Waves Flashcards
What is a progressive wave?
a moving wave that carries energy from one place to another without transferring material
What is a transverse wave?
a wave where the oscillations are perpendicular to the direction of energy travel
Label a transverse wave (7)
should label:
-crest
-wavelength
-amplitude
-equilibrium position
-trough
-direction of energy transfer
-time period
What are some examples of transverse waves? (4)
-surface water waves
-electromagnetic waves
-seismic s-waves
-guitar strings
What are longitudinal waves?
a wave where the oscillations are parallel to the direction of energy transfer
Label a longitudinal wave (5)
should label:
-amplitude
-wavelength (from the middle of one rarefaction to another)
-rarefaction
-compression
-direction of energy transfer
What are rarefactions in longitudinal waves?
regions of low pressure due to particles being spread further apart
What are compressions in longitudinal waves?
regions of high pressure due to particles being close together
What are some examples of longitudinal waves? (4)
-sound waves
-ultrasound waves
-seismic p-waves
-waves through a slinky coil
What do we mean by frequency?
the number of cycles a wave undertakes per unit time
What do we mean by wavelength?
the minimum distance between two parts of a wave oscillating in phase e.g. crest to crest
What do we mean by amplitude?
the maximum displacement from equilibrium
What do we mean by wave speed?
the distance a part of the wave form moves divided by the time taken to get there
What do we mean by period/time period?
the time taken for a point on a wave to pass through an entire cycle
What is the formula for wave speed? Give units
c=fλ
wave speed (ms^-1)= frequency (Hz) x wavelength (m)
What is the formula for frequency? Give units
f= 1/T
frequency (Hz)= 1/time period (s)
What do we mean by phase?
the stage of a particles’ motion in its sinusoidal cycle
What do we mean by phase difference?
the difference in the fraction of a cycle completed between two oscillating particles
What are the 2 formulas for phase difference? Give units and provide both formulas explaining when they should be used.
Δ ϕ = 2π x (d/λ) —> for points on a single wave
phase difference (rad or degrees)= 2π(OR 360) x (distance (m)/wavelength (m) )
Δ ϕ = 2π x (t/T) —> for two waves meeting w/ same frequency
phase difference (rad or degrees)= 2π(OR 360) x (time lag for wave 1 to get into phase with wave 2/time period (s) )
2π rad in degrees
360 degrees
1/2π rad in degrees
90 degrees
1π rad in degrees
180 degrees
3/2π rad in degrees
270 degrees
What are E-M (electromagnetic) waves?
consists of electric and magnetic waves travelling together oscillating perpendicular to the direction of energy transfer
in phase w/ one another
What are all E-M waves? What other qualities do they have?
transversal waves
-all have speed of 3 x 10^8 ms^-1 in a vacuum
Write down the E-M spectrum in order from lowest frequency. What else changes as you go down? (7)
radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays
Remember
Me
I
Very
Useful
Xylophone
Great
lowest–>highest
wavelength decreases as you go down
List the colour spectrum in order of decreasing wavelength(7)
red, orange, yellow, green, blue, indigo, violet
highest–>lowest
What is polarisation?
where particle oscillations occur in only one of the directions which is perpendicular to the direction of energy transfer
What wave can polarisation occur through? Why?
can occur only in transverse waves as they oscillate in any plane perpendicular to the direction of energy transfer
What stops polarisation?
when oscillations change from one plane to another, the waves become unpolarised (oscillations go in all directions)
Why can’t longitudinal waves be polarised?
they oscillate parallel to the direction of travel
How can waves become polarised?
-through a polariser or polarising filter
-this only allows oscillations in a certain plane to be transmitted
When can waves become polarised?
only when reflected, refracted or scattered
Describe what happens to light intensity being polarised through a polarising filter as the filter rotates
-a maximum (high intensity) occurs when the transmission axis of the filters are parallel (at 180 or 360/0 degrees rotated)
-a minimum (no light) occurs when the transmission axis of the filters are perpendicular (at 90 or 270 degrees rotated)
Draw the graph showing how the light intensity varies with the angle of a polarising filter
-x axis showing angle of polarising filter
-y axis showing intensity of light
-high intensity at 0, 180 and 360 degrees
-no intensity at 90 and 270 degrees
What are some applications of polarisers?
-polarised sunglasses
-polaroid photography
-aerials
Explain how polarised sunglasses work and what their uses could be
-contain lenses with polarising filters with transmission axes that are vertically/horizontally oriented, so blocks partially polarised light to pass through
-useful in reducing the glare on the surface of the water and objects under the surface of the water can be viewed more clearly
Explain how polaroid photography works
-work in the same way as polaroid sunglasses
-enables photographers to take photos of objects underwater
-glare is eliminated by the polarising lens
Explain how aerials work
-radio/television services are broadcast horizontally/vertically-polarised
-therefore, the reception aerial needs to be mounted horizontally/vertically for signals to be transmitted
How do stationary (standing) waves form?
produced by the superposition of two progressive waves of the same frequency, wavelength and amplitude travelling in opposite directions (usually achieved from the reflection of a travelling wave)
What are some examples of stationary waves?
-stationary microwaves which are formed by reflecting a microwave beam at a metal plate (to find antinodes/node use a microwave probe)
-stationary sound waves which are formed by placing a speaker at one end of a closed glass tube, lay powder across the bottom of the tube, it will be shaken at the antinodes and settle
at the nodes.
Describe the energy for a stationary wave compared to a progressive wave
stationary: store energy
progressive: transfer energy
Describe the amplitude for a stationary wave compared to a progressive wave
stationary: each point has a different amplitude depending on the amount of superposition
progressive: all points have same amplitude
Describe the phase difference for a stationary wave compared to a progressive wave
stationary: points between nodes are in phase, points on either sides of nodes are out of phase
progressive: points exactly a wavelength apart are in phase
Describe the wave speed for a stationary wave compared to a progressive wave
stationary: each point on the wave oscillates at a different speed, overall the wave doesn’t move
progressive: the wave speed is the speed at which a wave moves at through a medium
What are nodes?
points on a stationary wave which have no amplitude
What are antinodes?
points on a stationary wave which have maximum amplitude
What is the principle of superposition?
when two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point
What does constructive interference/reinforcement mean?
when waves meet and their displacement is at the same direction, they superpose (combine) to make a bigger displacement
e.g. a crest + a crest superpose to make a supercrest, same with a trough
What does destructive interference/cancellation mean?
when a wave with positive displacement meets one with a negative displacement, they cancel out
What are harmonics? Give an example
harmonics are the different wave patterns that stationary waves create
e.g. When a stationary wave is fixed at both ends, the simplest wave pattern is a single loop made up of two nodes and an antinode (called the first harmonic)
How would you find the frequency from a harmonic? Give the equation for the first harmonic
depends on the length of the string L (m) and the wave speed (ms^-1)
for a string of length L, the wavelength of the lowest harmonic is 2L (there is only one loop of the stationary wave, which is a half wavelength) so the equation for frequency:
f =c/λ
and the wavelength= 2L so can be written as f=c/2L
Draw the first three harmonics and their frequency formulas
1st: string on fixed end, two nodes and one antinode, 1/2 wavelength
f= c/2L
2nd: string on fixed end, three nodes, two antinodes, 1 wavelength
f= c/L
3rd: string on fixed end, four nodes, three antinodes, 1 1/2 (3/2) wavelengths
f= 3c/2L
How can you distinguish wavelengths on a harmonic/stationary wave?
the distance between adjacent nodes is always 1/2 a wavelength
Give another way to find the wave speed from a stationary wave
c=√ T/μ
Give another way to find the frequency for the first harmonic and give units
f= 1/2L x √ T/μ
√ T/μ –> tension (N) /mu (kgm^-1)
How can you work out μ?
divide mass by length of string
Harmonics on a string
RP1- describe the graph and find the equation y=mx
-value of 1/frequency on y-axis and length on x-axis
-gradient= 2/c
equation:
1/f= 2/c x L
What are the three interactions waves do?
-refract
-reflect
-diffract
What is refraction?
waves change direction crossing an interface between media when they have different wave speeds (e.g. different optical density/refraction index for light)
note: frequency is constant, wavelength changes after diffraction
What is something to know about reflection?
angle of incidence= angle of reflection
What is diffraction?
the spreading of a wave front around an obstacle or through a gap
What is needed for diffraction to occur?
the wavelength must be approx. the width of the gap
What happens when the gap is smaller than the wavelength in diffraction?
most of the wave is reflected
What happens when an obstacle is wider compared to the wavelength in diffraction?
the wider the obstacle, the less diffraction occurs
What is path difference
the difference in the distance travelled by two waves
How do you work out path difference?
path difference= distance travelled from source 1- distance travelled from source 2
What does a coherent light source have?
same: frequency, wavelength
fixed phase difference
What does monochromatic mean?
single frequency/small range of wavelengths
Why is a laser useful in showing interference and diffraction?
produces monochromatic light so diffraction and interference patterns are more defined
RP2: Describe young’s double slit experiment
2 coherent sources (2 slits) interfere with each other to produce interference patterns (both constructive and destructive)
RP2: What can be seen from the young’s double slit experiment?
dark fringes (points of destructive interference) and light fringes (points of constructive interference)
Young’s double slit
RP2: When do light and dark fringes occur?
-light fringes occur when the path difference is a whole number (2λ, 3λ, 4λ etc.) , both sources are in phase
-dark fringes occur when the path difference is a whole number+1/2 wavelength (2 1/2λ, 3 1/2λ etc.) ,both sources are in anti-phase
Young’s double slit
RP2: How can we find the slit spacing/fringe separation/distance between screen and slits/wavelength used?
W= λD/s
where:
w= fringe spacing/m
D= distance between screen and slits/m
λ= wavelength used/m
s= slit separation/m
Describe the interference pattern created when white light is diffracted through a single slit
a white central maximum surrounded by a spectrum of outer fringes with decreasing intensity from violet to the zero order and red at the furthest
Compare the width of a central maximum to the outer fringes
CM is twice the width of the outer fringes
Describe the effect on the CM when increasing the slit width or incident wavelength
increasing slit width: CM is narrower and more intense
increasing λ: CM is wider and less intense
How can we find the width of the CM for a single slit?
W= 2λD/a
where:
W= width of CM
D= distance between slit and screen/m
λ= wavelength used/m
a= slit width
What are some safety precautions when lasers are used?(6)
-wear safety goggles
-don’t shine the laser at reflective surfaces
-display a warning sign
-never shine the laser at a person
-never look the laser at eye level,
-use your hand to see where it is
TRUE/FALSE: Only light can produce interference patterns
FALSE
both sound and E-M waves can produce interference patterns
How can interference patterns be produced with sound waves?
two speakers connected to the same signal generator, the intensity of the wave can be measured using a microphone to find the maxima, and minima
What is a diffraction grating?
a slide containing many equally spaced slits which are very close together
What happens when monochromatic light is passed through a diffraction grating compared to a double slit? Why?
-the interference pattern is much sharper and brighter than it would be after being passed through a double slit
-because there are many more rays of light reinforcing the pattern
What is the formula associated with diffraction gratings?
d sinθ= nλ
where:
d= distance between the slits
θ= the angle to the normal made by the maximum (zero order)
n= the order
λ= the wavelength
How do you find out the distance for a diffraction grating?
1/n
where n= lines per mm
What happens when λ increases in a diffraction grating?
the distance between the orders will increase because θ is larger due to the increase in diffraction as the slit spacing is closer in size to the wavelength, this means the pattern will spread out
How can we find the distance of the fringes/orders from the zeroth order?
tan θ= x/d
where:
θ= the angle made by the zeroth order to the nth order
x= the order
d= distance between the slits
What is the max. possible value for sin θ? (for a diffraction grating)
the max. value is1, therefore any values of n, which give sin θ as greater than 1 are impossible
Explain how we can get the values for d sinθ= nλ
https://upload.wikimedia.org/wikipedia/commons/a/ab/Youngs_slits.png
-the path difference between two adjacent rays of light is one wavelength
-a right angle triangle is formed, with side lengths d and λ
-you can see the upper angle in the triangle is θ (because the lower angle is 90-θ°)
What is a refractive index?
a property of media which measures how much it slows down light passing through it/how optically dense it is
How can we calculate the refractive index of a media?
refractive index= dividing the speed of light in a vacuum (C) by the speed of
light in that substance (Cs)
n = C/Cs
remember: speed of light in a vacuum= 3 x10^8 ms^-1
Define surface normal
an imaginary line perpendicular to a surface between two materials at a point of incidence
What does it mean when a material has a higher refractive index?
known as more optically dense
What is the refractive index of air?
1
When does refraction occur?
when a wave enters a different medium, causing it to change direction, either towards or away from the normal depending on the material’s refractive index
How can we remember which way refraction occurs from air to glass?
TAG
towards normal air to glass
(as light in air passes through to glass, the direction of light changed direction towards the normal)
When light enters a more optically dense medium does it bend towards or away to the normal? What about if it enters a less optically dense medium?
more optically dense- towards the normal
less optically dense- away from the normal
State Snell’s law and draw a diagram showing this
n1 sinθ1 = n2 sinθ2
where:
n1= the refractive index of material 1
n2= the refractive index of material 2
θ1= the angle of incidence of the ray in material 1
θ2= the angle of refraction of the ray in material 2
https://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/Snell%27s_law3.svg/300px-Snell%27s_law3.svg.png?20140526191916
What do we mean by the critical angle?
when the angle of refraction is exactly 90° (equal to the angle of incidence), and the light is refracted along the boundary, the angle of incidence has reached the critical angle (θc)
How can we work out the critical angle?
sin θc = n2/n1
where n1 > n2
What is total internal reflection?
complete reflection of a ray of light within a medium
What are the two conditions required for total internal reflection (TIR) to occur?
-the ray must start in a more optically dense medium than it is travelling towards (n1>n2)
-the angle of incidence must be greater than the critical angle (θ1>θc)
Draw three diagrams showing the different paths of light rays that can occur at different angles of incidence when n1>n2. Label the angles, refractive indexes and what the diagram shows
https://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/Total_Internal_Reflection.png/799px-Total_Internal_Reflection.png?20150821043859
Describe a useful application of TIR
step index optical fibres
-these are flexible, thin tubes of plastic or glass which carry information in the form of light signals
-they have an optically dense core surrounded by cladding with a lower optical density allowing TIR to occur
What is the purpose of cladding in a step index optical fibre? (5)
-protects the core from damage
-prevents signal degradation through light escaping the
core, which can cause information to be lost
-allows TIR as n,core > n,cladding (n1 > n2)
-prevent crossover of signal in a bundle of fibres
-increases θc to reduce pulse broadening and modal dispersion
Draw a diagram showing the use of TIR in real-life applications
https://upload.wikimedia.org/wikipedia/commons/thumb/b/b3/Optic_fibre-numerical_aperture_diagram.svg/580px-Optic_fibre-numerical_aperture_diagram.svg.png?20101126090923
What are the reasons why signal degradation occurs in optical fibres? (2)
-absorption
-dispersion
What is absorption and why does it cause signal degradation?
where part of the signal’s energy is absorbed by the fibre, reducing the amplitude of the signal, which could lead to a loss of information
What is pulse broadening?
when the received signal is wider than the original which can cause an overlap of signals leading to information loss
What is dispersion and why does it cause signal degradation?
this causes pulse broadening therefore can overlap signals causing loss of information
What are the types of dispersion?
-modal
-material
What is modal dispersion?
light rays enter the fibre at different angles (all at same speed) and therefore take different paths. this leads to the rays taking a different amount of time to travel along the fibre, causing pulse broadening.
How can we reduce modal dispersion?
can be reduced by making the core very narrow, therefore making the possible difference in path lengths smaller
What is material dispersion?
caused by using light consisting of different wavelengths, meaning light rays will travel at different speeds along the fibre, which leads to pulse broadening
How can we reduce material dispersion?
use monochromatic light
How can we reduce both absorption and dispersion?
using an optical fibre repeater, which regenerates the signal during its travel to its destination
Draw a diagram showing an optical fibre pulse once absorption and dispersion has occurred
https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/04/Absorption-Pulse-Broadening.png
State the advantages of optical fibres over traditional copper wires(5)
-signal can carry more information as light has a high frequency
-no energy lost as heat
-no electrical interference
-cheaper
-very fast
