Waves Flashcards
What is a progressive wave?
Vibrations through the air that carries energy from one place to another without transferring any material
How is a wave produced?
Caused by something making particles or fields oscillate at a source
What happens to the energy at the source of a wave?
It decreases as the wave transfers energy away from the source
What is reflection?
When a wave bounces back when it hits a boundary
What is refraction?
When a wave changes direction as it enters a different medium
Why does a wave change direction (refracts) when it enters a different medium?
Because the wave speeds up or slows down
What is diffraction?
When a wave spreads out as it passes through a gap or round an obstacle
What is the displacement of a wave?
How far a point on the wave has moved from it’s undisturbed position
What is the SI units of the displacement of a wave?
Metres
What is the amplitude of a wave?
The maximum displacement of a wave (e.g. distance from undisturbed position to peak or trough)
What is the SI units of the amplitude of a wave?
Metres
What is the wavelength of a wave?
The length of one whole wave cycle (e.g. distance from peak to peak or trough to trough)
What is the period of a wave?
The time taken for one whole wave cycle
What is the SI units of the wavelength of a wave?
Metres
What is the SI units of the period of a wave?
Seconds
What is the frequency of a wave?
The number of whole wave cycles per second passing a given point
What is the SI units of the frequency of a wave?
Hertz
What is the phase of a wave?
A measurement of the position of a certain point along the wave cycle
What is the phase difference of a wave?
The amount by which one wave is behind another
What is the phase and phase difference of a wave measured in?
Degrees or radians as an angle or in fractions of a cycle
How do you work out the phase difference of 2 different waves?
Work out the difference between 2 identical points on each wave (e.g. the peak of both waves)
What equation links frequency and time period?
F = 1 / T
What is the wave speed?
The speed that a wave travels at
What is the equation for wave speed?
Wave speed = Frequency x Wavelength
What speed do all electromagnetic waves travel at through a vacuum?
3x10^8 ms^-1
What is a transverse wave?
A wave where the oscillations are at right angle to the direction of energy propagation
Are EM waves transverse or longitudinal?
Transverse
How do transverse waves travel?
As vibrations through magnetic and electric fields
What are 3 examples (except EM waves) of a transverse wave?
Waves on a string
Waves on water
Seismic S-waves
What are the 2 types of graphs you can draw to represent transverse waves?
Displacement-Distance graph
Displacement-Time graph
What is a longitudinal wave?
A wave where the oscillations are parallel to the direction of energy propagation
What is an example of a longitudinal wave?
Sound waves
What does a sound wave consist of?
Alternate areas of compression and rarefaction of the medium it’s travelling through
Why can’t sound waves travel in a vacuum?
Because it has areas of compression and rarefaction
Is a Seismic P-wave transverse or longitudinal?
Longitudinal
How do you find the wavelength of a longitudinal wave?
It’s the distance between compressions
What is a polarised wave?
A wave that only oscillates in one direction (as normal waves oscillate in all directions on the plane perpendicular to the direction of motion)
What is a polarising filter?
A filter that only transmits vibrations of a wave in one direction (the plane of transmission)
What is the plane of transmission?
The only direction of vibrations that a polarising filter transmits
What will you see if you hold up 2 polarising filters at right angle to each other?
Nothing as no light will get through
What does the transmission axis of a polarising filter show?
The only direction of vibrations which can pass through the filter
What is the only type of wave that polarisation can occur?
Transverse waves
What are 2 uses of polarisation in the real world?
Glare reduction
Improving TV and radio signals
When does superposition occur?
When 2 waves pass through each other
What is superposition?
The combination of displacements experienced when 2 2 waves pass through each other
What can the superposition of 2 waves result in?
Interference
What are the 2 types of interference?
Constructive
Destructive
What is constructive interference?
When 2 waves interfere to make a wave with a larger displacement
What is destructive interference?
When 2 waves interfere to make a wave with a reduced displacement
What is total destructive interference?
When 2 waves meet with equal and opposite displacements meet and cancel each other out completely
Describe what is meant by 2 waves in phase?
Where 2 points on a wave are in phase if they’re both at the same point in a wave cycle
What does a phase difference of 180° (π radians) mean?
The 2 waves are out of phase
What is a stationary wave?
A wave that is the superposition of 2 progressive waves travelling in opposite directions with the same amplitude, frequency and wave speed
How much energy is transmitted by a stationary wave?
None, because the 2 waves are carrying equal amounts of energy but in opposite directions
What happens at the resonant frequency?
The wave forms a harmonic
What is a node for a stationary wave?
A point at which the amplitude of the vibrations is 0
What is an antinode for a stationary wave?
A point at which the amplitude of the vibration is at maximum
Describe what happens to a length of string at a resonant frequency?
An exact number of wavelengths fit onto the length of string forming a harmonic
What type of interference happens at a node?
Total destructive interference
What type of interference happens at an antinode?
Constructive interference
Describe the first harmonic in terms of number of wavelengths and the number of ‘loops’?
One half wavelength fits on the string
1 loop with a node at each end
Describe the second harmonic in terms of number of wavelengths and the number of ‘loops’?
Two half wavelengths fit on the string (1 wavelength)
2 loops with a node in the middle and at each end
Describe the third harmonic in terms of number of wavelengths and the number of ‘loops’?
3 half wavelengths fit on the string
3 loops with 4 nodes in total
How many wavelengths will fit on a string at the A harmonic?
A/2 wavelengths
What is the equation for mass per unit length of the string, μ?
μ = Mass / Length
What is the SI unit for mass per unit length?
Kg m^-1
What is the equation for the tension of a string?
T = Mass x Gravity
What is the SI unit of tension?
Newtons
What is the equation for calculating the frequency of the first harmonic?
f = (1 / 2 x length) x (√(tension / mass per unit length))
What is the affect of a longer string length on the resonant frequency?
It means a lower resonant frequency, because the half-wavelength is longer
What is the affect of a higher mass per unit length of string on the resonant frequency?
It means a lower resonant frequency, because waves travel more slowly down the string
What is the affect of a greater tension on the string for the resonant frequency?
A higher resonant frequency, because waves travel faster on a more taught string
What is it called when waves spread out as they come through a narrow gap or go around narrow obstacles?
Diffraction