Waves Flashcards
What does an optical fibre do?
It transmits information using an electromagnetic wave, which travels down a fibre by means of repeated reflections off the surface of the fibre.
Define Amplitude
A wave’s maximum displacement from the equilibrium position
Define Wavelength
The length of one whole oscillation (e.g. the distance between 2 peaks)
Define Phase
the position of a certain point on a wave cycle
Define Phase difference
How much a particle/wave lags behind another particle/wave
Define period
Time taken for one full oscillation
What can optical fibres be used for?
Cable TV networks, Communication networks and medical imaging (endoscopes)
How do optical fibres transmit information?
By converting electrical signals into digital pulses of light or infrared.
How are waves caused?
By a vibrating source
What is a progressive wave?
an oscillation or vibration that transfers energy and information without transferring material
What is a progressive made wave of?
Particles, a medium or a field that is oscillating
What is a wavefront?
Joins points on a wave that are at the same point of the cycle as their neighbour
What is a mechanical wave?
A wave that requires a medium for it to travel.
Define frequency
The number of complete oscillations (1 wavelength) passing through a point per second.
What makes particles in phase?
When they move in the same direction with the same speed. They have the same displacement and they are separated by a whole number of wavelengths
What makes particles in antiphase/ completely out of phase?
They move in opposite directions at the same speed, they have opposite displacements and they are separated by a whole plus a half number of wavelengths
When a wave reflects off a surface, what is its phase change?
180° or π
What is a longitudinal wave?
A wave where the particles oscillate parallel to the direction of energy transfer
What are longitudinal waves made up of?
Areas of compressions and rarefactions
What are some examples of longitudinal waves?
Sound, primary seismic and pulses
What is a transverse wave?
A wave where the particles oscillate perpendicular to the direction of energy transfer
What are some examples of transverse waves?
All Electromagnetic waves, secondary seismic
How fast do transverse waves travel in a vacuum?
3x10^8
How do sound waves travel through a solid?
through intermolecular bonds or interatomic bonds
Does sound move faster through solids or gases? why?
Solids,
because the bonds are stiff and the atoms are close together.
In gases, it travels by molecules colliding so the speed of sound depends on the speed of the molecules
Why doesn’t sound travel through space?
Because sound is a mechanical wave so it requires a medium for it to travel
list the electromagnetic spectrum in order from longest wavelength to shortest
Radio, microwaves, infrared, visible, ultraviolet, x ray, gamma
How do radio waves transmit information?
BY superimposing information from the programme onto carrier waves changing either its amplitude or frequency. The receiver is tuned to the same frequency as the carrier wave which then converts it back into sound and images
What are some things to consider when using electromagnetic waves for communication?
how much will be absorbed into the atmosphere
how much the wave will broaden due to diffraction
Why do transmitters and receivers need to be in a straight line of sight?
Because microwaves have a shorter wavelength than radio waves and diffract less
How can stationary microwaves be formed?
By reflecting a microwave beam at a metal plate and then using a microwave probe to find the nodes and antinodes
How can stationary sound waves be formed?
By placing a speaker at one end of a closed glass tube, lay powder across the bottom of the tube and it will be shaken at the antinodes and settle at the nodes.
How does visible and infrared radiation transmit data?
Through optical fibres because they only travel a short distance in air
What electromagnetic waves aren’t used in transmitting information and why?
Gamma and x-rays because they have shorter wavelengths and do not penetrate the atmosphere
Define polarisation
When the wave only oscillates in one plane
What does Polarisation provide evidence for?
The nature of transverse waves because polarisation can only occur if a wave’s oscillations are perpendicular to the direction of travel
Is light polarised when transmitted?
nope
What type of wave can’t be polarised?
longitudinal
What is crossing the polarisers?
When 2 polarising filters are placed but one is rotated 90° so by the time the light has passed through the filters, none comes out the other side.
How does polarisation by reflection work?
At some angles of incidence, the only reflected rays are rays whose electric fields oscillate in one direction
Name some applications of polarisation?
sunglasses, Outdoor television, FM radio
How do sunglasses use polarisation?
They reduce glare by blocking partially polarised light reflected from water and tarmac as they only allow oscillations in the plane of the filter, making it easier to see
How do TV and Radio signals use polarisation?
They are usually plane polarised by the orientation of the rods on the transmitting ariel, so the receiving ariel must be aligned in the same plane of polarisation to receive the signal at full strength
What happens a wave travels from one medium to another?
They change speed
What is refraction?
When light travels at any angle other than along the normal, the change in speed when entering a different medium causes a change in direction
What doesn’t change when a ray refracts?
Frequency
What happens to the wavelength and speed when travelling into a medium with a greater RI?
The wave slows down and the wavelength gets shorter
What happens when a wave slows down when entering a medium?
It changes direction TOWARDS the normal
What happens when a wave speeds up when entering a medium?
It changes direction AWAY FROM the normal
What is refractive index?
It measures how much light slows sown when passing through the material so it is the ratio of the waves speed between 2 materials
What is total internal reflection?
The complete reflection of waves back inside a medium at a boundary with a second material in which the wave travels faster
When does total internal reflection occur?
When the angle of incidence is greater than the critical angle and the incident RI is greater than the RI of the material at the boundary
What is the critical angle?
The angle of refraction for which the angle of incidence is 90°
What is an optical fibre made of?
a core with a high RI and cladding with low RI
What does the cladding in an optical fibre do?
It protects the core from damage and prevents signal degradation through light escaping the core
What can signal degradation be caused by?
Absorption- part of the signal is absorbed by the fibre reducing the amplitude of the signal
Dispersion- This causes pulse broadening where the received signal is broader than the original transmission. These can overlap causing loss of information. (Modal or material)
What is an optical fibre?
A thin glass or plastic fibre that transmits light or infrared radiation
What is a step index optical fibre?
by choosing suitable materials for the core and cladding, only certain wavelengths can travel through the fibre by total internal refection
What is material dispersion?
Caused by light consisting of different wavelengths meaning light rays will travel at different speeds along the fibre which leads to pulse broadening.
How can material dispersion be minimised?
using monochromatic light
What is modal dispersion?
When rays enter the fibre at different angles so take different paths, this leads to rays taking a different amount of time to travel along the fibre causing pulse broadening
Is a material with a higher RI, more or less optically dense?
More
How can absorption and dispersion be minimised in an optical fibre?
By using an optical fibre repeater which regenerates the signal during its travel to its destination
How can modal dispersion be minimised?
By using a core with a smaller diameter
What is superposition?
When 2 waves of the same type overlap and interact and the total displacement is the vector sum of the two waves.
How does noise cancellation work?
The incoming wavelengths are inverted and transmitted so that they overlap and there is deconstructive interference
What is interference?
The superposition of waves from two coherent sources of waves.
When is interference deconstructive?
If waves are in antiphase (180° out of phase)/ One has positive displacement and one has negative displacement
What is total deconstructive interference?
When the two waves have equal but opposite displacements
When is interference constructive?
If waves are in phase/ have displacement in the same direction
What makes waves coherent?
When they have a fixed phase difference, same wavelength and same frequency and a constant phase relationship
Why are lasers used for diffraction?
Because they are coherent and monochromatic so they form clear interference patterns
What does a constant phase relationship mean?
When light from 2 coherent sources meet, and can produce maxima (constructive) or minima (deconstructive)
What is path difference?
The difference in distance travelled by the two waves produced
What is path difference measured in?
Wavelengths
Describe young’s double slit experiment
Uses 2 coherent sources produced from a single source to make it have a fixed path difference or a laser which then passes through parallel slits which are about the same size as the wavelength. Each slit acts as a coherent point source making the pattern of light and dark fringes. Interference occurs because the waves overlap and superpose.
What does Youngs double slit experiment prove?
the wave nature of light because diffraction and interference are wave properties
When do light fringes occur in the double slit experiment?
when the path difference is a whole number of wavelengths (constructive interference)
When do dark fringes occur in the double slit experiment?
When the path difference is a whole plus a half wavelength
What is a stationary wave?
A wave formed by the superposition of two progressive waves of the same frequency, wavelength and amplitude travelling in opposite directions.
What is a node?
When the amplitude of the two progressive waves moving in opposite directions always cancel out so the particle doesn’t oscillate at all when the wave is completely out of phase (deconstructive)
What is an antinode?
The point on a stationary wave that is always at a maximum when the waves meet in phase (Constructive)
What is the lowest frequency at which stationary waves can form?
The frequency of the first harmonic
What is the distance between adjacent nodes (or adjacent antinodes)?
Half a wavelength
What is a harmonic?
A mode of vibration that is a multiple of the first harmonic ( half a wavelength)
How do you find the frequency of other harmonics based off of the first one?
Multiplying the frequency of the first harmonic by the number of antinodes that are seen. (e.g. x2 for the second harmonic)
Do stationary waves transfer energy?
Nope
When is the amplitude greatest for a stationary longitudinal wave?
At the open ends of the tube
What is diffraction?
When a wave passes through a gap or moves past an obstacle with a similar order of magnitude and spreads out
When is diffraction the greatest?
When the gap is the same size as the wavelength
Describe a single slit diffraction pattern
A bright central fringe which is could the width of all the other fringes with alternating dark and bright fringes on either side
Describe the diffraction pattern of light
There is a bright central fringe surrounded by other less-bright fringes either side. These fringes are maxima and are due to constructive interference
Describe the diffraction pattern of white light
The maxima are wider and the diffraction pattern is less intense and there is a central white fringe with alternating bright fringes which are spectra, violet is closest to the central maximum and red is the furthest
Why do you get a colour spectra when diffracting white light?
Because the different wavelengths of light are all diffracted by different amounts so a spectrum is seen.
What colour of light has the shortest wavelength?
Violet
How would you vary the width of the central maximum for white light diffraction through a single slit?
If you increase the slit width, the amount of diffraction decreases so the central maximum becomes narrower and its intensity increases
Increasing the wavelength increases the amount of diffraction therefore central maximum becomes wider and its intensity decreases
What safety precautions must be followed when using lasers?
Wear laser safety goggles
Don’t shine the laser on reflective surfaces
display warning signs
Never shine the laser at a person
Don’t use at eye level
How can you show interference with sound waves?
By using two speakers connected to the same signal generator and the intensity can be measured using a microphone to find the maxima (like light fringes) and minima (like dark fringes)
What is a diffraction grating?
Has thousands of very closely equally spaced slits
What is the difference between the diffraction pattern seen with monochromatic light through a diffraction grating and through double slits? Which is more accurate?
the interference pattern is sharper and brighter through a diffraction grating . This is because many more rays are reinforcing the pattern therefore measurements of slit widths are more accurate and easier to get
What happens when light passes straight through the slits?
The path difference between the two slits is zero and it the central maxima also known as the zeroth order.
When is the first order maximum seen?
When the light leaving two adjacent slits has a path difference of one wavelength
What are some uses of diffraction gratings?
separating light of different wavelengths, Telescopes, CDs, DVDs
What are some applications of diffraction gratings?
Splitting up light from stars to get line absorption spectra
X-ray crystallography which are x-rays directed at a thin crystal sheet which acts as a diffraction grating to form a diffraction pattern, this is because the wavelength of x-rays are similar size to the gas between atoms. This is used to measure the atomic spacing in certain materials
