Waves 1.3

Waves 1.3

The Interaction of Waves

Reflection

• Reflection occurs when a wave strikes an object and bounces off of it.

• All types of wavesincluding sound, water, and light wavescan be reflected.

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The Behavior of Waves

Reflection

Reflection

• How does the reflection of light allow you to see yourself in the mirror?

• It happens in two steps. First, light strikes your face and bounces off.

• Then, the light reflected off your face strikes the mirror and is reflected into your eyes.

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The Law of Reflection

• The beam striking the mirror is called the beam of incident.

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• The beam that bounces off the mirror is called the reflected beam.


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• The line drawn perpendicular to the surface of the mirror is called the normal.


• The angle formed by the incident beam and the normal is the angle of incidence.

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• The angle formed by the reflected beam and the normal is the angle of refection.


• According to the law of reflection, the angle of incidence is equal to the angle of refection.

• All reflected waves obey this law.

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Refraction

• Refraction is the bending of a wave caused by a change in its speed as it moves from one medium to another.

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• If the wave is traveling at an angle when it passes from one medium to another, it changes direction, or bends, as it changes speed.

• When a wave passes from one medium to anothersuch as when a light wave passes from air to waterit changes speed.

• Light waves travel slower in water than in air.

• This causes light waves to change direction when they move from water to air or air to water.

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• When light waves travel from air to water, they slow down and bend toward the normal.

Refraction of Light in Water

• When light waves travel from water to air, they speed up and bend away from the normal.

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Refraction of Light in Water

Diffraction

• Diffraction occurs when an object causes a wave to change direction and bend around it.

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• Diffraction and refraction both cause waves to bend.

• The difference is that refraction occurs when waves pass through an object, while diffraction occurs when waves pass around an object.

Diffraction

• When waves strike an object, several things can happen.

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• The waves can bounce off, or be reflected.• If the object is transparent, light waves

can be refracted as they pass through it.

• Waves also can behave another way when they strike an object. The waves can bend around the object.

Diffraction

• Waves also can be diffracted when they pass through a narrow opening.

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• After they pass through the opening, the waves spread out.

Diffraction of Radio Waves

• AM radio waves have longer wavelengths than FM radio waves do.

• Because of their longer wavelengths, AM radio waves diffract around obstacles like buildings and mountains.

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• As a result, AM radio reception is often better than FM reception around tall buildings and natural barriers such as hills.

Interference • When two or more

waves overlap and combine to form a new wave, the process is called interference.

• Interference occurs while two waves are overlapping.

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Constructive Interference • In constructive interference, the waves

add together.

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•The diagram labeled A shows two identical waves (same amplitude, same wavelength) traveling in the same direction at the same time.

•If the two waves travel along the same path at the same time, they will behave as one.

Constructive Interference • In constructive interference, the waves

add together.

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•What will the combined wave look like? •The crests of the first wave will occur at the same place as the crests of the second wave.•The energy from the two waves will combine. •Thus the amplitude of the new wave will be twice the amplitude of either of the original waves.

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• The amplitude of the new wave that forms is equal to the sum of the amplitudes of the original waves.

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B. When the crests of one wave align with the troughs of another, they cancel each other out.

C. If one wave travels a little behind the other, they combine both constructively and destructively at different places.

Destructive Interference

• In destructive interference, the waves subtract from each other as they overlap.

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• This happens when the crests of one transverse wave meet the troughs of another transverse wave.

Standing Waves

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•If you tie a rope to a doorknob and continuously shake the free end, waves will travel down the rope, reflect at the end, and come back.

•The reflected waves will collide with the incoming waves.

•When the waves meet, interference occurs.

Standing Waves

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•If the incoming wave and the reflected wave combine at the right places, the combined wave appears to be standing still. •A standing wave is a wave that appears to stand in one place, even though it is really two waves interfering as they pass through each other. •If you make a standing wave on a rope, the wave looks as though it is standing still.

Standing Waves

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At certain points, destructive interference causes the two waves to combine to produce an amplitude of zero, as in the diagram below. These points are called nodes.

•The nodes always occur at the same place on the rope. •The crests and troughs of the standing wave are called antinodes. •These are the points of maximum energy.

Resonance 10.3


•Have you ever pushed a child on a swing? •At first, it is difficult to push the swing. •But once you get it going, you need only push gently to keep it going. •When an object is vibrating at a certain frequency, it takes very little energy to maintain or increase the amplitude of the wave.

Resonance 10.3


•Most objects have a natural frequency of vibration.

•Their particles vibrate naturally at a certain frequency.

•Resonance occurs when vibrations traveling through an object match the object’s natural frequency.

•If vibrations of the same frequency are added, the amplitude of the object’s vibrations increases.

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•If an object is not very flexible, resonance can cause it to shatter.

•For this reason, marching troops are told to break step as they cross a bridge.

•If they all march across the bridge in perfect step, it is possible that the pounding could match the natural frequency of the bridge.

•The increased vibration could cause the bridge to collapse.

http://www.youtube.com/watch?v=qbOjxPCfaFk&feature=player_detailpage



Waves 1.3

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