ripple tank apparatus
TRANSCRIPT
{ ThE SEIENEESBUREE,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 byenergy 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 BandsOne (1) Light Support Bar One (1) Motor Assembly with leads
Four (4) Foam wave dampers One (l) Dowel (for generating straightOne (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 supplyOne (1) High Power Light Source (0 to 6 VDC)
(The Science Source #14700) One (1) Stopclock or watchTwo (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 2Assembly diagram for ripple tank.
Assembly:
1) Prior to assembly, and each subsequent use of the ripple tank, clean both surfaces of thetank's glass bottom.
2) Adjust the levelers on each of the four legs so that all legs are the same length. Screw one leginto 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 theripple 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 thetank. When the tank is level, lock the adjustable feet in position by tightening the hex nut oneach foot.
5) Thread the light source support rod into the center hole on the back edge of the ripple tankframe. Clamp a high power light source onto the support rod near the top, using thePole CatrM included and described with the High Power Light Source. The light sourceshould provide a point source for best results.
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6) Insert the threaded end of a ripple bar hanger into the holes in each side of the ripple tankframe. Suspend the ripple bar from the hangers with rubber bands. Adjust the heigtrt of thehangers until the ripple bar (or poppet beads) just touch the surface of the water. Fasten thehangers in place with wingnuts.
Wet the foam dampers under running water. Squeeze them during this process to insure theyare thoroughly wet. Place the wet dampers in the ripple tank, one along each edge of theframe. 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 convenientfocusing reference for the light source. Adjust the height of the light source on the supportrod to obtain the sharpest image of the focusing strip on the viewing screen. This adjustmentwill provide the sharpest image of the wave patterns generated with the ripple bar. Thefocusing strip also provides a handy guide for calculating the actual wavelength of the waterwaves:
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 anumber of experiments. Blocks may be cut to a variety of lengths and shapes with a saw or autility 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 pointwhere your finger touched the water. Does the.speed of the pulse depend at all on how quicklyyour finger contacted the water? You should find that the speed of the pulse is constantregardless of the way you touch the water.
Place the dowel in the water and move it (roll it) forward a small amount. Notice the shapeof the wave front. This time you should see a straight wave (or a plane wave) travel across thetank. Is the speed of this wave front the sameas the circular wave? Does the speed of the wavechange 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 ofthe wave will be larger but the wave's speed will remain the same.
8)
1.
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Place a long flat obstruction near the middleof the tank. This can be constructed by layingparaffin blocks in a row across the tank. Make awave pulse by dipping your finger into the water.What happens as the pulse strikes the barrier?What does the reflected wave look like? Is itcircular like the incident pulse? Try differentplaces. Try to locate the virtual source (behindthe barrier) of the reflected pulse. How does theangle of incidence compare with the angle ofreflection? Remember that the wave front ismoving outward at the same speed in alldirections from the point where you touched thewater.
Figure 2Reflection of circular waves.
use the dowel to try plane waves at differentangles. How does the angle of incidencecompare with the angle of reflection?
Try replacing the straight barrier with alength of metal bent into the form of a parabola.Create a straight pulse that is incident along theaxis of the parabola. How is this straight pulsereflected from the surface of the parabola? Dothe reflected waves meet in a single point? Thisintersection point is referred to as the focalpoint for this parabolic reflector. Reflect a fewstraight pulses from the parabolic mirror and tryto locate this focal point. When you havelocated this focal point, create a circular pulseby sticking your finger into the water at thispoint. What do the reflected waves look like?
Are they straight waves? Large telescopes use parabolic mirrors (reflectors) to collect light that istraveling as straight waves from stars far away and focuses this light on a single point. Radioantennas are also parabolic and collect "light" in the form of radio waves. Another example ofparabolic reflectors are television dish antennas used to receive radio waves from satellites thattransmit television programming. Flashlights and automobile headlights use parabolic reflectorsto take light from the bulb that is located at its focal point and reflect it outward in a strongparallel beam.
2. Reflections
Figure 3Reflection of straight waves.
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3. Traveling waves
All periodic waves can be described by the relation
v=l,v
where the speed of the wavethe frequency of the wavethe wavelength
This relationship can be investigated using the ripple tank. To generate periodic waves set upthe straight wave generator, connect the motor to a variable DC source, and add water to a depthof about 5 to 8 millimeters. Practice using an adjustable stroboscope until you can "stop" themotion of the waves. l,ower the frequency of the wave generator and have a partner help you findthe frequency of the stroboscope's rotation as you visually "stop" the wave motion. You areinterested in finding the number of stroboscope slots that pass in front of your eye in a givenperiod of time as you view the "frozen" wave pattern. For example, 10 slots per second is afrequency of 10 Hertz. The frequency of the stroboscope should be measured from the slowestspeed required to "stop" the wave motion. This frequency will then correspond to the frequencyof the wave motion. Can you explain why?
Once you have visually stopped the waves with the stroboscope, have your partner positiontwo pencils or rulers on the screen below the ripple tank; the pencils being parallel to the wavesand several wavelengths apart.
To measure the wavelength you must measure the distance between the pencils, count thewaves that fall between them, and then account for any difference in scale between the image onthe screen and the actual waves in the tank. This can be done by the method described the set upprocedures above. Calculate the wave speed for several different values of wave generatorfrequency. I{ow do these values compare?
You may wish to repeat this experiment using different water depths. Try measuring the twoextremes of deep and shallow water. When you do this, see if the wavelength or velocity changewith water depth, if it does, how does it change?
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A
Deep Water
In our earlier experiments, we found that the speed of the water waves varied with the depthof the water. Therefore, water of different depths represent media with different indices ofrefraction, 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 spacersto 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 2millimeters 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 predictwhat will happen to the waves as they move from the deep water to the shallow water? Will thefrequency change? Will the wavelength change? Will the speed change? Use a low frequencysetting on the wave generator and test your predictions.
Next, turn the glass plate so the incident waves will strike the boundary at some angle. Dothe waves travel straight over the glass plate or do they bend (or refract) when they cross theboundary between the two regions? Keeping the frequency of the waves constant, measure theangle ofrefraction for several different angles ofincidence.
Figure 4Straight waves entering a different media.
4. Refraction
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Figure 5Refraction of straight waves.
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Place a small smooth paraffin block in the ripple tank about l0 cm in front of the straightwave generator. Do the waves continue in their straight path on both sides of the block? How farbehind the block does its "shadow" remain in existence? Change the frequency of the wavegenerator and observe the region near the edges of the block. How is the shadow affected byincreasing the frequency? Decreasing thefrequency? what frequency range will producethe sharpest shadow?
Replace the block with an open slit. This canbe constructed by placing two lengths of paraffinacross the tank and leavin g a small gap betweenthem near the middle of the tank. When straightwaves are incident on the opening, what do thewaves exiting the opening look like? Are they itittstraight waves, or are they curved? Keeping thefrequency constant, change the width of theopening. How does this affect the shape of theexiting waves? Repeat this experiment usingseveral different frequencies for the incidentwave.
6. Interference
Figure 6Waves blocked by a small object.
5. Waves and obstacles
Figure 7Waves passing through a narrow slit.
Figure 8lnterference pattern in ripple tank
Replace the straight wave generator with a point source generator. How will two circularwaves of the same frequency interact? Place the point sources approximately 5 centimeters apart
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and start the generator. How would youdescribe the resulting wave pattern? Noticelines of cancellation where the wave crests ofone source align with the wave troughs of theother source. How does this pattern change withincreasing frequency? Do the nodal lines movecloser together or further apart? What is theeffect of moving the points closer together?Notice the inverse relationship between thefrequency and the width of the interferencepattern, &s the frequency increases the patternwidth decreases. As the spacing of the twosources decreases the pattern width increases.By applying the principles of superposition, itcan 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 nftmaxima, L is the distance from the sources to the "screen", n is the maxima number beginningfrom the center of the pattern, l, is the wavelength used, and d is the distance between the twopoint 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 tothe measured value.
How would a change in the phase of one point source with respect to the other change theresulting interference pattern? For this experiment you will need an variable phase wavegenerator to replace the ripple bar. Begin with the two point sources in phase and generate aninterference 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. Actualexperiments will vary with needs of students and the method of instruction. While any one iseasily 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 callour toll free number 1-8fi)-299-5469, or e-mail us: [email protected]. Our FAXnumber is: 1-207-832-7281.
Figure IGeometry of constructive interference,
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