4.4 Waves Flashcards
What is a transverse wave? (2)
Oscillations are perpendicular to the direction of energy transfer
The particles vibrate at 90° to the motion of the wave
What is a longitudinal wave? (2)
Oscillations are parallel to the direction of energy transfer
The particles vibrate in the same direction as the wave
What are examples of transverse waves? (4)
Water waves
EM waves
Strings
S-waves
What are examples of longitudinal waves? (2)
Sound waves
P-waves
What is displacement?
The distance of a particle from the equilibrium position (s), metres
What is amplitude?
Maximum displacement from equilibrium (A), metres
What is wavelength?
Distance between two identical points on adjacent waves (λ), metres
What is period?
Time taken for one wavelength to pass a given point (T), seconds
What is frequency?
Number of wavelengths passing a given point per unit time (f), Hz
What is wave speed?
Distance travelled by wave per unit time (v/c), ms^-1
How can an oscilloscope be used to determine frequency? (4)
An oscilloscope plots a graph of PD against time
Horizontal squares (timebase) represent a certain time interval
The height represents PD
From the timebase the period can be determined and used to find the frequency
What is the equation linking frequency and time period?
f = 1/T
What is the wave equation?
v = fλ
How is the wave equation derived? (2)
Since v = d/t, in the time for one cycle a wave travels one wavelength (v = λ/t)
As T = 1/f:
v = λ / (1/f) so v = λf
What is a history graph? (2)
Displacement-time
Shows how the position of a single particle series with time as a wave passes through (time period)
What is a snapshot graph? (2)
Displacement-distance
Shows entire wave at one moment (wavelength)
What is phase difference?
The difference in the relative positions of two or more particles in the same wave or the relative points in the cycle of two or more different waves
What is phase? (2)
Describes the current point in the cycle of something that changes periodically, expressed either in radians or degrees
One complete wave = 2π radians
How can the direction a particle is moving be determined?
Draw the wave as it would look a short amount of time later in order to see whether specific particles are moving up or down
What is antiphase?
A phase difference of 180°
What is refraction? (4)
Refraction occurs when a wave passes from one median to another
If the wave slows down it refracts towards the normal and vice versa
There is always partial reflection
Refraction affects wavelength but not frequency
What is reflection? (3)
Reflection occurs when a wave changes direction at a boundary between two media
The angle of incidence = the angle of reflection
The wavelength and frequency remains the same
What is diffraction? (3)
Diffraction occurs when a wave passes through a gap or around an obstacle
Speed, wavelength and frequency stay the same
Diffraction effects are most significant when the gap is similar in size to the wavelength
What is dispersion?
When white light enters a prism it is refracted, first as it enters the prism and again as it leaves Longer wavelengths (red) are refracted less than shorter wavelengths (purple)
What is polarisation? (2)
The particles oscillate along one direction only, confining the wave to a single plane
The plane of oscillation is both where the particles oscillate and the direction they travel in
Which waves can be polarised?
Transverse waves
Why can’t longitudinal waves be plane polarised?
The oscillations are already limited to one plane (the direction of energy transfer)
What is partial polarisation?
When transverse waves reflect, there are more waves oscillating in a particular plane but the wave is not completely polarised
What is the relationship between intensity and distance? (3)
Intensity (Wm^-2) = Power (W) / Area (m^2)
When a wave travels from a point source, they spread across the surface area of a sphere: I = P / 4πr^2
Intensity is proportional to 1/(r^2)
What is the relationship between intensity and amplitude? (3)
Intensity is proportional to amplitude^2
Further away from a source, intensity decreases, causing a decreasing amplitude which in turn leads to a reduced average speed.
Halving the amplitude, halves the speed which causes the kinetic energy to be four times as small
What is an electromagnetic wave? (3)
A transverse wave
They require no medium to travel through
Electric and magnetic waves oscillating at right angles to each other
Wavelengths - Radio Waves
> 10^6 metres to 10^-1 metres
Wavelengths - Microwaves
10^-1 metres to 10^-3 metres
Wavelengths - Infrared
10^-3 metres to 7x10^-7 metres
Wavelengths - Visible Light
7x10^-7 metres to 4x10^-7 metres
Wavelengths - Ultraviolet
4x10^-7 metres to 10^-8 metres
Wavelengths - X-rays
10^-8 metres to 10^-13 metres
Wavelengths - Gamma Rays
10^-10 metres to <10^-16 metres
How are X-rays and gamma rays classified? (2)
Not by wavelength due to overlapping ranges
By origin: X-rays are emitted from fast-moving electrons and gamma rays come from unstable atomic nuclei
What speed do EM waves travel at?
3x10^8 ms^-1 through a vacuum
What is a polarising filter?
Each filter only allows waves with a particular orientation to pass through
What are Polaroid filters? (2)
Plastic films that polarise light
Two filters on top of each other will let varying amounts of light through as they are turned through 360° depending on which direction the films are polarising the light
How are microwaves polarised?
Using a metal grille
What is intensity?
The energy incident per square metre of a surface per second
When is there no refraction?
When a wave enters perpendicular to the new medium (along the normal)
What is optical density measured using?
The refractive index
What is the refractive index?
The ratio of the velocity of light in a vacuum to its velocity in a specified medium
How is absolute refractive index found?
n = c / vm
n: refractive index (>1 except for air)
c: speed of light in a vacuum
vm: speed of light in the specified medium
How is relative refractive index calculated?
n2 / n1 = v1/v2
What is Snell’s law? (2)
The ratio of the wines of the angles of incidence and refraction is equivalent to the ratio of the velocities in the two media
nsinθ = constant
What is the critical angle?
The largest angle at which refraction out of a denser medium is just possible
What is total internal reflection?
When all light hitting the boundary between two media is reflected back into the incident material
What are the conditions for total internal reflection to take place? (2)
Incident substance has a larger refractive index
Angle of incidence is greater than the critical angle
What is the principle of superposition?
When two or more waves of the same type exist at the same place, the resultant wave will be found by adding the displacements of each wave
How does a steady interference pattern form?
Two waves with a constant phase difference and the same frequency interfere over a period of time (coherent sources)
What is path difference? (2)
The difference between two identical points on interfering waves
Measured by number of wavelengths
What is constructive interference? (2)
When crests (or troughs) of coherent waves coincide, creating an amplified wave The interfering waves are in phase (nλ)
What is destructive interference? (2)
When the crest of one wave aligns with the trough of another the displacements cancel out
The waves are in anti phase (n+1/2 λ)
How is the path difference of a two source interference pattern found?
Number of wavelengths from one source to desired point - number of wavelengths from other source to point
How is two source interference created with sound waves? (3)
Set up speakers a known distance apart and emit sound waves
Walk along a straight line a set distance from the speakers and cover the other ear
Maxima and minima intensities will be heard
How is two source interference created with microwaves? (2)
Set up metal sheets in front of a transmitter to form a double slit with a gap equal to the wavelength of the microwaves to cause maximum diffraction
Move the receiver along a fixed line in front of the double slit and record the points where maximum and minimum readings are found
How is wavelength calculated from a two source interference pattern?
λ = ax / D
λ - wavelength (m)
a - gap spacing (between double slit) (m)
x - fringe spacing (maximum intensity to maximum intensity) (m)
D - distance from gap to measuring plane (m)
What is the Young double slit experiment using light? (4)
A colour filter can be used to make a source of light monochromatic
A single slit makes the light waves in phase
A double slit creates the interference pattern
Fringes are created on a screen, showing that light behaves like a wave
What happens when white light is used in the Young double slit experiment?
The central white fringe is surrounded by coloured fringes
What does single slit diffraction look like?
A bright central band, double the width of alternating bands on either side
What does double slit diffraction look like?
Uniform fringes where light waves interfere (single slit diffraction superimposed on top)
What is a stationary wave?
When two waves of the same frequency (usually the same amplitude) are travelling in opposite directions in the same medium, they interfere with each other
What is a node? (2)
A point on a stationary wave where destructive interference takes place Zero amplitude (minimum displacement)
What is an antinode? (2)
A point on a stationary wave where constructive interference
Maximum displacement/amplitude
How are stationary waves formed on stretched strings? (3)
A signal generator vibrates the string, forming a wave
The created wave reflects off of the clamp at the end of the string and interferes with the initial wave
Only forms when string is equal to or longer than the wavelength of the wave
How are stationary waves formed using microwaves? (2)
A microwave produced by a transmitter reflects off of a metal plate
A receiver detects the nodes and antibodies formed
What is the phase difference along a stationary wave? (2)
In between adjacent nodes all particles are in phase with each other
On different sides of a node the particles are in antiphase (π radians)
How are stationary waves formed in an air column? (3)
Open tube - antinodes at both ends
One closed end - antinode at open end, node at closed end
Closed tube - nodes at both ends
What are the similarities between progressive and standing waves? (2)
Comprised of oscillations and vibrations
Particles have a frequency, period of oscillation and amplitude
How is energy transfer different between progressive and standing waves? (2)
Progressive - in direction of wave
Standing - no net energy transfer
How is wavelength different between progressive and standing waves? (2)
Progressive - distance between two adjacent points oscillating in phase
Standing - twice the distance between adjacent nodes (original λ)
How is phase difference different between progressive and standing waves? (2)
Progressive - changes across one complete cycle
Standing - in phase between adjacent nodes, antiphase on the other side
How is amplitude different between progressive and standing waves? (2)
Progressive - all parts have the same amplitude assuming no energy is lost
Standing - maximum displacement at antinode, zero at node
Strings - What is the fundamental (1st harmonic)? (2)
1/2 a wavelength
Two nodes
Strings - What is the 1st overtone (2nd harmonic)? (4)
1 wavelength
3 nodes
Half wavelength of fundamental
Double frequency of fundamental
Strings - What is the 2nd overtone (3rd harmonic)? (4)
3/2 wavelengths
4 nodes
Third of the wavelength of fundamental
Triple the frequency of fundamental
