NAME______________________________________
WAVES
Introduction
A wave is a disturbance caused by the movement of energy from a source through some medium
(solid, liquid or gas). An ocean wave is the alternating rise and fall of a portion of water. Waves
can be produced by the wind, gravitational pull of the moon and sun, or seismic activity. When a
wave is produced, energy is transferred from the source (wind, for example) to the water.
Energy is transferred from particle to particle of water and the wave form moves. Waves that
move from the area where they were formed are called progressive waves. This laboratory will
familiarize you with the characteristics of progressive ocean waves.
Wave Parameters
All waves have certain features that can be measured and used to describe the wave: wave
height, wavelength, amplitude, period and frequency. From these measurements, the speed of
the wave (celerity) can be calculated.
Wave crest = highest part of the wave above the average water level
Wave trough = the valley between wave crests below average water level
Wave height (H)= the vertical distance between a wave crest and adjacent trough
Wave length (L)= horizontal distance between successive wave crests (or any identical points)
Amplitude = the vertical displacement of water above or below the standing water level. It is
equal to half the wave height (A =1/2 H)
Wave period (T) is the time (in seconds) it takes for two successive crests (or one wavelength)
to pass a given point
Wave frequency (F) is the number of waves passing a fixed point per second. Frequency is the
inverse of period (F = 1/T or T = 1/F).
Celerity is the speed at which the wave form travels. It can be calculated by dividing
wavelength by period (C = L/T)
Exercise 1 – Measuring Wave Parameters
1. Partially fill the wave tank with water. Place the wave tank so that one end is raised and 10 to
20 cm of the bottom of the tank is exposed on the higher end. This exposed bottom will act as a
shore and allow the waves to break. Make all measurements about midway between the shore
and the wave source.
2. With a china-pencil, lightly mark the standing water level along the side of the tank.
3. Using an ice cube bin as a plunger, raise and lower the bottom of the bin in the water every ½
second at the deep end of the tank to generate regular waves. Do not let the bin come out of the
water or hit the bottom of the tank.
4. As a wave passes through the mid-point of the tank, mark the crest and trough heights. Stop
the waves. Measure the wave height in centimeters and record the value in the data table below.
Measure the depth of the water at the point where you took your measurements and record it.
5. Generate more waves as before. At exactly the same time you mark the crest of one wave,
have your lab partner mark the crest of an adjacent wave. Measure and record the wavelength.
6. Determine the number of waves that pass the midpoint of the tank in 30 seconds. Determine
the frequency for one second (divide by 30). Record the data. Repeat this three times and
calculate and record the average frequency.
7. Calculate the period of the waves using the formula T = 1/F. Record the data.
8. Calculate the celerity of the waves using the formula C = L/T. Record the data.
9. Repeat steps 1 – 8, this time generating waves every two seconds.
Wave Parameter
½ second wave
2 second wave
Height
Amplitude
Water Depth
Wavelength
Frequency –trial 1
–trial 2
–trial 3
Average Frequency
Period
Celerity
Wave motion in Deep and Shallow Water
In deep water the wave form moves rapidly but individual molecules of water do not. As the
wave form passes through a parcel of water, molecules of water move forward and up with the
crest and then backward and down with the trough. The circle paths followed by the water are
called orbits. The wave form moves because energy is transferred from one water molecule to
the next in these orbits. The diameter of the orbit at the surface is equal to the wave height. The
orbit’s diameter decreases with depth.
The maximum depth to which a passing wave imparts motion is equal to ½ the wavelength.
Below this depth, the water is not affected by the wave and will not move. Deep water for a
given wave is defined as water having a depth greater than 1/2L for that wave. Whether a wave
acts as a deep-water wave or not depends on the wavelength and the depth of water that the wave
is moving through. Waves will move toward shore and go from being deep-water waves to
being shallow water waves. Celerity of deep water waves can be calculated using the
following formula:
C= 1.56T
Shallow water for a given wave is defined as a water depth of less than or equal to 1/2L. In
shallow water, frictional drag form the bottom slows the wave and distorts the circular orbits into
flattened ellipses. The ellipses become flatter with depth and at the very bottom, the water
simply moves back and forth. As waves move into shallow water, the wavelength decreases and
height increases as waves are squeezed together. Eventually the waves become too high for their
length and they break. When waves break, the orbit is broken and remaining wave energy is
expended, transporting water onto the shore. Gravity causes the water to return to the ocean.
Celerity of shallow-water waves can be calculated as: C = 3.1 d (where d = water depth)
Exercise 2 Calculating Celerity of Waves
1. Using the data from Exercise 1 of this lab, determine if the waves generated were deep- or
shallow-water waves at the point where measurements were made.
½ second wave _________________
2 second wave___________________
2. Using the appropriate formula (given above), calculate the celerity of each wave in cm/sec:
½ second wave _________________
2 second wave___________________
3. Compare the results with those obtained in Exercise 1. How well do they compare? Why
might they differ?
Exercise 3 Movement of Water in Waves
Part A:
1. Using the wave tank set-up from Exercise 1... Place a small cork in the center of the tank,
about 20 cm from the bin used to generate waves. Mark the position of the cork on the side of
the tank.
2. Generate waves by raising and lowering the bin every ½ second as in Exercise 1. Continue
this for about 45 seconds or until the cork becomes stuck to the side of the tank. Mark the final
position of the cork on the side of the tank. Count the number of waves generated and measure
the distance the cork travels.
Number of waves___________________
Distance cork travels_______________cm
Describe the motion of the cork in the water.
Did the cork move forward at the speed of the wave?
How far did the cork move with each wave (divide the total distance traveled by the number of
waves)?
_________________cm
If the particles move in circular orbits, why does the cork move forward?
Part B:
1. Place the cork in the center of the tank, about 10 cm from the shore.
2. Generate waves as in Part A. Continue until the cork is stranded on the beach. Start again if
the cork becomes stuck to the side of the tank. Count the number of waves and measure the
distance the cork travels.
Number of waves___________________
Distance cork travels_______________cm
Describe the motion of the cork in the water.
How far did the cork move with each wave (divide the total distance traveled by the number of
waves)?
_________________cm
How does this distance traveled per wave compare to what was determined in question Part A?
Why is there a difference?