Unit 2.5 - The Properties of Waves Flashcards
Diffraction
The effect observed when waves pass an object…
When waves strike an object, part of the wave is slowed down, and so the wave front curves
When a wave strikes a space in a barrier, both sides curve
Which waves undergo diffraction?
ALL waves
What’s the word for the effect observed when waves pass an object?
Diffraction
What happens when waves strike an object?
Part of the wave is slowed down, and so the wave front curves
What happens wen a wave strikes a space in a barrier?
Both sides curve
How can we show the diffraction of waves in water?
Ripple tank
Draw and label a ripple tank
(See notes)
Draw a wavefront and label its wavelength
(See notes)
What’s the angle between the direction of a wave and the wavefront?
90 degree angle
Describe the waves from a narrow gap between barriers in a ripple tank
A lot of spread
Waves become more and more curved
Describe the waves from a wide gap between barriers in a ripple tank
A little spread
Waves flat in the middle and curve on sides
How come you can hear sounds around corners?
A doorway is a good diffraction of sound waves, and sound waves can bend
What type of waves bend the most?
The ones with the longest wavelengths
What do waves with a longer wavelength do more?
Bend more
What types of sounds are loudest and why?
The ones with the longest wavelengths
Curves more
What does the wavelength of a sound wave depend on?
The frequency
What does a longer wavelength mean in a sound wave and why?
It’s louder as the wave is curved more
What is the wavelength of sound usually, and what is this similar to?
1m, similar to the doorway in a house
Why is it not possible to see a light around corners in the same as as we can hear sound around corners?
The wavelength of light is a lot shorter - the waves bend less
In which situation would you get the most diffraction?
If the gap or the size of the barrier is roughly the same as the wavelength of the diffracting waves
What does a ‘lot of diffraction’ basically mean?
A lot of spread of the wave
What do you get if the gap or size of the barrier in the pass of a wave is roughly the same as the wavelength of the diffracting waves?
A lot of diffraction
Draw a diagram to explain how the diffraction of microwaves would be shown
(See notes)
What do waves do when they interact?
“Pass through” each other
What happens in the region where waves overlap?
They superpose
What does it mean that waves superpose?
They “add up”
Principle of superposition
When two waves interact, their combined displacement at any point is the vector sum of the individual waves at that point
What does the ‘vector sum’ part of the principle of superposition show us?
The directions of the individual displacements need to be taken into account
Who was the first to demonstrate interference effects using light and when?
Thomas Young in the 19th century
What was Thomas Young the first to do?
Demonstrate interference effects using light
What first suggested wave-light properties of light?
Interference effects
What did interference effects suggest in Young’s experiment?
Wave-like properties associated with light
Where the ideas of Thomas Young’s experiment immediately accepted?
No, not until much later on
Draw the layout of Thomas Young’s experiment
(See notes)
Draw how Thomas Young’s experiment can be adapted to using sound
(See notes)
How would the sound version of Thomas Young’s experiment wok?
As a person walked from A to B, they would notice that the sound intensity (volume) wold rise and fall as the constructive interferences alternate
Why would the volume rise and fall whilst walked in a straight line by loudspeakers?
The constructive and destructive interferences alternate
What do we need to think about when considering interference?
The phase of the waves arriving at the screen
What is interference caused by?
Having 2 parallel slits for the wave to pass through
What are bright fringes in interference caused by?
Constructive interference
Constructive interference
The waves superimpose in phase, so the difference in distance both waves have travelled must be a whole number of wavelengths
Describe the interference of waves if they meet in phase
Constructive
How must waves meet for it to be constructive interference?
In phase
Which part of a wave overlaps with what for constructive interference?
Crest with crest
What are dark fringes in interference caused by?
Destructive interference
Destructive interference
Thaw waves must have travelled a distance that is different by a number of half wavelengths
Which part of a wave interacts with what for destructive interference?
Crest with trough
Is it still constructive interference if a point is 4 wavelengths apart from one slit and 3 from the other?
Yes, because they’re still in phase as its a WHOLE wavelength
Waves travelling by a distance hat is different by a number of half wavelengths
Destructive interference
Difference both waves have travelled is a whole number of wavelengths
Constructive interference
What does constructive interference cause?
Bright fringes
What does constructive interference cause?
Dark fringes
Do wavelengths change during diffraction?
No
What should you NOT show with the wave that comes out of a gap in an exam question?
Gaps between the ends of the waves and the barrier
How does the frequency of resultant waves from superimposing compare to the incoming waves?
The same
Which two conditions must be true for a fringe pattern to be observed with waves passing through slits?
The two sources of waves must have a constant phase difference
The vibrations must be in the same line (parallel)
Phase difference
The difference in time between 2 similar events
Path difference
The difference in distance travelled by 2 similar waves
Coherent
Two waves are coherent if thy have a constant phase difference
Give an example of a light which is coherent
Laser light
Give an example of a light that ISN’T coherent
Light from a filament lamp
What’s the phase difference is the path difference is zero?
Zero
What’s the phase difference if the path difference is half a wavelength?
Pi
Definition of interference
The superposition of two (or more) coherent waves. When this happens, it causes reinforcement at some points and cancellation at other points.
The resulting effect is called a “system of fringes” or an “interference pattern”
What was the first experiment to show the wave nature of light and as opposed to what?
Young’s double slit experiment
As opposed to particle nature
What did Young’s double slit experiment show us?
That light has a wave nature, not particle nature
Who first completed the double slit experiment and when?
Thomas Young
1801
Draw how the double slit experiment is completed
(See notes)
What can be used as the light source in the double slit experiment?
Either…
Monochromatic light source and a single slit
A laser source
In which direction do waves diffract when maximum diffraction occurs and from where?
90 degrees from the ‘straight through’ direction
What would change about the diffraction of a wave if the frequency increased?
Wavelength decreases (becomes smaller than the gap)
Less spreading
Side beams
When do you get the most diffraction from a wave travelling through a slit?
If the wavelength is equal to or greater than the width of the slit
What must wave sources have for two source interference to be observed clearly?
Zero/Constant phase difference (coherent)
The same frequency
The same or similar amplitudes
Oscillations in the same direction
The same wavelength
2 slits
Who believed that waves have particle nature around the time of Young’s experiment?
Newton
Path difference
The difference in the distance travelled by 2 waves in arriving at a particular point on the screen
What is the maximum amplitude of a wave caused by?
Constructive interference
What is the minimum amplitude of a wave caused by?
Destructive interference
What does constructive interference show up as on the interference pattern?
A light fringe
What does maximum amplitude of a wave show up as on the interference pattern?
Light fringe
What does destructive interference show up as on the interference pattern?
Dark fringe
What does minimum amplitude/zero amplitude show up as on the interference pattern of a wave?
A dark fringe
What is the path difference in waves that undergo constructive interference?
A whole number of wavelengths
How do waves arrive if their path difference is a whole number of wavelength?
In phase
What is the path difference of waves that undergo destructive interference?
An odd number of wavelengths
How do waves arrive that have a path difference that’s an odd number of wavelengths?
In anti phase
What does “a” represent in the equation for Young’s experiment?
Slit separation
What does “y” represent in the equation for Youngs experiment?
Fringe separation
What does “D” represent in the equation for Young’s experiment?
Distance from the double slits to the screen
What do we count on a diagram to work out fringe separation?
The gaps between bright fringes, not the bright fringes themselves
Diffraction grating
A piece of glass with a large number of parallel and equidistant lines scratched on it
What do the gratings of a diffraction grating do to light?
Split it up into its constituent colours
What is white light diffracted according to with a diffraction grating?
Wavelength
What happens when white light is passed through a diffraction grating?
Light is “split up” into its constituent colours, and diffracted according to wavelength
So, all the colours of the spectrum can be observed
How can we observe all of the colours of the spectrum using white light?
Diffract it through a diffraction grating, where its diffracted according to wavelength
When will diffraction occur on a diffraction grating?
When light is reflected onto the grating or if its projected through the grating
What effect does wavelength have on the amount of diffraction of light?
Longer wavelength = greater diffraction
Which light is deviated most - either or blue light - using the same diffraction grating and why?
Red light gets deviated more
Longer wavelength = greater diffraction
Draw and label a diagram of white light hitting a grating to diffract light
(See notes)
What can be observed when using a grating with more lines per mm (reducing d) to diffract light?
∆y is larger = beams are further apart (due to more constructive interference)
Bright fringes are sharper = bright maxima are very arrow = the angle can be measured accurately
Under which conditions is ∆y larger for diffracted light and why?
When using a grating with more lines per mm (reducing d)
More destructive interference
What is using a grating with more lines per mm reducing?
d
Under which conditions are the bright fringes sharper through a diffraction grating and why is this advantageous?
Using a grating with more lines per mm (reducing d)
The angle can be measured accurately
What does it mean if bright fringes are sharper after passing them through a grating with a lower value for d?
Bright maxima are very narrow
When are diffraction orders produced?
When monochromatic light is incident normally on a diffraction grating
What are diffraction orders?
A pattern of narrow, bright fringes
Monochromatic
Single wavelength
Normally
At right angles
Draw and label a diagram to represent diffraction orders
(See notes)
What do we label the different diffraction orders produced by a diffraction grating?
(X)th order maximum
What do we call the beam of light that travels in a straight line through the diffraction grating?
Straight through beam
What are diffraction orders caused by?
Constructive interference from light waves arriving in phase
How much light waves arrive for constructive interference to occur?
In phase
What number of maxima is there always for diffraction orders and why?
An odd number
Middle maximum, and an equal amount on both sides
What do wavelets do as they’re circular?
Overlap
Why would waves be circular?
By the diffraction process
What does nearly every part of every wavelet do to their wavelets when passed through 2 slits?
Nearly every part of every wavelet will overlap with another wavelet that is in anti phase, causing destructive interference
What causes destructive interference?
Waves overlapping in antiphase
What’s the only direction in which constructive interference can occur?
The direction that the pass difference from each slit is a whole number of wavelengths
What’s the distance between slits for the first order maximum?
A single wavelength
What unit to we work in using the diffraction grating equation?
Metres
Derive the diffraction grating equation
(Lovely triangle drawing drawn I’m sure)
d = slit separation
BC = path difference
θ = angle to bright spot
Sin θ = BC
——
AB
path difference
———————-
slit separation (d)
For constructive interference, path difference is nλ (a whole number of wavelengths)
Sin n λ
——
d
dsin(θ) = n λ
What’s the diffraction grating equation?
dsin(θ) = n λ
What is d in dsin(θ) = n λ?
Slit separation
Symbol and unit for slit separation
d
Metres
What’s the method for finding the total number of maxima? (diffraction orders)
The limit of θ = 90 degrees
So, there’s a limit to the number of orders that can be observed (has a maximum)
n λ = dsinθ
Sin 90 = 1
n= d
—
λ
nmax ≤ d
—
λ
Why is there a limit to the diffraction orders that can be observed?
The limit of θ = 90 degrees
What is sin 90 equal to?
1
Equation for finding the total number of maximum diffraction orders
nmax ≤ d
—
λ
What do we NOT do to the value of the maximum order for diffraction orders and why?
Round the answer up
It would surpass 90 degrees
How do we work out the total maxima if we have the maximum order?
Think - 1 on the middle, the value for maximum order on both sides
How do we calculate d for the diffraction grating equation?
1/lines per mm
Answer divided by 1000 to be in metres
How would we re-arrange the diffraction grating equation to find theta for example?
Use SHIFT sin
coherent light
waves or wave sources have a constant phase difference between them and therefore must have the same frequency
examples of coherent wave sources
lasers, speakers emitting the same frequency sound wave, two dippers on a beam in a ripple tank
incoherent light
emit waves that have no phase relationship
examples of incoherent wave sources
lightbulb, LED, person splashing wildly in water
what must it follow that a coherent light source is?
monochromatic
monochromatic
single wavelength
stationary waves
caused by the superposition of identical progressive waves that are travelling in opposite directions
what do we mean by ‘identical waves’ in the definition for stationary waves?
same amplitude and period
how can stationary waves be produced?
by reflecting progressive waves from a source back towards the source
nodes
points of zero amplitude on a stationary wave
antinodes
points of maximum amplitude on a stationary wave
what are nodes caused by in a stationary waves?
destructive interference
what are antinodes caused by in stationary waves?
contructive interference
draw and label the nodes and antinodes on a stationary wave
(see notes)
what’s the distance between the nodes (or antinodes) in a stationary wave?
half a wavelength
what results in the stationary wave being produced when reflecting a progressive wave from a source back to the source?
the interference between the incident wave and the reflective wave
what’s the relationship between the wavelength of a stationary wave and the progressive wave that causes it?
the same
what does a stationary wave have the same wavelength as?
the progressive wave that caused it
compare progressive and stationary waves in terms of energy
progressive - energy is transferred at the wave-front
stationary - energy is captive (i.e - not transferred)
compare progressive and stationary waves in terms of amplitude
progressive - every particle along the path is vibrated to the amplitude
stationary - varies from zero at the nodes to a maximum at the antinodes
compare progressive and stationary waves in terms of frequency
progressive - all particles vibrate with the frequency of the wave
stationary - all particles vibrate with the frequency of the wave except those at the nodes (which don’t oscillate at all)
what do some of the particles in only a stationary wave have at all times?
a displacement of zero
compare progressive and stationary waves in terms of wavelength
progressive - the distance between adjacent particles having the same phase
stationary - twice the distance between adjacent nodes or adjacent antinodes
compare progressive and stationary waves in terms of phase
progressive - all particles within one wavelength have a different phase
stationary - all particles between 2 adjacent nodes have the same phase but are in antiphase with those in the next loop
Equation for the speed of sound
c = f λ
What’s c = fλ the equation for?
The speed of sound
Why does every particle between two adjacent nodes in a stationary wave have the same phase?
The wave moves up and down together
Fundamental stationary wave
The lowest frequency stationary wave
What does a fundamental stationary wave look like?
1 loop
Wavelength of a fundamental stationary wave
2L
(Think - node to node is half a wavelength, and this type of wave literally just has 2 nodes - 1 on each end)
Harmonic
The amount of “loops” in a stationary wave
What would a stationary wave with 2 “loops” be known as?
The 2nd harmonic
Harmonic of a fundamental stationary wave
1st harmonic
In the equation V^2 = T x L
——
m
If T is reduced by a factor of 4 (i.e - a quarter of the previous value), what factor does V change by?
1/4 of its value
√1/4
1/2
Decreases by a factor of 2
What do we mean when we say that waves diffract?
Spread out
What’s the same between 2 coherent waves?
Frequency
Under which situation would we get lots of diffraction orders?
If the skit separation is large compared to the wavelength
What’s the difference between the interference pattern with Young’s experiment and using a diffraction grating? Explain
(Using a diffraction grating…)
Beams are further apart and there’s fewer of them (more slits and so more destructive interference)
Bright fringes are sharper as the bright maxima are very narrow
What happens to the intensity of light a the distance from the “straight through” of diffracted light increases? Why?
The intensity decreases
Most intensity “straight through”
What will happen to fringe separation as wavelength increases? Why?
Increasing wavelength will increase fringe separation
(Directly proportional)
How can we increase fringe separation?
Increase distance from screen
Why must constructive interference occur in deriving the diffraction grating equation?
For waves to arrive in phase to cause a bright spot
Which word should we always use when describing a stationary wave?
Reflecting
Do path differences need to be inserted into the equation?
Yes because it IS the path difference equation
