Ripple tank apparatus

1. { ThE SEIENEE
SBUREE,Ripple Tank, Large #L54I"5
OpaRATING INSTRUCTIONS
Purpose:
To investigate the motion of waves and its application to reflection, refraction,
interference, and diffraction using surface waves in water of variable depth and surrounded by
energy absorbing beaches to reduce troublesome reflections.
Contents:
One (l) Ripple Tank One (1) Parabolic reflector
. One (1) Glass Plate Four (4) Paraffin blocks
One (1) Plastic Viewing Screen Three (3) Spacers
Four (4) kgs with levelers Two (2) Angled aluminum rods
One (1) Ripple Bar Two (2) Poppet beads
Two (2) Ripple Bar hangers Two (2) Rubber Bands
One (1) Light Support Bar One (1) Motor Assembly with leads
Four (4) Foam wave dampers One (l) Dowel (for generating straight
One (l) Rubber Stopper pulses)
Materials Supplied under The Science Additional Required Materials:
Source #15415 A&8, Required of all others:
One (1) Variable voltage power supply
One (1) High Power Light Source (0 to 6 VDC)
(The Science Source #14700) One (1) Stopclock or watch
Two (2) Adjustable Hand Strobes One (1) Meter Stick
(The Science Source #l45OZ) .. One (1) Variable Phase Wave Generator
(The Science Source #15490)
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Figure 2
Assembly diagram for ripple tank.
Assembly:
1) Prior to assembly, and each subsequent use of the ripple tank, clean both surfaces of the
tank's glass bottom.
2) Adjust the levelers on each of the four legs so that all legs are the same length. Screw one leg
into each corner of the underside of the ripple tank frame. Tighten firmly by hand.
3) Spread the white plastic viewing screen oti a smooth level surface (table top or floor). Set the
ripple tank on this sheet.
4) Irvel the tank by pouring approximately 5 to 7 mm of water into the bottom of the tank.
. Adjust the leveling screw on each leg until the water depth is constant at all locations in the
tank. When the tank is level, lock the adjustable feet in position by tightening the hex nut on
each foot.
5) Thread the light source support rod into the center hole on the back edge of the ripple tank
frame. Clamp a high power light source onto the support rod near the top, using the
Pole CatrM included and described with the High Power Light Source. The light source
should provide a point source for best results.
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3. 7)
6) Insert the threaded end of a ripple bar hanger into the holes in each side of the ripple tank
frame. Suspend the ripple bar from the hangers with rubber bands. Adjust the heigtrt of the
hangers until the ripple bar (or poppet beads) just touch the surface of the water. Fasten the
hangers in place with wingnuts.
Wet the foam dampers under running water. Squeeze them during this process to insure they
are thoroughly wet. Place the wet dampers in the ripple tank, one along each edge of the
frame. Use care to install them proper side up if they are to fit well at the corners
A 5 cm strip of Yz inch masking tape in the bottom of the tank serves as a convenient
focusing reference for the light source. Adjust the height of the light source on the support
rod to obtain the sharpest image of the focusing strip on the viewing screen. This adjustment
will provide the sharpest image of the wave patterns generated with the ripple bar. The
focusing strip also provides a handy guide for calculating the actual wavelength of the water
waves:
Meas",
I-ength of focusing. strip
Measured length of strip shadow(screen)
9) The four paraffin blocks are utilized to make sharp-edged barriers and apertures (slits) in a
number of experiments. Blocks may be cut to a variety of lengths and shapes with a saw or a
utility knife.
Pulses
A good place to begin with a ripple tank is to investigate single waves of water or pulses.
Lightly touch the surface of the water near the middle of the tank. Study the wave you'r" created.
Notice that the wave front is circular and travels outward with a constant speed from the point
where your finger touched the water. Does the.speed of the pulse depend at all on how quickly
your finger contacted the water? You should find that the speed of the pulse is constant
regardless of the way you touch the water.
Place the dowel in the water and move it (roll it) forward a small amount. Notice the shape
of the wave front. This time you should see a straight wave (or a plane wave) travel across the
tank. Is the speed of this wave front the sameas the circular wave? Does the speed of the wave
change if you push the water more or less quickly with the dowel? If the speed doesn't change,
what does change? If you push the water more quickly with the dowel, the size or amplitude of
the wave will be larger but the wave's speed will remain the same.
8)
1.
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4. Place a long flat obstruction near the middle
of the tank. This can be constructed by laying
paraffin blocks in a row across the tank. Make a
wave pulse by dipping your finger into the water.
What happens as the pulse strikes the barrier?
What does the reflected wave look like? Is it
circular like the incident pulse? Try different
places. Try to locate the virtual source (behind
the barrier) of the reflected pulse. How does the
angle of incidence compare with the angle of
reflection? Remember that the wave front is
moving outward at the same speed in all
directions from the point where you touched the
water.
Figure 2
Reflection of circular waves.
use the dowel to try plane waves at different
angles. How does the angle of incidence
compare with the angle of reflection?
Try replacing the straight barrier with a
length of metal bent into the form of a parabola.
Create a straight pulse that is incident along the
axis of the parabola. How is this straight pulse
reflected from the surface of the parabola? Do
the reflected waves meet in a single point? This
intersection point is referred to as the focal
point for this parabolic reflector. Reflect a few
straight pulses from the parabolic mirror and try
to locate this focal point. When you have
located this focal point, create a circular pulse
by sticking your finger into the water at this
point. What do the reflected waves look like?
Are they straight waves? Large telescopes use parabolic mirrors (reflectors) to collect light that is
traveling as straight waves from stars far away and focuses this light on a single point. Radio
antennas are also parabolic and collect "light" in the form of radio waves. Another example of
parabolic reflectors are television dish antennas used to receive radio waves from satellites that
transmit television programming. Flashlights and automobile headlights use parabolic reflectors
to take light from the bulb that is located at its focal point and reflect it outward in a strong
parallel beam.
2. Reflections
Figure 3
Reflection of straight waves.
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5. 3. Traveling waves
All periodic waves can be described by the relation
v=l,v
where the speed of the wave
the frequency of the wave
the wavelength
This relationship can be investigated using the ripple tank. To generate periodic waves set up
the straight wave generator, connect the motor to a variable DC source, and add water to a depth
of about 5 to 8 millimeters. Practice using an adjustable stroboscope until you can "stop" the
motion of the waves. l,ower the frequency of the wave generator and have a partner help you find
the frequency of the stroboscope's rotation as you visually "stop" the wave motion. You are
interested in finding the number of stroboscope slots that pass in front of your eye in a given
period of time as you view the "frozen" wave pattern. For example, 10 slots per second is a
frequency of 10 Hertz. The frequency of the stroboscope should be measured from the slowest
speed required to "stop" the wave motion. This frequency will then correspond to the frequency
of the wave motion. Can you explain why?
Once you have visually stopped the waves with the stroboscope, have your partner position
two pencils or rulers on the screen below the ripple tank; the pencils being parallel to the waves
and several wavelengths apart.
To measure the wavelength you must measure the distance between the pencils, count the
waves that fall between them, and then account for any difference in scale between the image on
the screen and the actual waves in the tank. This can be done by the method described the set up
procedures above. Calculate the wave speed for several different values of wave generator
frequency. I{ow do these values compare?
You may wish to repeat this experiment using different water depths. Try measuring the two
extremes of deep and shallow water. When you do this, see if the wavelength or velocity change
with water depth, if it does, how does it change?
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6. A
Deep Water
In our earlier experiments, we found that the speed of the water waves varied with the depth
of the water. Therefore, water of different depths represent media with different indices of
refraction, similar to the differing indices of refraction for air and glass or glass and water.
We can study refraction using a ripple tank by placing a glass plate in the ripple tank. Use spacers
to raise the surface of the glass plate above the bottom of the tank by at least 1.5 centimeters.
When you fill the tank with water, be sure that the glass plate is covered by no more than 2
millimeters of water and that the water depth is uniform over the entire plate.
First, align the long edge of the glass plate so it is parallel to the ripple bar. Can you predict
what will happen to the waves as they move from the deep water to the shallow water? Will the
frequency change? Will the wavelength change? Will the speed change? Use a low frequency
setting on the wave generator and test your predictions.
Next, turn the glass plate so the incident waves will strike the boundary at some angle. Do
the waves travel straight over the glass plate or do they bend (or refract) when they cross the
boundary between the two regions? Keeping the frequency of the waves constant, measure the
angle ofrefraction for several different angles ofincidence.
Figure 4
Straight waves entering a different media.
4. Refraction
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Figure 5
Refraction of straight waves.
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7. Place a small smooth paraffin block in the ripple tank about l0 cm in front of the straight
wave generator. Do the waves continue in their straight path on both sides of the block? How far
behind the block does its "shadow" remain in existence? Change the frequency of the wave
generator and observe the region near the edges of the block. How is the shadow affected by
increasing the frequency? Decreasing the
frequency? what frequency range will produce
the sharpest shadow?
Replace the block with an open slit. This can
be constructed by placing two lengths of paraffin
across the tank and leavin g a small gap between
them near the middle of the tank. When straight
waves are incident on the opening, what do the
waves exiting the opening look like? Are they ititt
straight waves, or are they curved? Keeping the
frequency constant, change the width of the
opening. How does this affect the shape of the
exiting waves? Repeat this experiment using
several different frequencies for the incident
wave.
6. Interference
Figure 6
Waves blocked by a small object.
5. Waves and obstacles
Figure 7
Waves passing through a narrow slit.
Figure 8
lnterference pattern in ripple tank
Replace the straight wave generator with a point source generator. How will two circular
waves of the same frequency interact? Place the point sources approximately 5 centimeters apart
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8. and start the generator. How would you
describe the resulting wave pattern? Notice
lines of cancellation where the wave crests of
one source align with the wave troughs of the
other source. How does this pattern change with
increasing frequency? Do the nodal lines move
closer together or further apart? What is the
effect of moving the points closer together?
Notice the inverse relationship between the
frequency and the width of the interference
pattern, &s the frequency increases the pattern
width decreases. As the spacing of the two
sources decreases the pattern width increases.
By applying the principles of superposition, it
can be shown that the direction of the n'h
maxima is given by:
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1 0 xl
{<
=n,[ L.l
sin(@,) =xlL = n'.1"/d
where @n is the angle to the n6 maxima, x is distance from the center of the pattern to the nft
maxima, L is the distance from the sources to the "screen", n is the maxima number beginning
from the center of the pattern, l, is the wavelength used, and d is the distance between the two
point sources. Use the above formula (also for Young's double slit experiment) by measuring x,
L, n, and d and calculating the wavelength. Compare this calculated value of the wavelength to
the measured value.
How would a change in the phase of one point source with respect to the other change the
resulting interference pattern? For this experiment you will need an variable phase wave
generator to replace the ripple bar. Begin with the two point sources in phase and generate an
interference pattern. While the generator is running change the phase of one of the sources.
Notice how this affects the pattem. Does the spacing between nodal lines change?
Time Allocation:
To prepare this product for an experimental trial should take less than twenty minutes. Actual
experiments will vary with needs of students and the method of instruction. While any one is
easily concluded within one class period, several periods would allow for a full exploration.
Feedback:
If you have a question, a comment, or a suggestion that would improve this product, you may call
our toll free number 1-8fi)-299-5469, or e-mail us: info@thesciencesource.com. Our FAX
number is: 1-207-832-7281.
Figure I
Geometry of constructive interference,
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