NIS – PHYSICAL SCIENCE

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Lecture 42 – Lecture 43The Behavior of Waves

Ozgur Unal

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ReflectionEveryday you look at yourself in the mirrorYou probably heard the echo of your voice somewhereHow are these possible?

Reflection occurs when a wave strikes an object and bounces off it.Light waves are reflected twice (from your face and from the mirror) when you see your face in the mirror.Echoes form when sound waves reflect off a surface.

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ReflectionAccording to the law of reflection, the angle of incidence is equal to the angle of reflection.

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RefractionWhy does the pen in the glass look broken?

The speed of a wave depends on the medium it is moving through.When a wave passes from one medium to another it changes its speed.If the wave is traveling at an angle when it passes from one medium to another, it changes direction, or bends, as it changes its speed.Refraction is the bending of a wave caused by a change in its speed as it moves fro mone medium to another.

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RefractionExample: Light waves travel more slowly in water that in air.This causes light waves to change direction when they move from water to air or air to water.

Objects that are under water seem closer to the surface than they really are.This is because of refraction of light rays as they pass from water to air.

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DiffractionHow can you describe the change in these waves as they pass around the rocks?

Diffraction occurs when an object causes a wave to change direction and bend around it.Diffraction and refraction both causes waves to bend.The difference is that refraction occurs when waves pass through an object, while diffraction occurs when waves pass around an object (or, through an opening).

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DiffractionThe amount of diffraction that occurs depends on how big the obstacle or opening is compared to the wavelength.

If the obstacle is smaller than wavelength, the waves bend around it.If the obstacle is larger than the wavelength, the waves do not diffract much.Explain why we cannot see a person talking around a corner although we can hear her.

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InterferenceWhen two or more waves overlap and combine to form a new wave, the process is called interference.This new wave exists only while the two original wave continue to overlap.

There are two ways that the waves can combine:Constructive interferencevsDestructive interference

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InterferenceConstructive interference:In constructive interference the waves add together.

Destructive interference:In destructive interference the waves subtract from each other as they overlap.

Can compressional waves, such as sound, have interference?

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Standing Waves

A standing wave is a special type of wave pattern that forms when waves equal in wavelength and amplitude, but traveling in opposite directions, continuously interfere with each other.The places where the two waves always cancel are called nodes.The nodes always stay in the same place on the rope. Meanwhile, the wave pattern vibrates between the nodes.Example: Standing waves in a violin string.

www.walter-fendt.de/ph14e/stwaverefl.htm

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ResonanceThe tuning forks of different type vibrate at certain frequencies called natural frequencies.All objects have their own natural frequencies.Natural frequencies depend on the object’s size, shape and the material it is made from.

An object can be made to vibrate at its natural frequency.Example: A tuning fork with natural frequency of 512 Hz starts vibrating by a sound wave with a frequency of 512 Hz.The process by which an object is made to vibrate by absorbing energy at its natural frequency is called resonance.

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ResonanceSometimes, resonance can cause disasters see the handout