wave motion. all waves in the ocean, sound and light can interact this way moving through time and...
TRANSCRIPT
Wave motion
ALL WAVES In the ocean, sound and light can
interact this way
• Moving through time and space– Wave types
• Effects of waves on each other– Interference– Standing waves– The Doppler effect– Bow waves– Shock waves
SPEED (MOVING THROUGH TIME)
If you were watching a wave come past, it speed is how fast it passes a stationary point…
Wave speed =
distance travelled ÷ time
Another way of calculating
wave speed = frequency wavelength
SPEED
Example: If a train of freight cars, each 10 metres long, rolls past at the rate of 2 cars each second, what is the speed of the train
If using: Wave speed = distance travelled ÷ time
= (2 x 10 metres) ÷ 1 second
= 20 metres per second
If using Wave speed = frequency wavelength
= 2 Hz (cars per second) 10 m (each car)
= 20 metres per second
SPEED(MOVING THROUGH SPACE)
Electromagnetic waves
do not need material – they really go through space!
Notice all the types of radiation in the electromagetic spectrum
SPEED
In space ( a near perfect vacuum)
all electromagnetic radiation travels at the same speed!
symbol c.
300,000,000 metres per second! (exactly 299,792,458 m/s)
SPEED(MOVING THROUGH SPACE)
Other kinds of waves go through materials – called media
The speed of sound in air is not constant, but depends on air density – it varies from 330 m/s to 350 m/s
What is going on?
Think of the little dots as gas molecules.
compressed air = gas molecules are close together
rarified air = gas molecules are far apart (are rare)
As the tynes of the tuning fork move out – they compress a sphere of air
As they move in, they stretch or rarify the air
Bands of alternating compressed and rarified air create sound waves!
Can you now explain why sound travels better in COLD, DENSE air?
Notice that at any particular air temperature, the speed of sound is the same no matter what the frequency
Frequency (Hz) Wavelength (λ) Wave speed (m/s)
160 2.13 340
264 1.29 340
396 0.86 340
528 0.64 340
If this didn’t happen, we could go to a concert and hear the high notes before the low notes!
Calculations – how was this worked out?
Recall Wave speed = frequency wavelength
Frequency (Hz) Wavelength (λ) Wave speed (m/s)
160 2.13 340
264 1.29 340
396 0.86 340
528 0.64 340
Sound waves belong to a group of waves called longitudinal waves
Longitudinal waves move particles to and fro in the same direction as their energy travels
Compare Longitudinal and Transverse waves:
How would you define particle movement in a transverse wave?
Which of these examples are Longitudinal (L) and which are Transverse (T)?
• Ocean waves
• Light waves
• Sound waves
• Waves in a skipping rope
• ‘Mexican’ wave
• X-rays
• Microwaves
T
T
T
L
T
T
T
INTERFERANCE
You have already seen examples of interference in the video ‘Waves in the Ocean’
Look at the animated gifs and decide what the terms mean
INTERFERANCE Constructive interference
Constructive interference is when waves reinforce each other
INTERFERANCE Destructive interference
Destructive interference is when waves cancel each other out
INTERFERANCE
Moire interference patterns
Interference is a characteristic of all wave motion
You might see this sliding
two combs across each other
Standing wavesIf you tie a rope to a wall and shake it, after a while the wave is reflected back to form a standing wave, with stationary parts called nodes. Antinodes are halfway between nodes
This animation shows a standing wave resulting from the superposition of two plane waves traveling opposite directions.
The left side of the figure shows a representation of the molecular distribution. The darker areas represent areas where the density is greater indicating a compression. Midway between the compressions are regions of lower density indicating rarefactions.
The right side of the figure shows a plot of the resulting acoustic pressure or acoustic velocity profile.
The Doppler effect1.Suppose a stationary car horn blasts in still air (The pink lines indicate compressed air in sound)
2. In the second instance, suppose the car moves to the left as it sounds the horn
Blue shift – if the object moves in the direction of the spreading ripples, the wavelengths get shorter (in the case of light waves – colour looks ‘bluer’ (and therefore used by astronomers, to evaluate whether stars are moving towards us)
Red shift – if the object away from the direction of the spreading ripples, the wavelengths get longer (in the case of light waves – colour looks ‘redder’ (this is always on the opposite side of the blue shift).
Bow wavesThis is a variation of the Doppler effect caused when the object moves faster than the wave speed.
Shock wavesBow waves occur on surfaces – hence you see them off ships’ bows.
In air – the same wave is 3-dimensional – called a shock wave
Quiz: Bow wave or Shock wave?
waving
Goodbye!