Sound and Light
Waves
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· Recognize what is transferred along a wave as it is moving.
· Describe the two main types of waves.
· Define the parts of any wave.
· Explain frequency and period and how they relate to one another.
· Describe the relationship between velocity, wavelength, and frequency.
· Relate the mechanics velocity equation to the waves velocity equation.
· Explain the interaction of multiple waves.
· Describe refraction and diffraction of any wave.
Demonstrations and Notes
µ Stretch a small coiled spring across the room. Send a single pulse through the coil. Observe the pulse move through the spring.
wave pulse – a single disturbance that travels through a medium; coil is at rest, pulse moves through coil past a specific point, then returns to rest; can be a point, straight, etc.
µ Send a series of pulses through coil. Observe waves produced through coil.
traveling wave – a series of pulses at regular intervals; repeating pulses create a wave.
µ Place a roll of masking tape around the center of a coil. Send a series of pulses through the coil. Observe the motion of the tape roll.
Waves transfer energy only. No matter is transferred.
µ Hold pointer upright. Swing pointer left and right like a pendulum moving at a constant speed. Observe.
vibration – back and forth motion in time; oscillatory motion; simple harmonic motion; energy.
µ Turn pointer sideways and vibrate up and down. Replace pointer with a piece of chalk. Vibrate chalk up and down on blackboard. Walk parallel to blackboard while vibrating chalk to create a sine wave.
OR
µ Hang a funnel from a bar like a pendulum. Fill the funnel with sand and swing the funnel. Move paper under the funnel at a constant speed to produce a sine wave.
wave – back and forth motion in time and space; a vibration in space; waves transfer energy without transferring matter.
Demonstrations and Notes
µ Stretch a small coiled spring across the room. Create a wave through the coil.
transverse wave – a wave with vibrations perpendicular to the direction the wave is traveling; light waves; a snake’s movement.
µ Stretch a slinky across the table. Send pulses into slinky.
OR
µ Hang a slinky from a bar. Vibrate slinky up and down.
longitudinal wave – a wave with vibrations parallel to the direction the wave is traveling; sound waves; a worm’s movement.
torsional wave – a less common wave type; a wave that
has a twist-like motion; wave type that shook the
µ Fill a fish tank with water. Shake tank to create waves on surface of tank.
surface wave – a combination of transverse and longitudinal waves; water waves; used by a seismograph machine to locate earthquakes.
µ Fill a clear bowl with water. Tap center of water with finger or drops of water.
circular wave – round waves emanating from a center disturbance; water waves created from a drop of water when viewed from above.
µ Fill a clear pan with water. Roll a pencil back and forth in the water at one end of the pan.
straight wave – lines emanating from a parallel disturbance; water waves created in a wave pool when viewed from above.
Summary Review – Wave Basics and Wave Types
ü Waves transfer energy without transferring matter.
ü In transverse waves, the vibrations are perpendicular to the direction the wave is traveling.
ü In longitudinal waves, the vibrations are parallel to the direction the wave is traveling.
ü In surface waves, the vibrations are both perpendicular and parallel to the direction the wave is traveling.
Notes
Draw a transverse wave.
Label the parts as they are defined.
crest – point of maximum displacement.
trough – point of minimum displacement.
equilibrium position – zero position.
amplitude (A) – maximum displacement from equilibrium position; amount of energy in a wave; measured from equilibrium to crest (positive) or equilibrium to trough (negative).
wavelength (l) – distance between successive identical parts of a wave; measured from points on the wave where energy is transferring equivalently; one cycle.
Draw a longitudinal wave.
Label the parts as they are defined.
compression – area that is squeezed together; dark bands.
rarefaction – area that is spread apart; light bands.
amplitude (A) – measured from equilibrium position to maximum compression; the length of one compression.
wavelength (l) – measured from compression to compression or rarefaction to rarefaction or any two successive identical points on the wave.
Draw a circular wave.
Label the parts as they are defined.
source – vibration that is creating the wave; single object like water droplet.
crest – solid line.
trough – dashed line.
amplitude (A) – determined by thickness of line.
wavelength (l) – measured from crest to crest or trough to trough.
Draw a straight wave.
Label the parts as they are defined.
source – vibration that is creating the wave; long object like pencil.
crest – solid line.
trough – dashed line.
amplitude (A) – determined by thickness of line.
wavelength (l) – measured from crest to crest or trough to trough.
Discussion and Notes
frequency (f) – how often something occurs; the number of occurrences completed in a specific time; cycles per second; inverse of period.
hertz (Hz) – units of frequency; one cycle per second; when large, use kHz (kilo = 103 Hz), MHz (mega = 106 Hz), or GHz (giga = 109 Hz).
period (T) – time to complete one full cycle of motion; inverse of frequency.
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A train is going by.
Each boxcar is 10 meters long.
Every second, one boxcar passes.
Calculate the speed of the train.
The units of velocity are m/s, from the mechanics
equation
Therefore, the train is traveling at a speed of 10
m/s. If two boxcars pass every second,
the speed would increase to 20 m/s.
Think of the length of each boxcar as a wavelength (l) and
how often the boxcars pass as the frequency (f). The velocity can also be related to
wavelength and period (T) due to the inverse relationship between frequency and
period.
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Amplitude has no affect on wave speed. A change in amplitude does not change the
wavelength or frequency of a wave.
Summary Review – Wave Parts and Analyzing Waves
ü The frequency of a wave is the number of vibrations per second of any one point on a wave.
ü The period of a wave is the time needed to complete one full cycle of motion.
ü The wavelength is the shortest distance between successive identical points on a wave.
ü The velocity of a wave can be calculated from the equation v = f l.
ü The amplitude of a wave is its maximum displacement from an equilibrium position. The energy transferred by a wave is directly proportional to its amplitude.
Homework – Wave Parts and Analyzing Waves
1. If you triple the frequency of a wave, what happens to its period?
2. How far, in terms of wavelength, does a wave move in one period?
3. Calculate the frequency that corresponds to each of the following periods: 5 sec and 24 hours
4. Calculate the period that corresponds to each of the following frequencies: 10 Hz and 60 Hz
5. The crests on a long surface water wave are 20 meters apart. In one minute ten crests pass by. What is the speed of the wave?
6. If the speed of a longitudinal wave is 340 m/s and the frequency is 1000 Hz, what is the wavelength of the wave?
Demonstrations and Notes
µ Connect two slinky springs of different sizes together. Create a transverse wave with the larger slinky. Observe the wave patterns that are created in both slinky springs by the transfer of energy, focusing on the section where the two slinky springs are connected together. Observe the amplitudes of each wave and how their signs (positive or negative) compare to the original wave. Compare the frequencies of each wave. Repeat the process using the smaller slinky as the source wave.
medium – what a wave is traveling through; examples include water, air, coil, etc; a change can occur from one medium to another (air to water) or from extremes of the same medium (deep water to shallow water).
reflection – bouncing of a wave.
incident wave – original wave; first wave; source wave.
transmitted wave – resultant wave; new wave; wave in new medium.
reflected wave – reflection of energy due to a change in medium.
µ Connect a slinky to the wall. Send a pulse through the slinky. Observe what happens to the pulse when it reaches the wall.
absorption – to take in; opposite of reflection.
When the change in medium is small (string to rope), the
amplitude of the transmitted wave is similar to the amplitude of the original
wave and the amplitude of the reflected wave is minimal; hardly any energy is
absorbed.
When the change in medium is large (string to wall), the
amplitude of the transmitted wave is minimal and the amplitude of the reflected
wave is similar to the amplitude of the original wave; some energy is absorbed.
When the medium changes from a less dense to a more dense
medium, the reflected wave is inverted.
When the medium changes from a more dense to a less dense
medium, the reflected wave is erect.
Summary Review – Changes in Medium
ü The speed of a wave depends on the properties of the medium the wave travels through.
ü When waves are transferred between media, they are partially transmitted and partially reflected. The amount of reflection depends on how much the two media differ.
ü When a wave moves from a less dense to a more dense medium, the reflected wave is inverted. When a wave moves from a more dense to a less dense medium, the reflected wave is erect.
ü The only way to change the speed of a wave is to change the medium it travels through.
1. You are creating waves in a rope by shaking your hand back and forth. Without changing the distance your hand moves, you begin to shake faster. What happens to the amplitude, frequency, period, and velocity of the wave?
2. A pulse is sent along a spring. The spring is attached to a light thread. The thread is tied to the wall. What happens when the pulse reaches point A? Is the reflection at point A erect or inverted? What happens when the transmitted pulse reaches point B? Is the reflection at point B erect or inverted?
3. A pulse is sent along a thin rope. The thin rope is attached to a thick rope. The thick rope is tied to the wall. What happens when the pulse reaches point A? Is the reflection at point A erect or inverted? What happens when the transmitted pulse reaches point B? Is the reflection at point B erect or inverted?
4. If a wave moves from one medium with a high wave velocity to one with a low wave velocity, which of the following cannot change: frequency, amplitude, wavelength, velocity, and direction?
Demonstrations and Activity and Notes
µ Send pulses of equal amplitude through each end of a stretched slinky (both pulses should be sent on the same side of the slinky). Observe how the waves interact. Focus on changes in amplitude and wave direction. Send pulses of opposite amplitude through a stretched slinky from opposite ends (both pulses should have the same magnitude). Observe how the waves interact. Focus on changes in amplitude and wave direction. Repeat above with varying magnitudes of amplitude. Observe.
interference – the interaction of multiple waves traveling through the same medium.
constructive interference – the adding of waves to produce a single wave of larger amplitude. (adding)
in phase – doing the same thing at the same time; for example, reaching a maximum or equilibrium at the same time.
destructive interference – the adding of waves to produce a single wave of smaller amplitude. (subtracting)
out of phase – not doing the same thing at the same time; for example, when one is at a maximum, the other is at equilibrium; measured in degrees of a circle like 180°, 90°, etc.
µ Produce a standing wave of one wavelength. Describe the wave, focusing on locations that appear to repeat its motion and areas that seem to be not moving.
standing wave – wave in which parts of the wave are stationary; a wave that appears not to be moving; result of interference between incident wave and reflected wave.
node – point of no displacement on a standing wave; a stationary point.
antinode – point of maximum displacement on a standing wave; a crest or a trough.
µ Produce a standing wave of one wavelength. Locate the nodes along the wave. Grab the wave at the center node. Observe the motion of the wave. Locate the antinodes along the wave. Grab the wave at one of the antinodes. Observe the motion of the wave.
¹ Have a contest to see who can produce the most nodes.
Summary Review – Interference
ü Constructive interference is the adding of waves.
ü Destructive interference is the subtracting of waves.
ü Maximum destructive interference produces a node where there is no displacement.
ü Maximum constructive interference results in an antinode, a location of maximum displacement.
Demonstration and Notes
refraction – the bending of a wave due to a change in medium; changes the direction of the wave.
µ Fill a large beaker with water. Place a pencil in the beaker. Observe the pencil.
diffraction – the bending of a wave due to a slit or obstruction.
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l > d |
l < d |
Charlie trying to squeeze through small openings. Her face rounded out like a diffraction
pattern. A fat person trying to fit
through a small doorway. Their body
rounds as they force themselves through an opening that is too narrow.
If the wavelength (l) is shorter than or equal
to the slit width (d), then the wave passes through the slit with little to no
diffraction. 
If the wavelength is
longer than the slit width, diffraction occurs.
The more narrow the
slit width, the greater the diffraction.
Activity and Notes
¹ Give each group a rope. Have each group make a transverse wave on the floor with the rope. Count the number of waves created. Increase the number of waves by increasing the frequency. Determine what effect increasing the frequency has on the actual wavelength, not just the number of waves. (As f, ¯l) Determine the effect this has on velocity. (The velocity remains constant.) The velocity of all waves remains constant through the same medium.
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To affect the speed, only frequency and/or wavelength can
be changed. But to change one of those
without changing the other, the medium must change. Therefore, only a change in medium can change
the speed of a wave.
Summary Review – Bending of Waves and Making Waves
ü Refraction is the bending of a wave due to a change in medium.
ü Diffraction is the bending of a wave due to a slit or obstruction.
ü The velocity of all waves remains constant through the same medium.
Introduction – What’s Going On?
The two main types of waves are transverse waves (light) and longitudinal waves (sound). Both of these waves can be found when shaking a slinky or a coil. Lucky for us, slinky’s and coils are easy to come by and quite fun to play with. Many of the concepts we have talked about involving waves can be demonstrated shaking a slinky or coil on the floor. This lab contains six parts that review the concepts of wave types, wave behavior, interference, reflection, and mediums.
Working with one or two lab partners, stretch the slinky and coil no more than five meters in the hallway. Have fun shaking the slinky and coil, but be courteous to other classrooms, fellow students, and teachers.
Part I – Transverse and Longitudinal Waves
- Transverse Wave – Stretch out the slinky to about 5 meters long. Shake the slinky sideways on the floor to send a single pulse along the slinky. Send several pulses to create a standing wave. Note the direction the wave travels compared to the direction of motion of the slinky.
- Longitudinal Wave – Keep the slinky stretched out to about 5 meters long. Reach a short distance down the slinky’s length and gather the coils of the slinky toward you and then quickly release them. Note the direction the wave travels compared to the direction of motion of the slinky.
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DO NOT overstretch the
slinky! You will ruin it and have
to buy a new one. |
Part II – Waves in a Constant Medium
- Pulse – Create a transverse wave pulse in the slinky. Vary the amplitude of the wave pulse. Note the affect (if any) amplitude has on the speed of the wave pulse.
- Wave – Create a standing transverse wave in the slinky. Vary the amplitude of the wave. Note the affect (if any) amplitude has on the speed of the wave.
Part III – Frequency and Wavelength
- Frequency – Create a standing transverse wave in the slinky. Compare the frequency of the wave to the frequency of your hand shaking the slinky.
- Wavelength – Create a standing transverse wave in the slinky. Vary the frequency with which you shake the slinky. Compare the wavelengths created with the varying frequencies.
Part IV – Interference
- Constructive – Create transverse wave pulses from both ends of the slinky. Send the pulses toward one another simultaneously and from the same side of the equilibrium line. Observe the pulses where they meet and after they pass through one another. Vary the amplitude of each wave pulse and observe how the pulses interact.
- Destructive – Create transverse wave pulses from both ends of the slinky. Send the pulses toward one another simultaneously and from opposite sides of the equilibrium line. Observe the pulses where they meet and after they pass through one another. Vary the amplitude of each wave pulse and observe how the pulses interact.
Part V – Reflection
- Less to More – Hold one end of the slinky firmly to keep it from moving. Create a transverse wave pulse from the free end of the slinky. Observe the phase of the pulse as it approaches the rigid end and after the pulse reflects.
- More to Less – Connect one end of the coil to one end of the slinky. Create a transverse wave pulse from the open end of the coil toward the slinky. Observe the phase of the pulse as it approaches the slinky and after the pulse reflects.
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DO NOT allow the slinky
and coil to get tangle up! You will ruin them and
have to buy new ones. |
Part VI – Waves in a Changing Medium
- Less to More – Keeping the slinky connected to the coil, create a transverse wave pulse from the open end of the slinky toward the coil. Observe the phase of the pulse as it approaches the coil and after the pulse changes medium from the slinky to the coil. Observe what happens to the speed of the wave and its frequency when it changes medium. Note any other pulses created as a result of the change in medium.
- More to Less – Keeping the coil connected to the slinky, create a transverse wave pulse from the open end of the coil toward the slinky. Observe the phase of the pulse as it approaches the slinky and after the pulse changes medium from the coil to the slinky. Observe what happens to the speed of the wave and its frequency when it changes medium. Note any other pulses created as a result of the change in medium.
Part VII – Wrap Up
- Answer the following questions.
Ø When traveling through the same medium, how are velocity and wavelength of a wave affected if the frequency is increased? (2 points)
Ø When traveling through the same medium, how are velocity and wavelength of a wave affected if the frequency is decreased? (2 points)
Ø When a wave changes medium, what property is not affected? Explain. (2 points)
Ø Provide several examples of transverse waves in nature. (2 points)
Ø Provide several examples of longitudinal waves in nature. (2 points)
- Write a minimum of one paragraph for each part of this lab (six parts total). Explain your results and any conclusions you can draw from what you experienced. Include pictures (sketches) for Parts I, IV, V, and VI. If any of your observations contradict what you believe should have happened, explain the reason for the error. (30 points)
Properties of Water Waves
Introduction – What’s Going On?
The two main types of waves are transverse waves (light) and longitudinal waves (sound). Both of these waves can be found when looking at water waves. Lucky for us, water waves are the easiest waves to see and study. Many of the concepts we have talked about involving waves can be demonstrated using water in a ripple tank with a light source. This lab contains four parts that review the concepts of wave behavior, reflection, refraction, and diffraction.
Set up the ripple tank and accessories according to your teacher’s instructions. Fill the ripple tank with approximately 1500 mL of water. Use the adjustable support legs to level the water in the ripple tank. Lower the wave generator’s pulse bar approximately 2 cm into the water. Adjust the light source so it shines toward the center of the water and produces the sharpest image on the table surface below the ripple tank.
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ELECTRICITY AND WATER DON’T
MIX! If the Light Source should
happen to fall in the Ripple Tank, DON’T
Pick it Up! Unplug it then call your
teacher!! |
When describing (through words or drawings) the waves you create and see, focus on the incident waves (original) and reflected waves (returning). Sometimes the transmitted waves (new) are important, too, but keep your explanations simple and only include the information necessary to prove your point.
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Create circular waves by touching your finger to the
surface of the water and straight waves by tapping the top of the generator. |
Part I – General Behavior of Water Waves
- Spreading of Waves – Create a wave by touching your finger into the water. Describe the shape of the wave you create. How can you tell the speed of the wave is the same in all directions?
- Energy Transmission – Float a piece of cork in the water. Create a series of straight waves. Describe the motion of the cork. Based on your observation, does a wave transport matter, energy, or both?
- Wave Velocity – Create waves of varying frequencies by tapping the generator at different rates. Does the speed of the wave appear to change as the frequency changes? Tap the top of the generator lightly to produce a small amplitude wave. Tap the generator harder to create a large amplitude wave. What effect does amplitude have on the velocity of the wave?
- Think About It – Is the frequency of the waves in the tank the same as on the screen? Is the wavelength of the waves in the tank the same as on the screen?
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Incident waves are shown traveling from left to right
and drawn using solid lines. Reflected
waves are drawn using dashed lines.
Transmitted waves, when necessary, are also drawn with solid lines. |
Part II – Reflection
- Reflection Patterns of Straight Waves – Draw a picture of the reflection of a straight wave off the following barriers: straight (wood bar), concave (curved in metal bar), and convex (curved out metal bar). Where do the reflected waves off the convex barrier appear to be coming from?
- Reflection Patterns of Circular Waves – Draw a picture of the reflection of a circular wave off the following barriers: straight and concave. Where do the reflected waves off the straight barrier appear to be coming from? For the concave barrier, create the circular waves at the imaginary center of a circle that could be completed by the concave barrier.
- Law of Reflection – Place the straight barrier in the tank at an angle to the generator. Draw a picture of the reflection of a straight wave sent toward the barrier. How does the angle of the transmitted wave compare to the angle of the reflected wave?
Part III – Refraction
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Refraction is the bending of a wave due to a change in
medium. A change in medium exists
between varying depths of water. Use a
thin plastic plate to vary the depth of water in the ripple tank. Use large amplitude waves for best results. |
- Perpendicular Transmission – Place a plate in the ripple tank so the waves move straight from deep to shallow water. Explain and draw pictures of what happens to the following as the waves enter the shallow water: wavelength, frequency, velocity, and amplitude.
- Transmission at an Angle – Place a plate in the ripple tank so the waves travel through the deep water and enter the shallow water at an angle. Explain and draw pictures of what happens to the direction of the waves as they enter the shallow water. Are the waves bending inward or outward relative to the edge of the plastic plate? Locate and draw the reflected waves bouncing off the plate. Explain what effect a change in frequency has on the refraction of the waves?
- Shallow to Deep Water – Place a plate parallel and under the generator so the waves will travel from shallow to deep water. Explain and draw pictures of what happens to the following as the waves enter the deep water: wavelength, frequency, velocity, and amplitude. If the plates are moved at an angle, would the waves bend inward or outward relative to the edge of the plastic plate?
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If the refraction is difficult to see, try increasing the
amplitude of the incident wave. Be
careful how hard you hit the generator.
They are delicate and should be used with care and a controlled tap! |
Part IV – Diffraction
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Diffraction is the bending of a wave due to a slit or
obstruction. An obstruction is
anything partially blocking the path of a wave. A slit is an opening the wave can pass
through. |
- Diffraction Around a Corner – Place a plastic plate on its side to form a barrier. Place the plate parallel to the generator near the center of the ripple tank. Send a straight wave toward the barrier and draw a picture of the diffraction pattern created.
- Varying Slit Width – Place two plastic plates on their sides with the short ends facing each other to create a slit. Start with a large slit (10 cm). Create a straight wave toward the slit and draw the diffraction pattern created. Repeat this process using smaller slits (7 cm, 4 cm, and 2 cm) and draw the diffraction pattern each creates.
- Think About It – How does the degree of diffraction depend on the slit width? Does the wavelength or velocity of the wave change when it diffracts? What would the diffraction pattern look like if the slit was 20 cm wide?
Part V – Wrap Up
- Clean and Dry – Unplug and disconnect all the accessories before you begin to drain the ripple tank. Drain the ripple tank slowly without titling it (the legs will fall off). Do not pick up the ripple tank if there is any water in it. It will spill and make a mess! Dry everything off when the ripple tank is empty.
- Report Your Findings – Write a minimum of one paragraph for each part of this lab (four parts total). Explain your results and any conclusions you can draw from what you experienced. If any of your observations contradict what you believe should have happened, explain the reason for the error. (20 points)
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The lights are VERY HOT. Don’t touch them except
for their handles. |
1. What is the difference between transverse and longitudinal waves?
2. What is the difference between a pulse and a wave?
3. What is the difference between wave frequency and wave velocity?
4. Suppose you produce a wave by shaking one end of a spring back and forth. How does the frequency of your hand compare with the frequency of the wave?
5. Waves are sent a long a spring of fixed length. Can the speed of the waves in the spring be changed? Explain. Can the frequency of a wave in the spring be changed? Explain.
6. What is the amplitude of a wave and what does it represent?
7. When a wave reaches the boundary of a new medium, part of the wave is reflected and part is transmitted. What determines the amount of reflection?
8. What happens to the spring at nodes of a standing wave?
9. You repeatedly dip your finger into a sink full of water to make circular waves. What happens to the wavelength as you move your finger faster?
10. What happens to the period of a wave as the frequency increases?
11. Sketch what happens for each of the cases shown when centers of the two wave pulses lie on the dashed line so the pulses exactly overlap.
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1. The PPG building downtown sways back and forth with a frequency of about 0.10 Hz. What is the period of vibration?
2. An ocean wave has a length of 10 meters. A wave passes a fixed location every 2 seconds. What is the speed of the wave?
3. The time needed for a water wave to change from equilibrium to a crest is 0.18 seconds. What fraction of a wavelength is this? What is the period of the wave? What is the frequency of the wave?
4. A group of swimmers is resting in the sun on an off-shore raft. They estimate that 3 meters separates a trough and an adjacent crest of surface waves on a lake. They count 14 crests that pass by the raft in 20 seconds. How fast are the waves moving?
5. If you slosh the water back and forth in a bathtub at the correct frequency, the water rises first at one end and then at the other. Suppose you can make a standing wave in a 150 cm long tub with a frequency of 0.3 Hz. What is the velocity of the water wave?
6. If the speed of a wave is 3.0 ´ 108 m/s and the frequency is 1015 Hz, what is the wavelength?