurs when waves cross a boundary between one medium and another (of different density). Therefore there is a change of speed in the waves, which causes the waves to change direction. At the moment however, the most important type of wave to us is refraction, as this is the wave that is being used in our experiment.
Finally after researching all of this information, I can be certain that when the water depth changes, the wave speed will also change. Wave speed changes depending on the depth of water because the waves are controlled by gravity. The formula v² = gd proves that the deeper the water is, the faster the waves travel.
Equipment List
The Equipment being used in this experiment is as follows
- Plastic container (39cm in length)
- Stopwatch
- 1 metre ruler
- Unlimited amount of Water
Fair Testing
In an experiment like this one, testing can never be completely fair, but I can try to make it as fair and as error free as possible. Firstly I will measure exactly the depth of water that I need and make sure that I do not even go over or under by 1mm. Next, I need to keep a steady eye on the waves as they can travel quite fast and can sometimes be confusing as they get smaller. Therefore I have used my partner in the experiment to watch closely at when the final refraction takes place. Finally I will try to be as close as possible when stopping the stopwatch at the final refraction to ensure that this experiment is as fair as possible
Method
In the experiment, I am going to test six different depths of water, so therefore I will be repeating each depth of water three times which gives me a total of 18 results. Firstly, take the plastic container and measure it one cm with the metre ruler. Once filled to the exact depth, I will lift one end of the container and lower it to make a small wave, when the box is lowered to the desk, the stopwatch will start. I will watch the wave refract three times (this is the equivalent to the wave travelling up and down the container six times.) When three refraction’s have taken place I will stop the stopwatch and record the time. I will then repeat this three times, and then move on to a depth of 2cm, then 3cm, then 4cm, 5cm and finally 6cm. These tests will also be repeated 3 times and averaged. Finally once all the results are recorded in a table; I will be able to use the formula to find out the wavespeed.
The Preliminary Experiment
In my method above, I mentioned lifting the plastic container and lowering it again to create a wave. During the experiment, I started to wonder if the height you lift the container up would affect how fast the waves were to travel. Therefore I planned a smaller scale preliminary experiment to find out whether the height did affect it. I started of a height of 5cm and increased by 1cm each time, finishing at 10cm. I used a constant depth of 3cm. Below are the results that I found…
As you can see from these results, there is hardly any change in time when altering the height of the container and therefore the height does not affect the wavespeed. This is why I will not be altering the height in the main experiment, as it will have no effect.