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# Waves

## on Feb 24, 2010

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## WavesPresentation Transcript

• Waves
• Slinky Waves A wave is caused by a vibrating source, and travels outwards from the source. The particles of the medium through which a wave travels move about their equilibrium positions; they do not move along with the wave. Energy is transferred from place to place by a wave.
• Does the size of a pulse change as it travels along the spring?
• What is there was no friction?
• What happens when a pulse reflects at the fixed end? (we need to clamp the slinky at one end)
• How does the speed change if you change the tension? (does not work well, should be the same)
• What happens if pulses set off from opposite ends at the same time?
• What you are going to achieve today
• By the end of the lesson you will know
• What are the differences between transverse and longitudinal waves?
• What is a plane-polarised wave?
• What physical test can distinguish transverse waves from longitudinal waves?
• What is meant by the amplitude of a wave?
• Between which two points can the wavelength be measured?
• How is the frequency of a wave calculated from its period?
• What is phase?
• Important wave diagram
• Phase Dual pendulums to demo phase
• Phase Demo
• Phase on the oscilloscope
• V = f λ
• V = velocity (m/s)
• f = frequency (how many waves in a second)
• λ = wavelength (m)
λ c = f λ is used for electromagnetic waves as the velocity of light is always 3 x 10 8 m/s
• All go at the speed of light
• Speed, wavelength and frequency
• 1)Light travels 300,000 km in one second.
• a) How long does it take light to travel 300 m?
•   b) How long does it take light to travel 300 mm?
• 2)The speed of sound in a metal is 1000 m s–1.
• a.) What is the wavelength if the frequency is 2 kHz?
•   b.) What is the wavelength if the frequency is 2 MHz?
• 3)What is the frequency of sound waves of wavelength 100 mm?
• 4)What is the frequency of radio waves of wavelength 100 mm?
• 5)If you could 'freeze' ultrasound of wavelength 5 mm as it travels through tissue, how many waves would there be in a one metre length? This quantity is called the wave number . How is it related to wavelength?
• 6)If a thunder clap comes 1.5 s after the lightning strike, how far away is the strike? Explain your calculation.
The speed of sound in air 330 m/s speed of light 3 x 10 8 m/s. s = v ∆ t
• 1 microsecond b) 1 nanosecond
• 200 waves per metre; the wave number is the reciprocal of the wavelength.
6) s = v ∆ t = 330 x 1.5 = 495 m, light transit time is negligible
• 3D glasses
• The water tubs waves in another medium
• The speed of a water wave
• Create a water wave at one end of the tub and
• Time it to the other end
• Change the depth of water
• There should be a difference but it is very fast
• Speed = √g x depth (metres)
• Wave height
• (from trough to crest),
• Wavelength (from crest to crest),
• Period (time interval between arrival of consecutive crests at a stationary point),
• The direction of wave propagation.
• CHALLENGE! Demonstrate longitudinal and transverse waves In a group of 5 demo both types of wave