Name: ________________________________ Period: _______ Date: ________________
Partners: ________________________________________________________________
The Ripple Tank
Purpose
Another device for studying waves is a ripple tank. It has the advantage over the coiled spring in that
pulses are not restricted to a line. Your lab team will use the ripple tank to observe wave phenomena
and develop properties of waves.
Materials
Wave table
Eye Dropper
2 long straight metal barriers (12 cm)
1 short straight metal barrier (2.5 cm)
2 metal parabolas (DO NOT BEND!)
Timer & Ruler
Plastic container to store wave table accessories
1 roll of paper towels
Safety
This lab uses a strobe light. Students prone to seizures must inform their instructor prior to starting
the lab and may not be allowed to complete this lab. Water and electricity don’t mix well, and wet
floors in a dark room create a slipping hazard. Use the paper towels provided to keep your lab table
clean and dry at all times. If you run out of paper towels, ask your instructor for more. Items placed
in the water tank will get wet. When you finish using a piece of equipment, dry it off before putting it
back in the plastic container.
Be very careful not to rest your arms or wrists on the wave table! The equipment used in this
lab is very fragile, expensive and definitely not designed to support the weight of your arm.
Setting it up
The large grey box is the Ripple Tank Controller. Begin by flipping the orange switch located on the
lower right side of the controller to the on position. This will turn on the halogen lamp above the wave
table and the LCD readout on the controller. The instructions below explain how to use each of the
features on the Ripple Tank Controller. Read these instructions before proceeding with the lab.
Mode Knob:
This is the yellow knob located farthest to the right on the controller. The Mode knob has 3 positions:
1) Continue
By selecting this mode the controller will produce wave pulses in the range of approximately
5.5 to 25hz. Wave pulses are synchronized with the strobe lamp.
2) Pulsed
By selecting this mode the controller will produce wave pulses with a frequency of 1 hz
3) Manual
By selecting this mode it is possible to produce a single wave pulse by depressing the red
pulse button located to the right of the Mode knob.
Amplitude Knob:
Turning the knob clockwise increases the amplitude of the waves produced on all 3 mode settings.
Period Knob:
Turning this knob adjusts the period of the wave pulses produced when the controller is set to
Continue Mode. The period of the wave pulses is shown on the LCD display on the controller.
NOTE: The wave period is displayed in milliseconds (10-3 s).
The Ripple Tank
Section 1: Plane Waves
Background Information: (READ BEFORE PROCEEDING!)
When a single plane wave is produced, a bright line propagating away from the wave generator will
be observed. This is the crest of the wave. The brightness of the light will be the same at each point
on the line because the wave amplitude is the same at each point. The wavelength of a plane wave
is measured as the distance between two successive crests.
Straight Pulses:
Set the Mode Knob to manual. Straight pulses in the ripple tank are generated by pressing the red
button next to the Mode Knob. Do the pulses remain relatively straight as they move along the length
of the tank?
Do the waves appear to travel at a constant speed?
Set the wave period to maximum by turning the Period Knob all the way clockwise. Turn the Mode
Knob to the ‘Continue’ setting. Observe the speed of the wave pulses. Then try gradually decreasing
the period down to approximately 70 ms. Does the period of the wave pulses affect the wave speed?
(Be careful!!! I am not asking you if period affects wave frequency.)
Now I am asking you about wave frequency. How does changing the period of the wave pulses affect
the frequency with which the waves are produced? What is the relationship?
Turn the Mode Knob back to ‘Manual’ and allow the ripple tank to settle. Turn the Period Knob until it
reads approximately 40 ms. Turn the Mode Knob back to the ‘Continue’ setting and observe the
wave pulses produced. Gradually increase the period to approximately 140 ms. How does
decreasing the frequency of the wave pulses appear to change the wavelength of the waves? What is
the relationship?
Straight Reflection:
Use all three straight metal barriers arranged in a single line to create a straight barrier parallel to the
wave generator. Place the barrier at a distance of approximately 20 cm from the wave generator.
Create single wave straight pulses by setting the Mode Knob to ‘Manual’ and pressing the red button.
Observe the wave after it strikes the barrier. Describe it and sketch it. Use arrows to denote wave
direction before and after hitting the barrier, and label the original and reflected wave pulses.
Description:
Sketch:
Angled Reflection:
Move the straight barrier so that the waves strike it at an angle. The closest side of the angled barrier
should be approximately 7 cm from the wave generator. Observe angled reflections using all 3
settings on the Mode Knob before drawing any conclusions. Once you have observed angled
reflection using all three Mode Knob settings, describe the pattern you see and sketch it. Again, use
arrows to show the direction of the original and reflected wave. Complete this procedure BEFORE
making your sketch.
Description:
Sketch:
Reflect the wave pulses at several different angles by turning the barrier. How does the reflected
angle compare with the angle of incidence (the angle at which the wave approaches the barrier)?
Wave Speed:
Set the Mode Knob to Manual. Make sure the Amplitude Knob is set to maximum by turning the knob
all the way clockwise. Again, place your straight barrier parallel to the wave generator at distance of
approximately 20 cm. Using a stopwatch, time how long it takes a single wave pulse to travel to the
straight barrier and reflect back to the wave generator. For the most accurate results, the person
generating the pulses should also do the timing.
Repeat the above procedure, but decrease the Amplitude to approximately half by turning the knob
counter-clockwise. Time the wave and compare it to the bigger amplitude pulse.
Does amplitude change the wave speed? Explain why or why not below.
Remove your straight barriers from the ripple tank, dry them off with paper towels and return them to
the plastic container.
Section 2: Circular Pulses
Circular Pulses:
Use the eye dropper and let a few drops fall into the ripple tank from a few centimeters above the
water surface. What is the shape of the pulses created by the drops of water?
Do these waves appear to travel at a constant speed?
Try squeezing drops from different heights above the tank. Does changing the drop height of the
water droplets affect the speed of the waves produced?
What part of the wave is altered when changing the drop height? How do you know?
Circular Reflection:
Use your three straight metal barriers to again create a single straight barrier in the tank. Place your
straight barrier near the far end of the ripple tank. Using the eye dropper, create a few circular
waves. Describe the shape and sketch the reflection of the circular wave off the straight barrier.
Label the original and reflected wave pulses on your sketch.
Description:
Sketch:
Reflection & Virtual Source:
The reflected pulse appears as if it originated from a point behind the barrier. Can you locate this
“virtual source” of the reflected circular pulse? To help you locate this “virtual source”, first redraw
your sketch from above, but only include the reflected pulse. Extend the arc of the reflected pulses
behind the barrier. Make theses extended lines dashed lines and label the “virtual source”. Make
your sketch below.
Remove your straight barriers from the ripple tank, dry them off with paper towels and return them to
the plastic container.
Section 3: Parabolic Reflection
Positioning the Parabola: (Read before you proceed!!!)
When positioning either parabola in the tank, placing it farther from the wave generator will make it
easier for your lab team to make accurate observations; however, you must be able to see the
shadow of the parabola on the wave table screen.
Parabolic Reflection:
Place the larger of the aluminum parabolas in the ripple tank. Orient the parabola so that the center
of curve of the parabola is facing the wave generator. This is called the ‘convex’ side of the parabola.
Set the Mode Knob to ‘Manual’ and generate a few straight plane pulses. Observe the wave after it
strikes the barrier. Repeat the process with the smaller parabola. What is the shape of the reflected
pulse? Describe the pattern you see and sketch it. (HINT: the reflection is NOT in the shape of a
parabola)
Description:
Sketch:
Replace the smaller parabola in the ripple tank with the larger one, but this time orient the parabola
so that plane wave pulses travelling from the wave generator will travel into the curve of the parabola
(The edges of the parabola will be closer to the wave generator). This is called the ‘concave’ side of
the parabola.
Again, set the Mode Knob to ‘Manual’ and generate a few straight plane pulses. Observe the wave
after it strikes the barrier. Repeat the process with the smaller parabola. What is the shape of the
reflected pulse? Describe the pattern you see and sketch it. (HINT: again, the reflection is NOT in
the shape of a parabola)
Description:
Sketch:
The Focus (of a parabola):
Replace the small parabola with the large oriented with the concave side facing the wave generator.
Again, generate straight pulses that travel to the inside of the curve of the parabola. Find the point at
which the straight pulses reflected by the parabola meet. This is the focus of the parabola. You may
want to try setting the Mode Knob to the ‘Pulsed’ setting and also to the ‘Continue’ setting with
maximum period to help you locate the exact location of the focus of the parabola.
Return the Mode Knob to Manual and let the ripple tank settle. Use your eye dropper to generate
circular wave exactly at the focus of the parabola. What is the shape of the reflected wave pulses
when circular waves are generated at the focus of the parabola? Describe it and sketch it. Be sure
to include the location of the focus of the parabola and the shape of the reflected waves as they exit
the parabola. (HINT: You may want to have a team member mark the location of the focus on the
ripple tank screen it may make it easier to position the eye dropper directly above the focus)
Description:
Sketch:
Use the eye dropper to create circular waves at other points along the curve of the parabola. You will
see that the shape of the reflected pulses is NOT the same as the reflections of circular pulses
generated at the focus. Describe the pattern you see and sketch it.
Description:
Sketch:
Make sure both parabolas are removed from the ripple tank, dry them off with paper towels and return
them to the plastic container.
Section 4: Diffraction
READ BEFORE YOU PROCEED:
The Amplitude Knob must be set to maximum for all parts of Section 4 and Section 5. For the
remainder of the lab, you will be able to make more accurate observations by moving back away from
the wave table a distance of 2-3 meters.
Barrier Diffraction:
Place one of the long metal barriers in the ripple tank parallel to the wave generator at a distance of
approximately 8 cm. Turn the Mode Knob to the ‘Pulsed’ setting and observe the plane waves as
they pass the straight metal barrier. Focus on the edges of the straight barrier. You should see that
the edges appear to act like point sources creating circular waves. Once you see this effect, you are
ready to make observations of the plane waves as they pass the barrier.
You should observe that there is an area behind the barrier in which waves do not propagate
because they have been blocked. This area is called the ‘shadow zone’. You should also observe
that as the waves pass the barrier, they bend into the shadow zone. This bending of waves as they
pass a barrier is called DIFFRACTION.
Adjust the Period Knob to its maximum setting (thus setting the wave generator to its minimum
frequency). Set the Mode Knob to “Continue’ and observe the pattern of diffraction that occurs
beyond the barrier. Don’t forget to move back away from the lab table. Increase the wave frequency
to its maximum by turning the Period Knob to its minimum. You should see a noticeable difference in
the amount of diffraction that occurs between high frequency and low frequency settings. Make a
sketch below showing diffraction with high frequency waves and low frequency waves. Include the
wave pulses before they reach the barrier and show the diffraction of the waves after they pass the
barrier.
Low Frequency Diffraction:
What is the relationship between frequency and diffraction?
What is the relationship between wavelength and diffraction?
High Frequency Diffraction:
Single Slit Diffraction:
Place both of the long metal barriers in the ripple tank parallel to the wave generator at a distance of
approximately 8 cm. Leave a gap between the two barriers that is approximately 5 cm for the waves
to pass through (this is the slit). Set the Period Knob to its maximum value. Turn the Mode Knob to
the ‘Continue’ setting and observe the plane waves as they pass through the slit. You should
observe that the waves appear to spread out as they pass through the slit. This effect is caused by
wave diffraction occurring around both of the barriers.
Decrease the size of your slit from 5 cm to approximately 1.5 cm and observe the differences in the
amount of diffraction that occurs as the waves pass the slit. Make a sketch below showing the
diffraction of the waves passing through both large slit and a small slit. Include the wave pulses
before they reach the barrier and show the diffraction of the waves after they pass the barrier.
Diffraction through small slit:
Diffraction through large slit:
What is the relationship between the size of the slit and the amount of diffraction?
As the size of the slit width decreases, the waves that pass through the slit resemble plane waves
less and less. What do these plane waves appear to become as the slit width becomes very small?
You will use the concepts developed in this section of the lab to help complete the final section.
Leave your barriers in the water.
Section 5: Interference
Background: (Read before you proceed!!!)
When two point sources (circular waves) of the same frequency propagate in the same area, they
superpose and form patterns of interference called Moiré Patterns. These patterns are characterized
by nodal lines forming where the later surface is motionless. The nodes will alternate with peaks that
correspond to points where the waves arrive in phase with each other.
Interference
Add the small metal barrier to the ripple tank and place it in between the two large metal barriers
already in the tank. Position all three barriers parallel to the wave generator at a distance of
approximately 5 cm. Use your three barriers to make a double slit with the slit. The distance
between barriers (slit width) should be approximately 1 cm. Set the period of the wave generator to
approximately 80 ms.
Due to the diffraction of the waves as they pass through the slits, they will now act as point sources.
Set the Mode Knob to ‘Continue’ and observe the interference pattern that occurs. Specifically, look
for the nodal lines that form as the waves overlap. You will see these on the ripple tank screen as
lines that do not flicker because along these lines the water surface is calm. Make a sketch below of
the interference pattern seen on the ripple tank screen.
In the above sketch, label with an arrow an example of constructive interference and destructive
interference.
Remove your straight barriers from the ripple tank, dry them off with paper towels and return them to
the plastic container along with any other materials that have not been put away. Use the paper
towels to dry off any wet areas on your lab table and the surrounding floor, and turn off the power on
the Ripple Tank Controller. Congratulations!!! You have just completed the Ripple Tank Lab.