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# Waves unit (1)

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## Waves unit (1)Presentation Transcript

• Waves Unit Waves, sound and light
• Waves and Vibrations
• A vibration is a movement that occurs in a period of time.
• A wave is a movement that occurs in a period of time and also in space.
• A wave is a way of transferring energy.
• Time for a Cycle
• The time that it takes for a wave (or vibration) to complete one cycle is called a period (T).
• A cycle is when a process starts to repeat itself.
• A period is measured in units of time (seconds, most of the time).
• Simple Harmonic Motion
• The swinging of a pendulum (or a mass bouncing on a spring), the motion back-and-forth, is called simple harmonic motion.
• The result of this simple oscillation back-and-forth is a sine wave.
• Wave Characteristics
• There are some distinct parts of a wave called characteristics.
• The peak is the top part of the wave.
• The trough is the bottom part of the wave.
• More Characteristics
• The baseline is the line of 0 energy in the wave.
• The wavelength (  )is the distance from peak to peak of the wave.
• The amplitude is the distance from the baseline to the peak of the wave.
• More Characteristics
• Period (T) is the time that it takes for the wave to start over again, typically it is the time for one wavelength (  ) to pass by.
• Frequency ( f )is the number of  to pass by in a time frame.
• Frequency is measured in Hertz (Hz).
• Wave Speed
• It is calculated by dividing distance by time.
• With a wave, it is no different, we just change some of the symbols
• The velocity of a wave is calculated by dividing wavelength by period of the wave.
• v =  / T or
• v =  f
• v = d / t
• How is speed (velocity) calculated?
• Simple Calculations
• What is the speed of a wave that has a wavelength of 10.0 meters and it takes 2.0 seconds to go by?
• The solution is:
•  = 10.0 m
• T = 2.0 s
• v =  / T
• So,
• v = 10.0 m / 2.0 s
• v = 5.0 m/s
• What is the velocity of a wave that has a wavelength of 5.0 x 10 -3 meters and a frequency of 20.0 Hz?
• The solution is:
•  = 5.0 x 10 -3 m
• f = 20.0 Hz
• v =  f
• So,
• v = (5.0 x 10 -3 m)(20.0 Hz)
• v = 0.10 m/s
• What is the wavelength of a wave that travels at 4.00 m/s and has a period of 3.0 seconds?
• The solution is:
• v = 4.00 m/s
• T = 3.0 s
•  = v T
• So,
•  = (4.00 m/s)(3.0 s)
•  = 12 m
• What is the wavelength of a wave that travels at 200.0 m/s and has a frequency of 40.0 Hz?
• The solution is:
• v = 200.0 m/s
• f = 40.0 Hz
•  = v / f
• So,
•  = (200.0 m/s)/(40.0 Hz)
•  = 5.00 m
• What is the frequency of a pendulum if it swings back and forth with a period of 4.0 seconds?
• The solution is:
• T = 4.0 s
• f = 1 / T
• So,
• f = 1 / (4.0 s)
• f = 0.25 Hz
• What is the period of a wave if the wavelength is 20.0 m and the velocity is 10.0 m/s?
• The solution is:
•  = 20.0 m
• v = 10.0 m/s
• T =  / v
• So,
• T = (20.0 m)/(10.0 m/s)
• T = 2.00 s
• Types of Waves
• There are 2 basic types of waves.
• One is called a longitudinal wave.
• The other is called a transverse wave.
• Longitudinal Wave
• Longitudinal waves move the medium parallel to the motion of the wave.
• Longitudinal waves are compression waves.
• Longitudinal waves have the same characteristics as transverse waves.
• However, peaks are referred to as condensations and troughs are rarefactions.
• Transverse Waves
• Transverse waves move the medium perpendicular to the motion of the wave.
• Transverse waves are like ocean waves.
• A Different Type of Transverse Wave
• A different type of transverse wave is called the standing wave.
• This is a wave that travels back and forth between 2 points.
• N is called a node (null point).
• A is called an anode (anti-null point).
• How do waves interact?
• When waves interact, it is called interference.
• There are 2 types of interference.
• Constructive interference
• Destructive interference
• Constructive interference
• Constructive interference is when 2 or more waves come together and build themselves up.
• They add their collective amplitudes together.
• Destructive interference
• Destructive interference is when 2 or more waves tear each other down.
• They are off by “half a phase” and their amplitudes (peak and trough) subtract from each other.
• What happens if the interference keeps progressing?
• If the interference keeps going, then it is called resonance.
• Resonance is when the waves keep building upon each other.
• This occurs with constructive interference.
• What happens if the source of the wave is moving?
• If the source of the wave is moving, then the wave adjusts for the speed.
• It compresses or expands depending on the direction the source is moving relative to the detector of the wave.
• Doppler Effect with Sound
• If the object isn’t moving, then the sound generated stays constant.
• If the object is moving toward another object, then the sound is compressed.
• This gives the sound a higher frequency and a higher pitch.
• If the source is moving away from the observer, then the sound is stretched out as it reaches the observer.
• This gives the sound a lower frequency or a lower pitch.
• Doppler Effect with Light
• Similarly to sound, if the object is not moving, then the light emitted is normal.
• If the source of the light is moving close to the speed of light, then the light is compressed and it is given a higher frequency (more energy).
• However, if the source of the light is moving away from the observer at nearly the speed of light, then it will have a stretched out wavelength and lower frequency (less energy).
• This is also known as Red Shift.
• Light
• Light is known as an Electromagnetic wave.
• But, light is also known as a photon.
• Light is both a particle and a wave.
• This is called the Duality of Light.
• Light is a particle with a wave-like characteristic.
• More light
• Light does not need a medium to travel, neither do any of the EM waves.
• Light travels at it’s own speed.
• The speed of light is 3.00 x 10 8 m/s or 186,000 miles/sec.
• Light travels in straight lines.
• The photon is a packet of energy.
• Einstein showed that light was a particle.
• He was able to show that the momentum of the photon was able to move electrons, causing electricity to be produced.
• Waves Bounce
• When a sound wave bounces, we call it an echo.
• When a light wave bounces, we call it a reflection
• The law of reflection says that the reflected ray bounces off at the same angle, to the normal, as the incoming ray.
• Flat Mirror
• A flat mirror (flat surface) will reflect a true image.
• This image will appear as if it were a copy of the original.
• Concave Mirror
• Concave mirrors follow the law of reflections.
• Concave mirrors have the reflective surface on the inside of the curve.
• If an object is beyond the focal point of the concave mirror, then the image is smaller and inverted.
• If an object is within the focal point of the concave mirror, then the image is magnified.
• Convex Mirror
• A convex mirror has its reflective surface on the outside of the curve of the mirror.
• Therefore, the focal point is inside the mirror, and the object of the reflection can not reach that focal point.
• Images seen in a convex mirror appear as smaller, right-side up, and the view is spread out more.
• Refraction
• Refraction is when a light wave travels through a medium and bends.
• Whenever, light enters or exits a medium, the light rays bend.
• A medium is a material that waves travel through.
• As the incoming light enters the medium at an angle, the medium bends that light according to its “index of refraction”.
• This index is constant for each medium.
• The different mediums also cause the light to change speeds.
• This bending of the light gives us the illusion that the objects are bent or in different locations when they are in water.
• Lenses
• Like mirrors, there are 2 different types of lenses.
• A concave lens is curved so the open part is the inside of the curve.
• All lenses follow the same law of refraction.
• Light bends as it goes through a lens.
•
• Convex Lens
• Convex lenses have the medium within the curve of the lens.
• Unlike the concave lens, which bends the rays outward, convex lenses bend the rays inward to a single point.
•
•
• The Electro-Magnetic Spectrum
• The electro-magnetic spectrum is also referred to as the EM spectrum.
• This spectrum consists of waves that are driven by electric fields and magnetic fields that continuously create each other.
• Unlike other waves, the EM waves do not need a medium to propagate.
• All EM waves (including light) travel at 3.0 x 10 8 m/s or 186,000 miles/second.