Experiment 1
Reflection and Refraction
Overview: These instructions will first provide basic information on how to
set-up and use the ripple tank. Then you will make a few simple measurements and
answer a couple questions.
WARNING: If you suffer from photosensitive epilepsy, you should inform the
instructor and not be present during this lab.
Equipment:
Tanks: About 800 cm3 of water will produce a depth of 5 mm.
Yellow plastic barriers: These are about 5 mm thick.
Light source: Often the shadows of waves are easier to see and measure than the
waves themselves. But the shadows are larger than the actual waves, there are two
ways to correct for this:
(1) If t is the distance from the light to the tank and s is the distance from the light to
the shadows, the shadow waves are larger by a factor of (s/t). Multiplying shadow
lengths by (t/s) will give the actual lengths in the tank.
(2) The clear plastic ruler can be placed in the tank and measurements of the
shadow lengths using the ruler’s shadow will give correct centimeter lengths for the
waves in the tank.
Ripple Generator: The amplitude adjust changes how much the actuator arms move
up and down and hence the amplitude (height) of the generated waves. You will
want both arms set to the same amplitude.
The phase dial allows the arms to be out of phase (like one going up while the
other is going down). You will probably want the phase difference to be locked at
zero.
The frequency knob controls the frequency of the actuator arm’s up and down
motion. This will determine the frequency of the generated water waves. The
frequency knob is marked with the letters A (0 Hz??) through J (20 Hz??) in
approximately even intervals. (1 Hz = 1 full up-down-up motion per second or 1
cycle/sec)
The speed of water waves partially depends on the depth of the water, it can also
depend on the frequency of the waves (that is called dispersion). For frequencies
above about 10 Hz, the waves in these ripple tanks all move with the same speed
regardless of depth. So for experiments in which you wish to observe refraction of
waves, frequencies below 5 Hz should be used.
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Strobe light: It can be very hard to measure the length of moving waves, the strobe
light is a way to “freeze” the waves. The strobe light periodically flashes light onto
the waves, if the strobe frequency matches the wave frequency then each successive
wave is seen where the previous one was and it looks like the waves are frozen in
place. The can greatly simplify measuring distances and angles. For clearest viewing,
we will have the overhead lights off for most of the class period.
Procedures:
1. Measuring the wave speed.
Fill the ripple tank to one of the depths listed on the board (from 2 mm to 20 mm),
check off that depth so that each group will use a different depth. You’ll need to use
the strobe light, both to see the waves and to measure the frequency. Place blank
white paper on the table under the tank, this will be where you will see the wave’s
shadows.
Set the Ripple Generator to produce plane waves with a low frequency (you want
less than 5 Hz). Vary the amplitude as necessary to best see the waves (remember
there are amplitude controls on both sides). When the strobe has frozen (or nearly
frozen) the waves, measure the wavelength (we want the wavelength value of the
waves, not their shadows).
(Q1) List the depth used, wavelength measured, frequency reading, and calculated
wave speed on your data sheet. (v = λ∙f)
2. Reflection from a straight
barrier.
Make sure the water level is less
than 5 mm (less than the thickness
of the triangular yellow plastic
barrier that you will be using).
Place the barrier as shown. Use
maximum amplitude with the
actuator just touching the water
surface and a frequency between 5
and 10 Hz (maybe “C”).
Maybe using lines
traced onto paper in the
shadows, try to determine
the ray directions, normal,
and angles as shown. Note
that the incident ray
2
should be parallel to the side of the ripple tank.
(Q2) Summarize your results, list the factors that made these measurements
difficult or inaccurate.
3. Reflection using a curved barrier
Replace the straight barrier with the
circular barrier as shown.
(Q3) You should find that the waves
converge and pass through a common
point after reflecting from the curved
barrier (hard to see? try both with
and without the strobe light), what is
the distance from the curved barrier
to that convergence point (want wave
value, not shadow value)?
(Q4) Turn off the ripple generator.
Use the tip of your finger to tap the water at the position where the reflected waves
converged. What is the shape of the pulse after it is reflected off the barrier?
4. Refraction
As a wave passes from one medium to another, it changes speed. If it slows down,
the wave will bend toward the normal (as shown below). This bending is called
refraction.
[Finding the best combination of settings to demonstrate refraction is still a work in
progress, please share with the instructor what you found worked best.]
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As a starting point, fill the tank with water to a depth of about 10 mm, level the
tank if the water depth is not uniform. Use frequency A or B with the actuator
amplitude at maximum and the actuator as deep as possible without hitting the tank
bottom.
Use the yellow triangular barrier to create regions of greater and lesser water
depth in the tank (you can put coins under the barrier to raise it higher. Try to find a
barrier position such that you can see waves of different wavelengths in the deeper
and shallower regions.
5. Measure those wavelengths.
Measure the incident and refracted angles and also the incident and refracted
wavelengths. Then test the equation relating wavelengths and refraction angles
sin 1 1
(called Snell’s Law):
. How good is your data as compared to

sin 2 2
Snell’s Law?

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