Waves Flashcards
What are mechanical waves
Waves that pass through a medium (sound, seismic and waves on a string)
What are electromagnetic waves
vibrating electric and magnetic fields that progress through space without the need for a substance – the vibrating electric field generates a vibrating magnetic waves which generates an electric field etc…
Define and explain and give examples of longitudinal waves
direction of vibration of the particles is parallel to the direction of wave travel e.g. sound waves, primary seismic waves, compression waves on a slinky (compression and rarefaction)
Define and explain and give examples of transverse waves
Transverse waves – direction of vibration of particles is perpendicular to the direction of wave travel e.g. electromagnetic waves, secondary seismic waves and waves on a string
Transverse waves are plane-polarised
Transverse waves are plane-polarised if the vibrations stay in one plane only. If the vibrations change from one plane to another, the waves are unpolarised. Longitudinal waves cannot be polarised.
How can you prove if light is polarised or not
Light from a filament lamp or candle is unpolarised. If unpolarised light is passed through a Polaroid filter, the transmitted light is polarised as the filter only allows through light which vibrates in a certain direction. If unpolarised light is passed through two Polaroid filters, the transmitted light intensity changes if one Polaroid is turned relative to the other one. The filters are said to be crossed when the transmitted intensity is a minimum. At this position, the polarised light from the first filter cannot pass through the second filter is at 90 degrees to the alignment in the first filter
The plane of polarisation of an em wave e.g. light is defined as
the plane in which the electric field oscillates
How do polaroid sunglasses work
Polaroid sunglasses reduce the flare of light reflected by water or glass. Such light is partly polarised when it reflects and so its intensity is reduced using Polaroid sunglasses.
Define displacement
distance and direction from equilibrium position
Define amplitude
maximum displacement of a vibrating particle
Define wavelength
least distance between two adjacent vibrating particles (with equal and simultaneous velocity and displacement)
Define one complete cycle of a wave
One complete cycle of a wave is from maximum displacement to the next maximum displacement
Define period
the time taken for one complete wave to pass a fixed point = 1/frequency
Define frequency
number of complete waves passing a point per second, Hz
frequency =
c / lambda
The phase difference between two vibrating particles is
the fraction of a cycle between the vibrations of the two particles - 1 cycle = 360 degrees
Phase difference in radians =
2pi * distance / lambda
Wavefronts are
lines of constant phase which are perpendicular to the direction of wave travel
The angle between the incident ray and the plane mirror
is the equal to the angle between the reflected ray and the mirror
Refraction -
when a wave enters a new medium and changes speed, wavelength and direction
Diffraction -
occurs when waves spread out after passing through a gap or around an obstacle
The narrower the gap
the more the waves spread out
The longer the wavelength
the more the waves spread out
Why are satellite dishes bigger
So it reaches a bigger signal as more radio waves are reflected by the dish onto the aerial but a bigger dish reflects the radio waves to a smaller focus as it diffracts the waves less so the dish needs to be aligned more carefully than a smaller dish or it will not focus the radio waves onto the aerial.
Superposition
when two waves meet the total displacement at a point is equal to the sum of the individual displacements at that point
Supercrest
when two crest meet and reinforce each other
Supertrough
when two troughs meet and reinforce each other
Crest + trough =
When a crest meets a trough the resultant displacement is zero as the two waves cancel each other out
What happens when polarising filter rotated 360degrees
Intensity varies between two maxima and two minima in 360 degree rotation
Why is a stationary wave formed with two nodes
Progressive waves travel from centre to ends and reflect, Waves have same frequency and amplitude, Superposition occurs, Waves move in opposite directions superpose and cancel at nodes
How are maxima and minima formed?
Superposition/interference occurs and waves of equal frequency travelling in opposite directions are reflected off the metal plate. Maxima are formed where waves are in phase (and interfere constructively) and minima are formed where waves are in antiphase (and interfere destructively)
Compare stationary and progressive waves
Progressive: all particles equal amplitude and vibrate at equal frequencies, phase difference = 2pid/lambda
Stationary: amplitude varies (0 at node max at antinode) and all particles but nodes vibrate at equal frequencies, phase difference = no. of nodes between 2 particles * pi
3 equations
f = c/lambda 2L = lambda T = 1/f
Wavelength > gap Wavelength < gap Wavelength = gap
Large diffraction Small diffraction Maximum diffraction
Transverse wave (peak and troughs) examples -
Em waves (radio,micro,visible light), water waves, secondary seismic waves, light, radiowaves
Longitudinal wave (compression and rarefaction) examples -
Sound waves and primary seismic waves
Explain why it is important to correctly align the aerial of a TV in order to receive the signal -
Radio waves are polarised so aerial must be aligned in the same plane of the wave
Why does the knot become motionless when two equal waves come from either end?
Stationary wave formed by superposition Knot is at node and waves cancel at node/knot
When does refraction occur -
When a wave passes from one medium into another and the wave speed is different
Define diffraction
The spreading out of waves around an obstacle or through an aperture
Stationary waves don’t
transfer energy
Node =
fixed point of zero displacement
Antinode =
point of maximum displacement
nth harmonic =
lambda(n) = 2L/(n)
f(n) =
n * f
First/fundamental harmonic (two nodes either end antinode central)
n = 1
First overtone (node in centre so two loops)
n = 2
Second overtone (nodes at L/3 from either end and a central antinode)
n = 3
What happens during superposition of two waves
When a wave is reflected from a fixed end it undergoes a 180 degree, superposition between the incident and reflected waves cause destructive interference at the nodes and constructive at the antinodes
Small gap in diffraction
More diffraction and lower amplitude (as waves more spread out)
Why does small wavelength not reach over hills and valleys but large wavelength does
Large wavelength = wavelength of hills so diffracts a lot and reaches the valley whereas the small wavelength is smaller than the gap so little diffraction occurs and cannot reach over hills and into valleys.
Speed of transverse wave in a string depends on
the tension in the string and the mass per unit length
v^2 = Tension/mass
The fundamental frequency is
f = 1/2L * square root of T/M
The pitch is raised by
shortening the string, tightening the string, or using a lighter string
How are transverse waves polarised
transverse waves go into a filter as different planes but come out with a single plane so polarised
Longitudinal waves cannot be polarised because
Propagation is parallel to wave movement so no planes so cannot be polarised or unpolarised through a filter
radian =
360 / 2pi
Explain a test to prove light is not longitudinal
When unpolarised light passes through a polarising filter, about half of its energy will be absorbed. If you align two Polaroid filters and then steadily rotate one of them you will find that less light gets through until when you have rotated the second filter 90 degrees and light gets through. As you continue to rotate the filter the amount of light getting through the filter combination will increase until you get to the point when the total rotation of the filter is 180 degrees. When in this position you will have the same amount getting through as you had in the first place.
Formation of stationary wave (6)
Two progressive waves of equal frequency and wavelength are sent along a rope in opposite directions
At a node (point of zero displacement so particles dont move) two waves cancel (destructive interference) and are 180 degrees out of phase (antiphase) while at an anti-node (point of maximum amplitude) they reinforce (constructive interference) (in phase) and so superposition of the two waves occur.
Energy is not transferred along in a standing wave
Amplitude varies between nodes
The fundamental pattern of vibration has…
an antinode at the middle and a node at either end
The fundamental wavelength lambda =
2 * length of string
fundamental frequency =
c/2L
Wavelength of first overtone
lambda = length
Frequency of first overtone =
2f0 = c/L
= c/lambda
Second overtone has…
nodes a third of the length of the rope apart and antinode at middle
Wavelength of second overtone
2L/3
Frequency of the second overtone vibrations
f = c/lambda2
= 3c/2L
= 3f0
The time taken for a wave to travel along the string and back
t = 2L/c
The time taken for the vibrator to pass through a whole number of cycles =
x (whole number) / f
The length of the vibrating section of the strong L =
x*lambda/2
A vibrating dipper on a water surface sends out _________ ________. The ____ pass through each other continuously. Points of ______ are created where a _____ from one dipper meets a ____ from the other dipper. These points of _____ are seen as gaps in the wavefronts. Points of _____ are created where a ____ from one dipper meets a ____ from the other dipper or a ____ from one dipper meets a ____ from the other dipper.
A vibrating dipper on a water surface sends out circular waves. The waves pass through each other continuously. Points of cancellation are created where a crest from one dipper meets a trough from the other dipper. These points of cancellation are seen as gaps in the wavefronts. Points of reinforcement are created where a crest from one dipper meets a crest from the other dipper, or where a trough from one dipper meets a trough from the other dipper.
As the waves are continuously passing through each other at constant frequency and at constant phase difference, ____ __ _____ occurs at fixed positions. This effect is known as _____. _____ sources of waves produce an ______ pattern where they overlap, because they _____ ___ _____ ____ ___ ____ _____ _____. If the phase difference changed at random, the points of cancellation and reinforcement would move about at random, and no interference pattern would be seen.
As the waves are continuously passing through each other at constant frequency and at constant phase difference, cancellation and reinforcement occurs at fixed positions. This effect is known as interference. Coherent sources of waves produce an interference pattern where they overlap, because they vibrate at the same frequency with a constant phase difference. If the phase difference changed at `random, the points of cancellation and reinforcement would move about at random, and no interference pattern would be seen.
Antiphase means
Points on a wave which are always travelling in opposite directions to each other, one is rising while the other is falling
In phase means
points on a wave which are travelling in the same direction, rising and falling together
Constructive interference is where
the two waves are in phase with each other and constructively interfere to give a wave of greater amplitude
Destructive interference is where
the two waves are in anti-phase and destructively interfere to give a wave of zero amplitude
Stationary waves are formed by
two waves with equal frequency travelling in opposite directions
A rock is at the peak of a transverse wave describe its motion in one complete cycle
Oscillation perpendicular to the direction of wave travel
Oscillates from equilibrium to maximum positive displacement, back to equilibrium and then to negative maximum displacement and back to equilibrium
For a stationary wave, two nodes are at a distance of
0.5 * wavelength
Light shows wave properties through
diffraction through single slit
diameter of atom of order
10^-10m
calculated angle = angle towards normal but angle towards boundary =
90 - x
When path difference = 0 there is _______-
As distance from centre increases, the path difference increases so ________
When path difference =…
As distance then increases more….
maximum displacement
not in phase so displacement decreases
0.5*lambda antiphase so lowest displacement
more in phase so displacement increases
When a sound wave is quietest, path difference =
0,5 * lambda as antiphase
Spectrometer and grating application
spectral analysis of light/composition of starts and chemical analysis
When source of light enters diffraction grating what happens to light
white/greater intensity light goes straight on and diffracted beam of spectrum of light
Lambda (n) =
2L/(n)
f(n) =
n*f = cn/2L
Mu =
mass per unit length
What affects speed of string
Tension and mass per unit
derive from equation on data sheet
