0.3 Waves Flashcards
What is a progressive wave?
A wave which transfers energy from one place to another without transferring any material.
What is a wave caused by?
Something making the particles near the source vibrate.
What are transverse waves?
The oscillation of particles are perpendicular to the direction of energy transfer.
What are some examples of transverse waves?
All electromagnetic waves.
Water waves.
Earthquake S-waves.
What is the peak of a wave?
Wave’s maximum displacement from the equilibrium position (units are m).
What is the frequency of a wave?
The number of complete oscillations passing through a point per second, (units are Hz).
What is the wavelength of a wave?
The length of one whole oscillation (e.g. the distance between successive peaks/troughs) (units are m).
What are longitudinal waves?
The oscillation of particles are parallel to the direction of energy transfer.
What are examples of longitudinal waves?
Sound waves.
Earthquake P-waves.
What are the features of a longitudinal wave?
It has areas of compression and rarefaction.
What direction can waves travel at?
In transverse waves, particles are oscillating: up and down; left to right; and diagonally. There are an infinite number of angles that the particles could be oscillating at and still be oscillating perpendicular to the direction of the wave.
BUT
This is not true for longitudinal waves, where particles can only oscillate parallel to the direction of travel.
How do you test if a wave is transverse of longitudinal?
Test is it can be polarised.
What does it mean for a wave to be unpolarised?
The particles/field is oscillating in more then one plane. (most light sources produce unpolarised light)
What does it mean for a wave to be polarised?
The particles/field is oscillating in just one plane. (light from some screens is polarised.
Is it possible to polarise unpolarised light?
Yes, by using a polarising filter.
What does a polarising filter consist of?
Small openings which can be orientated horizontally or vertically.
What does a horizontal polarizer do?
Only lets waves oscillating horizontally through.
What does a vertical polarizer do?
Only lets waves oscillating vertically through.
If you were to measure the intensity of light passing through two polarising filters, whilst rotating the second filter what would you see?
No light passes through when one filter is vertical and the other horizontal.
Some light passes through when both filters are vertical.
Why is it important for TV aerials to be aligned properly?
The signals from TV transmitters are sent polarised.
This means the aerials on houses only have to be aligned in one plane.
So your aerial can lose signal if it gets blown by a big gust of wind.
Why are some sunglasses an example of using polarisation?
The lenses of some sunglasses are coated with a polarisation filter, which blocks some incoming light.
What are examples of surfaces that act as natural polarising filters?
Water
Glass
Tarmac
What is the symbol for wave speed?
C
What is the equation for wave speed?
Wave speed = frequency / wavelength
How can you measure wave speed using two microphones?
1) Two microphones are set up a fixed distance apart.
2) Both are connected to a computer.
3) The computer records when the first microphone heard the sound and then the second.
4) You now have a time and distance measurement.
Why is the two microphone example very accurate?
It doesn’t require any human intervention.
How do you measure wave speed timing an echo?
1) Someone makes a loud noise opposite a flat wall.
2) You measure the time it takes from the sound being made to you hearing the echo.
3) You then measure the distance to the wall.
4) BUT remember to double the distance, as the sound travelled there and back.
How do you measure wave speed timing a gunshot?
1) Have one person with a started pistol a large distance from someone with a timer.
2) Start the timer when you see the flash of the gun.
3) Stop it when you hear the gunshot.
4) Then measure the distance between the people.
What are the assumption is being make when measuring wave speed by timing a gunshot?
The light from the gun reaches you at the exact moment it was fired.
This is not true, light is very fast but does not travel instantaneously.
But over a short distance we can assume it does.
What is superposition?
The term used to describe two waves combining together.
What are the three main types of superpostion?
1) Constructive interference
2) Destructive interference
3) Total destructive interference
When is the only time total destructive interference occur?
When the amplitudes are the same.
Under what conditions is it possible to produce a stationary wave?
- If the superposition of two progressive waves, are travelling in opposite directions.
- With the same frequency/wavelength.
- And similar amplitude.
What is the main difference between progressive waves and stationary waves?
No energy is transferred.
As two progressive waves move through each other what happens?
They combine to form a single stationary wave.
When the two progressive waves overlap what will happen?
It will create a larger stationary wave via constructive interference.
When the two progressive waves line up so the peak of one wave is in line with the trough of another what will happen?
The stationary wave becomes flat via total destructive interference.
On stationary waves, what are the two types of points that will form?
Nodes
Anti-nodes
What are the features of nodes?
These are points of no displacement.
They constantly remain stationary.
Total destructive interference is always occurring here.
What are the features of anti-nodes?
These are points of maximum displacement.
They are where amplitude reaches a maximum.
Constructive interference is occurring here.
What is the distance between two nodes?
λ/2
What is the distance between two anti-nodes?
λ/2
What are three examples of stationary waves forming in real life?
1) String instruments - guitars, pianos (stationary transverse waves)
2) Wind instruments (stationary longitudinal waves)
3) Chladni plates, (shows stationary waves in 3D)
What is the easiest way to form a stationary wave?
Reflect the wave back on itslef.
When forming a stationary wave, what requirements must be met?
(1) Two progressive waves travelling in opposite directions.
(2) Same frequency/wavelength.
(3) Similar amplitude.
How are stationary waves demonstrated in a microwave oven?
A stationary wave forms within the oven.
This explains why your food needs to rotate, as no cooking will be done at the nodes.
What is phase difference?
The amount one point on a wave lags behind another point on a wave.
What is phase difference measured in?
Degrees or radians
What is a whole wavelength?
360 degrees or 2π.
When are points in phase?
When they have a difference of 0, 360, 720 … degrees.
This means they are at the same point of the wave cycle.
(E.g. Both at the peak)
When are points in anti-phase?
When they have a difference of 180, 540 … degrees.
This means they are at opposite points of the wave cycle.
(E.g. One at the peak, one at the trough)
On a progressive wave the phase difference between two points can be what value?
Any values
On a stationary waves two points can only be in phase at what values?
360 degrees or 2π.
On a stationary waves two points can only be in anti-phase at what values?
180 degrees or π.
When is the only time stationary waves form?
When a whole number of half wavelength can fit.
Therefore only certain frequencies will work.
What are harmonics or resonant frequencies?
What is the fundamental frequency?
The lowest possible frequency and fits just half a wavelength. (first harmonic)
What can affect the fundamental frequency of a string?
1) The length of the string
2) The tension in the string
3) The mass per unit length (heaviness) of the string
Assuming wave speed is constant, if you reduce the wavelength what will happen to the frequency?
The frequency increases.
What is the relationship between fundamental frequency and wavelength?
Fundamental frequency is inversely proportional to wavelength.
If you increase the tension, what will happen to the speed of the wave?
Wave speed will increase.
If there is a lower tension, what happens to the wave speed and frequency?
Slower waves
Lower frequency
If there is a higher tension, what happens to the wave speed and frequency?
Faster waves
Higher frequency
What equation can you use to find the tension of the string?
Tension = mass x acceleration due to gravity
T = m x g
If the wire is heavier, what will happen to wave speed?
Wave speed decreases.
How much diffraction can you expect if the gap is significantly larger then the wavelength?
No diffraction
How much diffraction can you expect if the gap is a bit larger then the wavelength?
Some diffraction
How much diffraction can you expect if the gap is equal to the wavelength?
Maximum diffraction
How much diffraction can you expect if the gap is small then the wavelength?
None, waves cannot pass through
What is monochromatic light?
Light of a single wavelength/frequency.
What is meant by coherent?
Light with a fixed phase difference between waves.
What are bright fringes cause by?
Constructive interference
What is a bright fringe?
Where waves from across the width of the slit arrive at the screen in phase.
What are dark fringes caused by?
Total destructive interference.
Where is the brightest part of the pattern?
Where the light is the most intense - central maximum (centre)
What effect would increasing the width of slit (while using the same wavelength of light) have on the width and brightness of the central maximum?
Increasing the slit width narrows the central maximum and makes it brighter:
- because there is less diffraction as the gap is now much bigger than the wavelength,
- It will also be brighter,
- Same number of photons
- but spread over a smaller area
What effect would increasing the wavelength of the light (while using the same slit size) have on the width and brightness of the central maximum?
Increasing the wavelength would increasing the wavelength widen the central maximum and make it dimmer:
- Because there is more diffraction as the wavelength is now closer in size to the slit size
- It will also be dimmer
- the same number of photons
- but spread over a larger area
What is two-source interference?
When the waves from two sources overlap with each other.
How can the two-source interference be achieved?
1) with two speakers producing sound waves
2) two pebbles dropped into a pond
3) a laser beam shone through two small slits
Two-source interference only works if the waves are?
Coherent: light has the same frequency and a fixed phase difference
What is the maxima?
Where a peak meets a peak.
- points of maximum displacement
- constructive interference
Or when a trough meets a trough.
What is a minima?
Where a peak meets a trough.
- no displacement
- total destructive interference
What is path difference?
A measure of how much one wave has travelled further than another.
This explains why you get bright spots (maxima) and dark spots (minima).
In young’s double split equation, what does w stand for?
Fringe spacing (m)
In young’s double split equation, what does In young’s double split equation, what does λ stand for?
Wavelength of light (m)
In young’s double split equation, what does D stand for?
Distance to screen (m)
In young’s double split equation, what does s stand for?
Split separation (m)
Young’s double split equation only works if…
D is much, much bigger than s.
What is Young’s Double split equation?
w = (λD)/s
What safety precautions should you follow when using a laser?
1) Never shine towards a person
2) Wear laser safety goggles
3) Avoid shining at a reflective surface
4) Have a warning sign
5) Turn the laser off when not needed
What are the features of one slit diffraction?
Produces very blurry interference patterns.
Central maximum twice the size.
What are the features of two slit diffraction?
Produces a more intense pattern of maxima and minima due to interference.
What are the features of diffraction grating?
Light diffracts trough multiple slits.
Produces a very intense pattern.
What is a diffraction grating?
- Contains lots of equally sized slits very close together.
- Hundreds of slits per millimetre.
How do you calculate difference between two slits?
d = 1 / N
N = number of lines per meter
What are orders?
Bright spots formed by a diffraction patterns.
What would happen to the pattern if you used a longer wavelength of light?
Increasing the wavelength widens the pattern, because there is no more diffraction as the wavelength is now closer in size to the slit size.
What would happen to the pattern if you used a grating with a fewer slits per mm?
If you have fewer slits per mm then it means each individual slit is now larger, the pattern will become narrower because there is less diffraction as the gap is now much bigger then the wavelength.
Is there a limit to how many orders you can get?
Yes there us, when light is diffracted through a slit can never be diffracted by more than 90 degrees.
When white light is shone through a diffraction grating, what pattern will you get?
Central maximum will appear white.
All subsequent maxima will be continuous spectra.
Red will appear in each maxima furthest from the centre and blue/violet will appear closest.
What is refraction?
When a wave passes from one medium to another.
Causing the wave to change speed.
Also changing the direction.
What is the refraction index?
The different speeds visible light travels at depending on the transparent material it is passing through.
The ratio of how fast light travels in a vaccum compared to how fast it travels in that medium.
What is the refractive index of air?
1
What happens when a wave passes into a denser material?
It causes the wave to slow down and the wavelength to reduce.
What happens when light passes from one medium to another?
It changes speed and direction.
The amount its direction changes depends on the refractive indices of the mediums.
How can the amount direction changes, be calculated?
Snell’s law :
n1 Sin01 = n2 Sin02
What happens when light goes from a low refractive index to a higher refractive index?
Bends towards the normal.
What happens when light goes from a higher refractive index to a lower refractive index?
Bends away from the normal.
What happens to SOME of the light when it goes from a higher refractive index to a lower refractive index?
It gets reflected.
The angle of reflection is the same as the angle of incidence.
According to snell’s law, what happens to the angle of refraction as you make the angle or incidence larger?
It will also get larger.
What is the critical angle?
When the angle of refraction is 90 degrees, light will not escape material 1.
What are the three main facts about the critical angle?
1) Can only beachieved when going from a higher refractive index to a lower refractive index.
2) When the angle of refraction is 90 degrees.
3) It is diffreent depending on the two materials.
What is the equation to calculate the critical angle?
n1 Sin0c = n2
What are the two conditions needed for total internal reflection?
1) Light is travelling from a higher to a lower refractive index material.
2) The angle of incidence is greater than the critical angle between the two materials.
What do fiber optic cables do?
= Make use of total internal reflection.
= To send messages at the speed of light.
= From one side of the planet to the other.
What is the features of the core - fiber optics?
Has a very high refractive index.
What is the features of the cladding - fiber optics?
Has a very low refractive index?
What are the advantages of sending messages as light through fiber optic cables?
1) Signal can carry more information as it has higher frequency.
2) Light doesn’t heat up fibre so no energy lost as heat.
3) No electricial interference.
4) Much cheaper to produce.
5) Signal can travel a long way quickly without much signal loss.
What causes some energy to be lost in fibre optic cables?
Scattering and absorption cuased by imperfections in the fiber, such as bubbles and dust.
This makes it harder to distinguish between 1’s and 0’s.
What would be the solution to prevent energy being lost in fiber optic cables?
Boost the signal every 100 Km’s or so before it becomes too weak.
What causes the pulse to get wider - broadening?
Dispersion of light.
What is modal dispersion?
Where one part of the signal travels further than another part (potentially due to enetering the fiber at different angles) meaning they don’t reach the end at the right time.
What is the solution to modal dispersion?
Use a signal boosters to correct this problem before it gets worse.
What can dispersion cause (not including broadening)?
Different wavelengths being refracted by different amounts.
What is material dispersion?
Signals to arrive at different times, due to different wavelengths are refracted by different amounts.
What is the solution to material dispersion?
Use monochromatic light so the amount of refraction is the same.
