# WAVES REVIEW

• Click on highlighted words as you come across them to complete activities.
• Review the VOCABULARY terms & definitions.

Light travels as waves. Waves can be described by their amplitude, wavelength and frequency. The speed of a wave can be calculated from its frequency and wavelength.

# WAVES have three important measurements:

• AMPLITUDE - height of a peak or trough
• WAVELENGTH - distance between any two peaks or troughs.
• FREQUENCY - number of waves that pass by in one second
What are waves?
Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred. Think of a Mexican wave in a football crowd: the wave moves around the stadium, while each spectator stays in their seat only moving up then down when it's their turn.
Some waves must travel through a substance. The substance is known as the medium and it can be solid, liquid or gas
Sound waves and seismic waves are like this.
• They must travel through a medium, and it is the medium that vibrates as the waves travel through.
Other waves do not need to travel through a substance. They may be able to travel through a medium, but they do not have to.
• Visible light, infrared raysmicrowaves and other types of electromagnetic radiation are like this. They can travel through empty space.
• Electrical and magnetic fields vibrate as the waves travel.
Longitudinal and Transverse waves
You should be able to describe the characteristics of transverse and longitudinal waves.

## Transverse waves

In transverse waves, the oscillations (vibrations) are at right angles to the direction of travel and energy transfer
• Light and other types of electromagnetic radiation are transverse waves. All types of electromagnetic waves travel at the same speed through a vacuum, such as through space.
Water waves and S waves (a type of seismic wave) are also transverse waves.

## Longitudinal waves

the oscillations are along the same direction as the direction of travel and energy transfer.
Sound waves and waves in a stretched spring are longitudinal waves.
• P waves (relatively fast moving longitudinal seismic waves that travel through liquids and solids) are also longitudinal waves.
Longitudinal waves show area of compression and rarefaction. In the animation, the areas of compression are where the parts of the spring are close together, while the areas of rarefaction are where they are far apart.

Suppose you fix a slinky spring at one end & push the other end back & forth.  Some parts of the spring, called compressions are squeezed together.  Other parts of the spring, called rarefactions are stretched out.  The compressions & rarefactions travel down the spring carrying energy.  This type of wave is called a longitudinal or compression wave.

Amplitude, Wavelength and Frequency
You should understand what is meant by the amplitude, wavelength and frequency of a wave.

## Amplitude

As waves travel, they set up patterns of disturbance. The amplitude of a wave is its maximum disturbance from its undisturbed position. Take care: the amplitude is not the distance between the top and bottom of a wave.

Amplitude and wavelength

## Wavelength

The wavelength of a wave is the distance between a point on one wave and the same point on the next wave. It is often easiest to measure this from the crest of one wave to the crest of the next wave, but it doesn't matter where as long as it is the same point in each wave.

## Frequency

The frequency of a wave is the number of waves produced by a source each second. It is also the number of waves that pass a certain point each second.
The unit of frequency is the hertz (Hz). It is common for kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) to be used when waves have very high frequencies.
• For example, most people cannot hear a high-pitched sound above 20 kHz, radio stations broadcast radio waves with frequencies of about 100 MHz, while most wireless computer networks operate at 2.4 GHz.

Check your understanding of this section by having a go at this activity.  Click wave

Wave speed
The speed of a wave is related to its frequency and wavelength, according to this equation:
v = f × λ
• v is the wave speed in meters per second, m/s
• f is the frequency in hertz, Hz
• λ (lambda) is the wavelength in meters, m.
All waves obey this wave equation. For example, a wave with a frequency of 100 Hz and a wavelength of 2 m travels at 100 × 2 = 200 m/s.
Check your understanding of the equation by having a go at this activity.

All waves obey this wave equation. For example, a wave with a frequency of 100 Hz and a wavelength of 2 m travels at 100 × 2 = 200 m/s.

# Refraction and Diffraction

The refraction follows a regular pattern. Check your understanding of refraction by having a go at the animation.
Waves can be refracted and diffracted.

## Refraction

Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.
There is one special case you need to know. Refraction doesn't happen if the waves cross the boundary at an angle of 90° (called the normal) - in that case they carry straight on.
• the phenomenon of refraction — that when light travels across the border of two transparent media (such as air, glass, Lucite, etc.), the path of light bends.
• light is made up of waves, and that waves at different wavelengths create different colors.
• a prism sorts a light beam into its various wavelengths, appearing as a rainbow of colored light.
• there are waves that cannot be seen by the human eye that affect our daily lives.
• For example, we wear sunscreen and sunglasses to protect our skin and eyes from ultraviolet waves.

## Diffraction

When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is called diffraction.
The extent of the spreading depends on how the width of the gap compares to the wavelength of the waves. Significant diffraction only happens when the wavelength is of the same order of magnitude as the gap. For example:
• a gap similar to the wavelength causes a lot of spreading with no sharp shadow, eg sound through a doorway
• a gap much larger than the wavelength causes little spreading and a sharp shadow, eg light through a doorway.

Reflection
Sound waves and light waves reflect from surfaces. When waves reflect, they obey the law of reflection:
the angle of incidence equals the angle of reflection
• The normal is a line drawn at right angles to the reflector
• The angle of incidence is between the incident (incoming) ray and the normal
• The angle of reflection is between the reflected ray and the normal.
You need to label an example of the LAW OF REFLECTION.

Smooth surfaces produce strong echoes when sound waves hit them, and they can act as mirrors when light waves hit them. The waves are reflected uniformly and light can form images The waves can:
• appear to come from a point behind the mirror, for example a looking glass
• be focused to a point, for example sunlight reflected off a concave telescope mirror.
Rough surfaces scatter sound and light in all directions. However, each tiny bit of the surface still follows the rule that the angle of incidence equals the angle of reflection.

## Wednesday, April 23, 2014

### WAVES - ANSWERS TO QUESTIONS 1-10

waves

1. Energy moves by waves.
2. False; particles only vibrate, they do not move along the wave.
3. Particles are being moved against a force. Work is being done on them and they are doing work on other particles.
4. Transverse and longitudinal.
5. Longitudinal.
6. Transverse.
7. Wavelength is the distance between two like parts of the wave.
8. Amplitude is the height of the wave.
9. “A” has the longer wavelength.
10. “B” has a larger amplitude.

# Sound is a Mechanical Wave

A sound wave is similar in nature to a slinky wave for a variety of reasons.
• First, there is a medium that carries the disturbance from one location to another. Typically, this medium is air, though it could be any material such as water or steel. The medium is simply a series of interconnected and interacting particles.
• Second, there is an original source of the wave, some vibrating object capable of disturbing the first particle of the medium. The disturbance could be created by the vibrating vocal cords of a person, the vibrating string and soundboard of a guitar or violin, the vibrating tines of a tuning fork, or the vibrating diaphragm of a radio speaker.
• Third, the sound wave is transported from one location to another by means of particle-to-particle interaction. If the sound wave is moving through air, then as one air particle is displaced from its equilibrium position, it exerts a push or pull on its nearest neighbors, causing them to be displaced from their equilibrium position. This particle interaction continues throughout the entire medium, with each particle interacting and causing a disturbance of its nearest neighbors. Since a sound wave is a disturbance that is transported through a medium via the mechanism of particle-to-particle interaction, a sound wave is characterized as a mechanical wave.

The creation and propagation of sound waves are often demonstrated in class through the use of a tuning fork. A tuning fork is a metal object consisting of two tines capable of vibrating if struck by a rubber hammer or mallet. As the tines of the tuning forks vibrate back and forth, they begin to disturb surrounding air molecules. These disturbances are passed on to adjacent air molecules by the mechanism of particle interaction. The motion of the disturbance, originating at the tines of the tuning fork and traveling through the medium (in this case, air) is what is referred to as a sound wave. The generation and propagation of a sound wave is demonstrated in the animation below.

Many Physics demonstration tuning forks are mounted on a sound box. In such instances, the vibrating tuning fork, being connected to the sound box, sets the sound box into vibrational motion. In turn, the sound box, being connected to the air inside of it, sets the air inside of the sound box into vibrational motion. As the tines of the tuning fork, the structure of the sound box, and the air inside of the sound box begin vibrating at the same frequency, a louder sound is produced. In fact, the more particles that can be made to vibrate, the louder or more amplified the sound. This concept is often demonstrated by the placement of a vibrating tuning fork against the glass panel of an overhead projector or on the wooden door of a cabinet. The vibrating tuning fork sets the glass panel or wood door into vibrational motion and results in an amplified sound.
We know that a tuning fork is vibrating because we hear the sound that is produced by its vibration. Nonetheless, we do not actually visibly detect any vibrations of the tines. This is because the tines are vibrating at a very high frequency. If the tuning fork that is being used corresponds to middle C on the piano keyboard, then the tines are vibrating at a frequency of 256 Hertz; that is, 256 vibrations per second. We are unable to visibly detect vibrations of such high frequency. A common physics demonstration involves slowing down the vibrations by through the use of a strobe light. If the strobe light puts out a flash of light at a frequency of 512 Hz (two times the frequency of the tuning fork), then the tuning fork can be observed to be moving in a back and forth motion. With the room darkened, the strobe would allow us to view the position of the tines two times during their vibrational cycle. Thus we would see the tines when they are displaced far to the left and again when they are displaced far to the right. This would be convincing proof that the tines of the tuning fork are indeed vibrating to produce sound.
Electromagnetic waves are waves that have an electric and magnetic nature and are capable of traveling through a vacuum. Electromagnetic waves do not require a medium in order to transport their energy.
Mechanical waves are waves that require a medium in order to transport their energy from one location to another. Because mechanical waves rely on particle interaction in order to transport their energy, they cannot travel through regions of space that are void of particles. That is, mechanical waves cannot travel through a vacuum.
• A ringing bell is placed in a jar and air inside the jar is evacuated. Once air is removed from the jar, the sound of the ringing bell can no longer be heard. The clapper is seen striking the bell; but the sound that it produces cannot be heard because there are no particles inside of the jar to transport the disturbance through the vacuum. Sound is a mechanical wave and cannot travel through a vacuum.

1. A sound wave is different than a light wave in that a sound wave is

a. produced by an oscillating object and a light wave is not.
b. not capable of traveling through a vacuum.
c. not capable of diffracting and a light wave is.
d. capable of existing with a variety of frequencies and a light wave has a single frequency.

Sound is a mechanical wave and cannot travel through a vacuum. Light is an electromagnetic wave and can travel through the vacuum of outer space.

# Energy Transport and the Amplitude of a Wave

How is the Energy Transported Related to the Amplitude?
The amount of energy carried by a wave is related to the amplitude of the wave.
• A high energy wave is characterized by a high amplitude
• A low energy wave is characterized by a low amplitude.

Remember, the amplitude of a wave refers to the maximum amount of displacement of a particle on the medium from its rest position
Amplitude is the height of the wave.
• The higher the amplitude, the higher the wave. Also,
• The higher the amplitude the higher the energy of the wave.

Can you see why?

If the wave has a high amplitude, how must the particles in the wave be moving? A lot, or a little? If you said a lot you’re right!
For a wave to have a high amplitude the particle has to be moving over a large distance (large being a relative term here, the distance may still be miniscule). The more the particle moves, the more work there is being done on the particle (work is force and distance). The more work there is, the more energy there is and so, a wave with a large amplitude has more energy then a wave with a small amplitude. If you've ever been in the ocean this may be more clear. Small little waves don’t have the energy to knock you over, but the larger ones...watch out! In sound, amplitude determines the loudness of the sound. In light, amplitude determines the brightness.

Consider two identical slinkies into which a pulse is introduced. If the same amount of energy is introduced into each slinky, then each pulse will have the same amplitude. But what if the slinkies are different? What if one is made of zinc and the other is made of copper? Will the amplitudes now be the same or different? If a pulse is introduced into two different slinkies by imparting the same amount of energy, then the amplitudes of the pulses will not necessarily be the same. In a situation such as this, the actual amplitude assumed by the pulse is dependent upon two types of factors: an inertial factor and an elastic factor. Two different materials have different mass densities. The imparting of energy to the first coil of a slinky is done by the application of a force to this coil. More massive slinkies have a greater inertia and thus tend to resist the force; this increased resistance by the greater mass tends to cause a reduction in the amplitude of the pulse. Different materials also have differing degrees of springiness or elasticity. A more elastic medium will tend to offer less resistance to the force and allow a greater amplitude pulse to travel through it; being less rigid (and therefore more elastic), the same force causes a greater amplitude.

## Tuesday, April 1, 2014

### A wave is a disturbance that moves along a medium from one end to the other. If one watches an ocean wave moving along the medium (the ocean water), one can observe that the crest of the wave is moving from one location to another over a given interval of time. The crest is observed to cover distance.  The speed of an object refers to how fast an object is moving and is usually expressed as the distance traveled per time of travel. In the case of a wave, the speed is the distance traveled by a given point on the wave (such as a crest) in a given interval of time. In equation form,

• If the crest of an ocean wave moves a distance of 20 meters in 10 seconds,then the speed of the ocean wave is ???
• 2.0 m/s
• On the other hand, if the crest of an ocean wave moves a distance of 20 meters in 10 seconds (the same amount of time), then the speed of this ocean wave is ???
• 2.5 m/s.
The faster wave travels a greater distance in the same amount of time.

### ***Sometimes a wave encounters the end of a medium and the presence of a different medium.

• For example, a wave introduced by a person into one end of a slinky will travel through the slinky and eventually reach the end of the slinky and the presence of the hand of a second person.
One behavior that waves undergo at the end of a medium is reflection. The wave will reflect or bounce off the person's hand. When a wave undergoes reflection, it remains within the medium and merely reverses its direction of travel.
• In the case of a slinky wave, the disturbance can be seen traveling back to the original end. A slinky wave that travels to the end of a slinky and back has doubled its distance. That is, by reflecting back to the original location, the wave has traveled a distance that is equal to twice the length of the slinky.

### Reflection phenomena are commonly observed with sound waves.

• When you let out a holler within a canyon, you often hear the echo of the holler.

The sound wave travels through the medium (air in this case), reflects off the canyon wall and returns to its origin (you). The result is that you hear the echo (the reflected sound wave) of your holler. A classic physics problem goes like this:

### Noah stands 170 meters away from a steep canyon wall. He shouts and hears the echo of his voice one second later. What is the speed of the wave?

• In this instance, the sound wave travels 340 meters in 1 second, so the speed of the wave is 340 m/s.

Remember, when there is a reflection, the wave doubles its distance. In other words, the distance traveled by the sound wave in 1 second is equivalent to the 170 meters down to the canyon wall plus the 170 meters back from the canyon wall.

### Variables Affecting Wave Speed - Think!!!!

What variables affect the speed at which a wave travels through a medium?

Does the frequency or wavelength of the wave affect its speed?
Does the amplitude of the wave affect its speed?
Or are other variables such as the mass density of the medium or the elasticity of the medium responsible for affecting the speed of the wave?

The speed of sound depends on the elasticity, density & temperature of medium

Elasticity - ability of a material to bounce back after being disturbed.
• rubber band is an elastic substance
sound travels more quickly in mediums that have a high degree of elasticity b/c when the particles are compressed, they quickly spread out again.
Densityof a medium is how much matter, or mass, there is in a gvine amount of space, or volume.

• ·       Sound travel more slowly in dense metals.

Temperature – room temperature of about 20*C, sound travels at about 340 m/s.

·         In a given medium, sound travel more slowly at lower temperatures & faster @ higher temperatures.

•   Factor Change in Factor Effect on Speed of Sound Elasticity Increase in elasticity Density Increase in density temperature Decrease in temperature

### SPEED OF A WAVE - Check Your Understanding

1. A teacher attaches a slinky to the wall and begins introducing pulses with different amplitudes. Which of the two pulses (A or B) below will travel from the hand to the wall in the least amount of time? Justify your answer.

2. The teacher then begins introducing pulses with a different wavelength. Which of the two pulses (C or D) will travel from the hand to the wall in the least amount of time ? Justify your answer.

3. The time required for the sound waves (v = 340 m/s) to travel from the tuning fork to point A is ____ .
 a. 0.020 second b. 0.059 second c. 0.59 second d. 2.9 second

4. Two waves are traveling through the same container of nitrogen gas. Wave A has a wavelength of 1.5 m. Wave B has a wavelength of 4.5 m. The speed of wave B must be ________ the speed of wave A.
 a. one-ninth b. one-third c. the same as d. three times larger than
5. An automatic focus camera is able to focus on objects by use of an ultrasonic sound wave. The camera sends out sound waves that reflect off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. The camera lens then focuses at that distance. Now that's a smart camera! In a subsequent life, you might have to be a camera; so try this problem for practice:
If a sound wave (speed = 340 m/s) returns to the camera 0.150 seconds after leaving the camera, then how far away is the object?
6. TRUE or FALSE:
Doubling the frequency of a wave source doubles the speed of the waves.
7. While hiking through a canyon, Noah Formula lets out a scream. An echo (reflection of the scream off a nearby canyon wall) is heard 0.82 seconds after the scream. The speed of the sound wave in air is 342 m/s. Calculate the distance from Noah to the nearby canyon wall.

8. Mac and Tosh are resting on top of the water near the end of the pool when Mac creates a surface wave. The wave travels the length of the pool and back in 25 seconds. The pool is 25 meters long. Determine the speed of the wave.

9. The water waves below are traveling along the surface of the ocean at a speed of 2.5 m/s and splashing periodically against Wilbert's perch. Each adjacent crest is 5 meters apart. The crests splash Wilbert's feet upon reaching his perch. How much time passes between each successive drenching? Answer and explain using complete sentences.