Waves Flashcards
What is a progressive wave?
A wave that transfers energy from one point to another without transferring any matter/material
What is a transverse wave?
A wave where the oscillations are perpendicular to the direction that the wave travels in (or the direction of energy propogation)
What is a longitudinal wave?
A wave where the oscillations are parallel to the direction that the wave travels in (or the direction of energy propogation)
What are some examples of transverse waves?
Electromagnetic waves, water waves, waves on a string and seismic S-waves
What are some examples of longitudinal waves?
Sound waves, waves on a spring and seismic P-waves
What is a peak in a wave?
The highest point on a transverse wave above the equilibrium position
What is a trough in a wave?
The lowest point on a transverse wave below the equilibrium position
What is a compression in a wave?
A region in a longitudinal wave where the particles are closely packet together, creating an area of high pressure
What is a rarefaction in a wave?
A region in a longitudinal wave where particles are spread apart, creating an area of low pressure
What is the wavelength of a wave?
The distance between two identical points on a wave, it is the length of one whole wave oscillation or wave cycle (e.g. the distance from peak to peak or trough to trough on a transverse wave and the distance from compression to compression or rarefaction to rarefaction on a longitudinal wave)
What is the amplitude of a wave?
The maximum displacement of a wave from its equilibrium position
What is the frequency of a wave?
The number of complete wave cycles passing a point per second
What is phase?
It describes the position of a point within the wave cycle (usually in degrees, radians or as a fraction of a wave cycle)
What is phase difference?
The amount by which one wave lags behind another (measured in degrees, radians or as fractions of a wave cycle)
How do EM waves travel in a vacuum?
They all travel at the same speed, regardless of their frequency or wavelength (the travel at the speed of light)
What is meant by the polarisation of a wave?
It is the process by which the oscillations of a transverse wave are restricted to a single plane, you can do this using a polarising filter
Why does polarisation only occur in transverse waves?
Because transverse waves have oscillations in all directions, allowing you to filter out oscillations in one plane however longitudinal waves have oscillations in one plane parallel to the direction of travel so you can’t filter out oscillations
What happens if a second polarising filter is rotated through 180° relative to the first filter?
The intensity of light passing through will decrease as you increase the angle from 0° to 90° and then at 90° no light will be able to get through then as you increase the angle from 90° to 180° the intensity of the light passing through increases and at 180° the intensity is at a maximum
(the same thing repeats from 180° to 360° where the intensity is at a minimum at 270° and at a maximum at 360°)
How does polarisation provide evidence for the transverse nature of EM waves?
Because only transverse waves can be polarised and so because light can be polarised it must be a transverse wave
How can polarisation be used in sunglasses?
Light that has been reflected off of surfaces becomes polarised in the horizontal direction which can cause glare so polaroid sunglasses help to reduce this glare by blocking out the horizontal reflected light helping to reduce the intensity of light entering your eye helping to improve visibility
How can polarisation be used in polaroid cameras?
When taking photos a polarising filter is placed in front of the lens to block any unwanted reflections (partially polarised light), by not allowing horizontal oscillations to pass through which helps to reduce glare and it can improve the quality of the image
How can polarisation be used to improve TV signals?
TV signals are polarised by the orientation of the rods on the transmitting aeriel so to recieve a strong signal you have to line up the rods on the recieving aeriel with those on the transmitting aeriel
How can polarisation be used to improve radio signals?
Radio signals are polarised by the orientation of the rods on the transmitting aeriel so if you try tuning in a radio and then moving the aeriel around, the signal will come and go as the transmitting and recieving aeriels go in and out of alignment
What is a stationary wave?
A wave that remains in a constant position, is created when two waves of the same frequency and amplitude travel in opposite directions and interfere with eachother
What is a node and how is one formed?
It is a point on a stationary wave where there is no displacement and it is formed when two waves destructively interfere with eachother and the total displacement of the two waves added togethter at that point is zero (they cancel out)
What is an antinode and how is one formed?
It is a point on a stationary wave where the displacement is at a maximum and it is formed when two waves constructively interfere with eachother meaning the total displacements of the waves combine to create a point of maximum displacement
What does it mean for two waves to be coherent?
They have a constant phase difference and the same frequency
What does it mean source to be monochromatic?
It is made up of waves of a single wavelength/frequency
What is two-source interference?
When the waves from two sources interfere to produce a pattern (to get a clear pattern the waves need to be monochromatic and coherent)
What is path difference?
The difference in distance travelled by two waves from their sources to a point
How do you work out the path difference in places where constructive interference has occured?
path difference = nλ
(where n = 0, 1, 2,…)
How do you work out the path difference in places where destructive interference has occured?
path difference = (n+1/2)λ
(where n = 0, 1, 2,…)
Why is a laser used in interference or diffraction experiments?
A laser produces a coherent, monochromatic source of light which is ideal for producing clear interference and diffraction patterns
What is interference?
When two or more waves overlap and combine
What is constructive interference?
When the overall amplitudes of two or more waves add up resulting in a larger wave. This occurs when waves are in phase with eachother
What is destructive interference?
When the overall amplitudes of two or more waves add up resulting in an overall amplitude of zero (the waves cancel eachother out). This occurs when waves are out of phase with eachother
What kind of pattern is seen when a monochromatic light source is passed through a single slit?
A diffraction pattern is produced with a central maximum with alternating dark and bright fringes on either side due to interference (the bright fringes are where constructive interference has occured and the dark fringes are where destructive interference has occured)
What happens if you shine white light through a single slit?
An interference pattern is observed but because white light contains many different wavelengths (colours) this causes each wavelength to diffract by different amounts. The resulting pattern is a central white fringe (where all the wavelengths constructively interfere) and then coloured fringes on either side
How does the width of a central maximum in a diffraction pattern vary with slit width?
Increasing the slit width decreases the amount of diffraction this means the central maximum is narrower, and the intensity of the central maximum is higher
What safety precautions should be taken when using lasers?
- Never shine a laser towards a person
- Wear laser safety goggles
- Avoid shining the beam at reflective surfaces
- Have a warning sign on display
- Turn off the laser when it is not needed
Draw the graphs of monochromatic red, green and blue light being shone through a single slit and compare them
Look in booklet for answers
Draw the intensity graph for white light being shone through a single slit
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How does changing the slit width affect a diffraction pattern?
A narrower slit means a wider and shorter central maximum and a wider slit means a taller and narrower central maximum
How does the wavelength of light used affect a diffraction pattern?
A longer wavelength means a wider and shorter central maximum and a shorter wavelength mean a taller and narrower central maximum
What is a diffraction grating?
An optical device with many closely spaced parallel slits that diffracts light
Draw what is observed when monochromatic light is shone through a diffraction grating
Look in bookelt for answer
Draw what is observed when white light is shone through a diffraction grating
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Derive the equation dsinθ = nλ
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What are some applications of diffraction gratings?
- Spectroscopy (to separate light into its wavelengths to identify elements based on their spectra)
- Astronomy (to analyse light from stars and galaxies, to determine their composition and movement)
- X-ray crystallography (studying atomic structures by analysing how X-rays diffract through crystals)
- CD/DVD readers (to read data encoded on a disk by diffracting light from a laser)
What is refraction and when does it happen?
It is when a wave changes direction due to it changing speed, this happens as it passes from one medium to another
How does path of a light ray change as it enters a material that is more difficult to travel through?
The light will bend towards the normal line
What is meant by the refractive index of a material?
A measure of how much light slows down or speeds up as it enters a material, it compares the speed of light in a vacuum to the speed of light in that material
How is optical density related to refractive index?
A material that is more optically dense has a higher refractive index
What does a refractive index of 3 mean?
That the speed of light travelling in a material is 1/3 of the speed of light in a vacuum
What does Snell’s law describe?
How light changes direction (refracts) as it crosses the boundary between two materials with different refractive indicies
Derive Snell’s law equation
Look in booklet for answer
What happens if light hits the boundary between two materials at 90° (along the normal)?
There will be no change in direction, the light will continue straight along the normal line but it’s speed will still change
What is meant by the ‘critical angle’ of a material?
It is the smallest angle of incidence at which the light travelling from a more optically dense material to a less optically dense material, is refracted along the boundary (at 90° to the normal). This means the angle of refraction is 90°
What happens if the angle of incidence is greater than the critical angle?
Then total internal reflection will occur
What are the two conditions total internal reflection to occur?
1) The refractive index of the first material must be greater than the refractive index of the second material (n1 > n2)
2) The angle of incidence must be greater than the critical angle (θi > θc)
What is an optical fibre?
Thin strands of glass or plastic that rely on total internal reflection to transmit light signals
What are the advantages of using optical fibres?
They can carry a lot of information, the light doesn’t heat up the fibre, there is no electrical interference, they are cheap and they can travel a long way very quickly (as light is used)
Draw and label the cross section of an optical fibre
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What is the purpose of the sheath in an optical fibre?
It acts as an outer protective layer that surrounds the cladding and prevents it from being damaged
What is the purpose of cladding?
It has a lower refractive index than the core so it keeps the light trapped in the core and it helps to protects the core
Describe what absorption is in optical fibres and why it occurs
When energy is lost as light travels through an optical fibre as some of the signal’s energy being absorbed by the material that the fibre is made from which results in the amplitude of the signal being reduced
Draw the effect of absorption in optical fibres
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What is pulse broadening?
The spreading out of a light pulse as it travels through an optical fibre
What is the effect of pulse broadening?
The broadened pulses can overlap eachother leading to information loss
Draw the effect of pulse broadening
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Describe what material dispersion in an optical fibre is and why it occurs
It is when some parts of a signal take longer to travel down an optical fibre because different wavelengths (colours) of light experience different amounts of refraction in a material (so some wavelengths travel slower than others), this results in pulse broadening
How can you stop material dispersion?
By using a monochromatic light source
Describe what modal dispersion is in an optical fibre is and why it occurs
It is when light rays take different paths down an optical fibre due to them entering at different angles, causing rays travelling down the centre to get to the end faster and others to arrive at different times, this results in pulse broadening
How can you reduce modal dispersion?
By using a single-mode fibre (which is one in which light is only allowed to follow a very narrow path)
How can you reduce the effect of signal degradation from absorption and dispersion?
By using optical fibre repeaters that can be used to regenerate rhe signal every so often
