Chapter 11 - Waves Flashcards
What are P waves and S waves?
Primary and secondary waves
What is a progressive wave and what do they transfer?
An oscillation that travels through matter but only transfers energy
What happens when a progressive wave travels through a medium?
The particles move from their original equilibrium post into a new position
The particles exert forces on each other so a displaced particle experiences a restoring force which brings it back to its original position
What is a transverse wave?
A wave in which the oscillations are perpendicular to the direction of energy transfer
What do peaks and troughs in a transverse wave show?
It shows where the particles are at a maximum displacement from their equilibrium position
What are some examples of transverse waves?
Waves on the surface of water
EM waves
S waves in earthquakes
Waves on stretched strings
What is a longitudinal wave?
A wave in which oscillations are parallel to the direction of energy transfer
What are some examples of longitudinal waves?
Sound waves
P waves in earthquakes
What are compressions and rarefactions?
Similar to peaks and troughs but are instead created by longitudinal waves (compression)
The particles are displaced and bounce off each other
What is displacement?
Distance from equilibrium position
Unit - m
What is amplitude?
Maximum displacement from equilibrium position
Unit - m
What is wavelength?
Minimum distance between two points in phase
E.g - from one peak to another or one trough to another
Unit - m
What is period of oscillation?
Time taken for one oscillation or one wavelength to pass a point
Unit - s
What is frequency?
Number of wavelengths passing a point per unit time
Unit - Hz
What is wave speed?
Distance travelled by the wave per unit time
Unit - ms^-1
What is the wave equation?
Wave speed = Frequency x Wavelength
What is a wave profile and what does it show?
A graph showing the displacement of the particles in the wave against the distance along the wave.
(A snapshot of the wave)
What can the wave profile be used for?
Determine the wavelength and amplitude of both types of wave
How do you work out where a particle will move from a wave profile?
Don’t move the particle but move the wave the particle will follow the peaks and troughs.
What is phase difference?
The difference between the displacements of particles along the wave or the displacements of particles in different waves.
It is measured in degrees or pi radians.
What does it mean when two particles are in phase?
They both reach maximum positive displacement at the same time.
They have a phase difference of 0
How do you describe two particles that are separated by a distance of one whole wavelength?
Phase difference of 2pi or 360 degrees
What does it mean when two particles are in antiphase?
One reaches maximum positive displacement as the other reaches maximum negative displacement
They have a phase difference of pi or 180 degrees
What is another type of graph showing waves (not wave profile) and what can it be used to determine?
Displacement time graph which shows how displacement of a particle varies as time changes
It can be used to determine the period and amplitude of both types of waves
What is reflection?
Reflection occurs when a wave changes direction at a boundary between two different media and remains in the original medium
Wavelength and frequency do not change
What is the law of reflection?
It states that the angle of incidence = angle of reflection
What is refraction?
Refraction occurs when a wave changes direction as it changes speed when it passes from one medium to another
Whenever a wave refracts there is always a partial reflection
Doesn’t change frequency but if the wave slows down then wavelength decreases
What happens if the wave slows down or speeds up at the boundary between two media?
Slows down - Refract towards the normal
Speeds up - Refracts away from the normal
What happens to sound and EM waves in a denser medium?
Sound waves speed up in a denser medium
EM waves slow down in a denser medium
What happens when water waves enter shallower water?
They slow down and the wavelength gets shorter
What is diffraction?
When waves travel through a gap or around an obstacle they spread out
Speed, wavelength and frequency do not change
Same gap and wavelength size means more diffraction
What is polarisation?
Make the particles of a wave oscillate along one direction only
Longitudinal waves cannot be plane polarised as they are already limited to one plane
What is partial polarisation?
When tranverse waves reflect off a surface and there are more waves oscillating in one plane compared to the other
What is the intensity of a wave?
The radiant power passing through a surface per unit area
What is the equation for intensity and what are its units?
Intensity = Power passing through surface / Cross sectional area of surace
Units - Watts per square metre
How do you work out intensity at a distance from the source?
Intensity = Total radiant power / 4 pi r squared
r = distance from source
What is the relationship between intensity and distance from the source?
Distance is inversely proportional to the square of intensity
(Double distance means intensity decreases by 2 squared which is 4)
What is the relationship between intensity and amplitude?
Intensity is directly proportional to the square of the amplitude
Double amplitude and intensity will quadruple
Do EM waves need a medium?
No they can travel through a vacuum
What is the electromagnetic spectrum?
In order of reducing wavelength (m)
10^6 - Radio Waves 10^-1 - Microwaves 10^-3 - Infrared 7x10^-7 - Visible 4x10^-7 - Ultraviolet 10^-8 - X rays 10^-10 - Gamma rays
What is the speed of an EM wave through a vacuum?
3x10^8 ms^-1
What is a polarising filter and why do we use it?
Most natural EM waves are unpolarised so the electric field oscillates in random planes 90 degrees to the energy transfer.
Each filter only allows waves with a particular orientation through
What does a horizontal polarising filter do?
Horizontally polarises EM waves. (It is drawn with vertical bars)
What is the real world use of polarising filters?
Sunglasses
In order to reduce interference in communications transmitters some are polarised vertically and other are polarised horizontally
What is a refractive index?
The relative speed at which light travels through the medium compared to the light at which light travels through a vacuum
What is the equation for refractive index?
Refractive Index = Speed of light through vacuum(3x10^8 m^s-1) / Speed of light through material
Refractive Index has no units
What equation describes what happens when light travels from one medium to another?
n1 sinθ1 = n2 sinθ2
n=refractive index
θ= Angle between the normal and the incident ray
What is total internal reflection?
When the light strikes the boundary at a large angle to the normal, it is totally internally reflected and no light is refracted out of the medium
What are the two conditions required for total internal reflection?
The light must be travelling through a medium with a higher refractive index as it strikes the boundary with a lower refractive index
The angle at which the light strikes the medium must be above the critical angle (which depends on refractive index)
What is the relationship between refractive index and the critical angle?
The higher the refractive index the lower the critical angle
How do you work out critical angle?
Inverse Sin (1/n) for air
or
Sin critical angle = refractive index of second medium / refractive index of incidence medium
