Physics Unit 1 - Waves in communication Flashcards
what is a wave?
-a movement of energy
-the medium the wave moves through doesn’t move, but energy does
what is a medium?
-a substance or material that contains particles
what is meant by periodic time / time period ( T )?
-the time it takes to complete one full wave
-measured in seconds ( s )
what is meant by frequency ( f )?
-the number of complete waves that pass a point per second
-measured in Hertz ( Hz )
what is the formula linking time period and frequency?
-T = 1 / f
-time period ( T ) in s = 1 / frequency ( f ) in Hz
what is meant by wavelength ( λ )?
-the distance between a point on one wave and the same point on the next wave
-measured in metres ( m )
what is meant by wave speed ( v )?
-the distance the wave travels in a given amount of time
-measured in metres per second ( m/s )
how is wave speed calculated?
-v = fλ
-wave speed ( v ) in m/s = frequency ( f ) in Hz x wavelength ( λ ) in m
what is meant by amplitude?
-the height of the peak / crest, or the depth of the trough, from the centre of the wave ( i.e. the maximum displacement )
what is meant by oscillation?
-going back and forth repeatedly between two positions or states
what is meant by a longitudinal wave?
-the particles oscillate parallel to the direction of propagation ( i.e. energy transfer )
-a series of compressions and rarefactions
what are some examples of longitudinal waves?
-sound waves
-ultrasound waves
-seismic P ( primary ) waves
what is the difference between compressions and rarefactions?
-compressions are where particles are closer together ( at the peak )
-rarefactions are where particles are further apart ( at the trough )
how is a longitudinal wave produced on a slinky spring?
-by stretching out the slinky parallel to the direction of propagation and releasing one end
what is meant by a transverse wave?
-the particles oscillate 90° to the direction of propagation
what are some examples of transverse waves?
-water waves
-electromagnetic waves
-seismic S ( secondary ) waves
how is a transverse wave produced on a slinky spring?
-by holding one end of the stretched slinky and moving your hand up and down, perpendicular to the direction of propagation
what is light made up of?
-transverse waves that oscillate at different angles relative to the direction of propagation
what is a polariser and what does it do?
-an optical filter which removes any oscillations that are at 90° to the filter
why can longitudinal waves not be polarised?
-their particles oscillate parallel to the direction of propagation, so there is only one oscillation direction
if unpolarised light passes through a horizontal polariser, will the light be polarised vertically or horizontally and why?
-horizontally
-the polariser removes the oscillations that are at 90° to the filter
what is Giga ( G ) in standard form?
-x10^9
what is Mega ( M ) in standard form?
-x10^6
what is kilo ( k ) in standard form?
-x10^3
what is milli ( m ) in standard form?
-x10^-3
what is micro ( µ ) in standard form?
-x10^-6
what is nano ( n ) in standard form?
-x10^-9
what is meant by phase difference?
-how much one wave is shifted relative to another wave
-waves can be in phase ( aligned peak to peak or trough to trough ), antiphase ( aligned peak to trough ) or out of phase
what is a wave the same as?
-a circle
-this means that 1 wavelength = 1 circle, and 1/2 λ = 1/2 circle etc.
how many degrees is one wavelength?
-360°
-this means that each quarter ( each middle to peak / trough and vice versa ) is 90°
what is the radian ( rad )?
-a unit of angle which is more accurate than degrees
how are degrees converted to radians?
-number of quarters x π / 2
-e.g. 180° = 2π / 2 rad
what is 90° in radians?
-1 quarter x π / 2
-so 90° = π / 2 rad
what is 180° in radians?
-2 quarters x π / 2
-so 180° = 2π / 2 rad
what is 540° in radians?
-6 quarters x π / 2
-so 540° = 6π / 2 rad
what is 720° in radians?
-8 quarters x π / 2
-so 720° = 8π / 2 rad
how is the phase difference between two waves calculated?
-draw a vertical line at the first peak of one wave
-draw a vertical line at the first peak of the other wave
-choose one wave and count the quarters between the two lines
what is meant by a constant / fixed phase relationship?
-the phase difference between the waves never changes
what is meant by coherent waves?
-waves with the same frequency and a constant phase difference
what is diffraction?
-the spreading of waves as they pass through a gap or go past an obstacle
what are wavefronts?
-lines ( or planes in 3D ) drawn to join points in a wave where all the oscillations are in phase
-they are spaced one wavelength apart ( phase difference of 360° or 2π rad ) and move forward in a direction perpendicular to the wavefront i.e. along a ray line
how does gap size affect diffraction?
-when the gap size is similar to the wavelength, more diffraction occurs and the waves spread out a lot, creating almost semicircular wavefronts
-when the gap size is much larger than the wavelength, the waves spread out less ( less diffraction )
can wavefronts pass through a gap that is much smaller than their wavelength?
-no, they can’t fit through
what is superposition?
-when two or more waves occupy the same space ( which is possible because they aren’t charged )
what are some examples of interference / superposition?
-large and small water ripples
-light and dark fringes ( visible light )
-loud and soft ( sound )
-good and poor reception ( radio / phone )
when are waves in phase?
-when their phase difference is 0, 2π, 4π, 6π rad etc. ( when they are in the same place or a whole number of wavelengths apart )
-they are aligned peak to peak or trough to trough
when are waves in antiphase?
-when their phase difference is π rad / 180° ( when they are half a wavelength apart )
-they are aligned peak to trough
what is constructive interference?
-when waves meet each other in phase, their amplitudes add together to produce large crests and troughs
-the resultant wave is bigger than either of the two original waves
what is destructive interference?
-when waves meet each other in antiphase, their amplitudes subtract to produce no crests or troughs
-the amplitude of the resultant wave is equal to the difference between the two original amplitudes
what is meant by path difference?
-the difference in distance that waves travel from their sources to get to the same point
-if the path difference is equal to a whole number of wavelengths ( including zero ), constructive interference ( a bright fringe ) will occur
-if the path difference is equal to half a wavelength, destructive interference ( a dark fringe ) will occur
what is a diffraction grating?
-a slide that has a large number of parallel, closely spaced slits
-the light passing through each slit is diffracted
how are bright and dark interference fringes produced by superposition?
-a path difference is created by using a diffraction grating to divide a light source so that separate rays of light travel different paths
-the path difference increases as the angle through which a grating scatters the light is increased
-when the path difference equals a whole number of wavelengths, the light rays are in phase and constructive interference ( a bright fringe ) occurs
-in between, at a path difference of λ/2, 3λ/2 etc. which is equal to half a wavelength, the light rays are in antiphase and destructive interference ( a dark fringe ) occurs
how does the wavelength of light affect diffraction?
-the larger the wavelength, the larger the angle at which the light spreads out / is diffracted
what happens when white light is diffracted?
-each bright fringe becomes a rainbow because the angle at which the light spreads out depends on its wavelength, and white light is made up of all the visible colours which each have a different wavelength
-constructive interference occurs when the path difference is a whole number of wavelengths ( in phase ), so each colour will form a bright line in a different place
-red has the highest wavelength so it is diffracted at a larger angle than all the other visible colours, forming a bright line furthest away from the zero order ( i.e. incident ray )
-the straight-ahead ( transmission ) beam has no path difference, so it is not split into a spectrum
what is the order of diffraction?
-zero order = the central maximum bright white beam which has the highest intensity
-first order = the first bright coloured fringes
-second order = the second bright coloured fringes
how is an emission spectrum formed?
-there is a specific energy difference between the energy levels in an atom / ion
-electrons in an atom occupy the orbital closest to the nucleus in which there is space ( known as the ground state )
-if the atoms are in a gas or vapour, they can be heated or an electrical current can be passed through them, and electrons in the ground state absorb a fixed amount of energy and are promoted to a higher energy level
-to stop being excited ( de-excite ) and fall back to their ground state, the electrons must re-emit the energy in the form of light
-this amount of energy is equal to the energy difference between the two energy levels
-the emitted light is passed through a slit to produce a coherent beam which is then passed through a diffraction grating, and different wavelengths are diffracted at different angles
-constructive and destructive interference occurs to produce coloured bands and dark regions
what is the formula linking energy and frequency of light?
-E = h x f
-energy ( E ) in J = Planck’s constant ( h ) in Js x frequency ( f ) in Hz
-Planck’s constant = 6.63 x 10^-34 Js
what are progressive / travelling waves?
-waveform travels, transferring energy from one place to another
what are stationary / standing waves?
-wave motions that store energy rather than transfer it to another location
how are stationary waves formed?
-when two progressive waves are travelling in opposite directions and are of the same frequency and speed, with a fixed end at both sides, e.g. when a wave is reflected off a boundary and undergoes a phase change of 180° / π rad ( waves in antiphase )
-these waves superpose producing points of constructive and destructive interference
what are nodes?
-points of zero amplitude along a stationary wave, caused by destructive interference where the two waves are in antiphase and hence cancel each other out
what are antinodes?
-points of maximum amplitude along a stationary wave, caused by constructive interference where the two waves are in phase and hence combine to give maximum displacement
what is resonance?
-when the amplitude of a system’s oscillations drastically increases due to gaining energy from an external driving force, the energy is stored in an oscillation or a stationary wave
-this occurs if the driving frequency ( frequency of energy from an external source ) is equal to the natural frequency
what are resonators?
-systems / devices which store wave energy by reflecting the wave back on itself to form a stationary wave
-they only efficiently receive energy from an external source that has a frequency close to their natural frequency
-e.g. the strings and body of a violin or the pipes of an organ
what is a harmonic?
-a wave which is added to the basic fundamental wave ( i.e. the first harmonic which has 2 nodes and 1 antinode )
-this vibration makes the air vibrate and the sound waves travel to our ears so we can hear it
-subsequent standing waves are called the second harmonic, third harmonic etc.
how many nodes and antinodes do the first four harmonics have?
-first = 2 nodes, 1 antinode
-second = 3 nodes, 2 antinodes
-third = 4 nodes, 3 antinodes
-fourth = 5 nodes, 4 antinodes
what are the three types of instrument?
-stringed instruments e.g. violin, guitar, cello
-wind instruments e.g. flute, clarinet, oboe
-percussion instruments e.g. drums, triangle, xylophone
how do stringed instruments work?
-they use stationary transverse waves in stretched strings to make a resonator
-the fixed ends of the string are always nodes
-the fundamental mode has one antinode, so L = λ / 2
-for higher harmonics, any number ( n ) of antinodes can fit in the length of the string, so L = λ / 2n
what factors can be changed to alter the pitch of the notes that stringed instruments produce?
-the tension of the string = more tension means faster speed and hence a higher note
-the thickness of the string = thicker strings have more mass per unit length which gives a slower speed and hence a lower note
-the length of the string = longer strings have less mass per unit length which gives a faster speed and hence a higher note
how do wind instruments work?
-pipe organs, woodwind and brass instruments all use longitudinal sound waves in a tube or pipe to make a resonator
-open ends of pipes are antinodes, so a pipe open at both ends has λ = L / 2n ( i.e. a complete set of harmonics as in a stringed instrument )
-closing one end of a pipe creates a node there, so the fundamental mode has L = λ / 4 and only odd numbered harmonics will fit into the pipe ( e.g. L = 3λ / 4, 5λ / 4 etc. )
-missing out the even harmonics makes for a brighter / harsher sound
what is the only way by which wind instruments can be tuned and why?
-by altering the length of pipe being used because the speed of sound in air is fixed for any given temperature and pressure
how do percussion instruments work?
-the sound comes from striking two things together
-they can be the simplest type of instrument because usually very few parts are needed to produce an amplified sound, e.g. when struck, the skin of the drum vibrates and the sound is amplified by the barrel
what is the formula for mass per unit length?
-µ = m / L
-mass per unit length ( µ ) in kg/m = mass ( m ) in kg / length ( L ) in m
what is the formula for wave speed in a string?
-v = √( T / µ )
-wave speed ( v ) in m/s = √ tension ( T ) in N / mass per unit length ( µ ) in kg/m
how is the formula for wave speed in a string rearranged so it can be put into a triangle?
-v^2 = T / µ
what is fibre optics?
-a technology that uses thin, flexible cylinders of glass or plastic to transmit data as pulses of light over long distances
what is the law of reflection?
-angle of incidence = angle of reflection
what is refraction?
-when waves ( e.g. light ) pass from one medium to another the speed of them changes, causing the angle they are travelling through to change
what happens when light enters a more dense medium?
-light slows down and bends towards the normal
-angle of incidence > angle of refraction
what happens when light enters a less dense medium?
-light speeds up and bends away from the normal
-angle of incidence < angle of refraction
what happens when light enters a more dense medium along the normal?
-light slows down but is not bent
what are the formulae for refractive index?
-n = c / v = sin i / sin r
-refractive index ( n ) = speed of light in air ( c ) in m/s / speed of light in material ( v ) in m/s = sine of angle of incidence ( i ) / sine of angle of refraction ( r )
-n = 1 / sin C
-refractive index ( n ) = 1 / sine of critical angle ( C )
what is total internal reflection?
-when light meets a less optically dense ( lower refractive index, n ) medium at an angle of incidence larger than the critical angle ( the least angle of incidence at which TIR occurs ), it is completely reflected back into the first medium rather than refracted
-the critical angle must be smaller than the angle of incidence ( i.e. incidence larger than critical )
-must be the boundary from an optically dense medium to a less dense medium so light speeds up and bends towards the normal
what happens when the angle of incidence is equal to the critical angle at a glass-air boundary?
-the angle of refraction is 90° i.e. the refracted ray is parallel to the boundary surface, so it disappears and instead there is total internal reflection
-sin C = 1 / n
how does light travel through an optical fibre?
-light from quite a range of angles is refracted on entering the cut end of a fibre so that it hits its inside surfaces at angles greater than the critical angle and so is totally internally reflected
what is the purpose of the structure of an optical fibre?
-a thin glass fibre core is surrounded with glass cladding and then covered with a protective layer ( buffer coating ), and often multiple fibres are bundled together with additional layers for strength and insulation
-light passes through the core, while the cladding layer acts as a boundary that keeps the light trapped in the core due to total internal reflection
-the cladding is less optically dense than the core which increases the critical angle so TIR occurs and there are fewer reflections, meaning the signal travels a shorter total distance and energy loss is reduced as all the light stays inside the core ( principle of conservation of light )
what are the applications of fibre optics?
-in telecommunication and broadband internet services, enabling fast, reliable data transmission
-in medicine, enabling high-resolution imaging and precise surgical procedures
-in scientific research e.g. astronomy and physics, enabling accurate data collection and analysis
-by the military and law enforcement for secure communication lines due to fibre optics’ resilience to electromagnetic inference and ease of surveillance detection
what are the two types of fibre optic cables?
-single-mode fibre ( SMF ) = used for high-speed data transmission and contain a narrow core that allows light to take one direct path along the cable, ensuring that a signal will retain its shape and intensity over long distances
-multi-mode fibre ( MMF ) = used for shorter-range transmissions and contain a large core that allows multiple light rays to propagate at the same time ( higher capacity ), but signals deteriorate over longer distances due to different light paths crossing and interfering with each other
what are the advantages of using fibre optics in communication?
-provide greater bandwidth which allows them to carry a higher data rate than traditional copper cables due to their higher frequencies and ability to be multiplexed
-suffer less signal loss ( attenuation ) over distance than in copper wires, enabling data to be sent over long distances without quality loss and need for amplification
-more secure from data theft as the data is carried as light pulses so cannot be tapped into
-more durable and resilient to physical disruption
-resistant to EM interference which improves the quality of the transmitted signals
compare the transmission of signals using optical fibres and satellites
-both use EM ( transverse ) waves
-both transmit signals over long distances
-both are subject to damage ( e.g. cables and dishes )
-both provide access to internet / broadband
-optical fibres use pulses of visible light or infrared in cables, whereas satellites use digital signals carried by microwaves and radio waves through the atmosphere to a satellite and back to a dish
-optical fibres have a higher bandwidth as they can be multiplexed to carry more data and use higher frequency EM waves, whereas satellites have a lower bandwidth
-optical fibres use TIR and don’t require line of sight, whereas satellites require the dish to be pointing directly at it
-optical fibres are more reliable due to very little interference and have better quality of signal, whereas the quality of satellite signal is affected by terrain and weather e.g. rain
-optical fibres have better security and are more difficult to hack, whereas satellites are less secure
how are optical fibres used in medicine?
-in diagnostic procedures due to their flexibility and thinness, allowing doctors to non-invasively explore the body ( endoscopy )
-in treatment procedures e.g. laser surgeries or photodynamic therapy ( PDT ) where light-sensitive drugs are activated by light delivered through a fibre optic to destroy cancer cells, allowing for accurate and minimally invasive treatment
how are optical fibres used in endoscopy to produce an image?
-light is piped in from an external source using a small bundle of optical fibres down to the distal ( remote ) end, where it illuminates and reflects off the area being investigated
-an analogue image is passed up through the second bundle of optical fibres and each fibre collects and carries the light for one pixel of the image seen on the screen
-light passes down the optical fibre by total internal reflection as it strikes the sides at angles greater than the critical angle, and the less dense cladding increases the critical angle so that there are less reflections down the length of the fibre and thus less energy is lost
what is the difference between analogue and digital signals?
-analogue = continuously variable and can have any value
-digital = encodes the information as a series of numbers in binary code ( 1s and 0s )
what are the advantages and disadvantages of digital signals?
-advantages = can travel longer distances as can be regenerated, more accurate and better quality as most interference can be eliminated, can be stored on a computer ( due to their use of binary code ) to prevent data loss as signal can be restored, higher bandwidth as can be multiplexed, more secure as can be encrypted
-disadvantages = time delay due to signal processing, data accuracy limited by sampling settings
what two factors limit the accuracy of digitised information?
-sampling rate = the number of times per second that the quantity ( e.g. voltage ) is measured
-sampling sensitivity = the smallest increment in the quantity that is measured and recorded
-the higher the sampling rate and sensitivity, the higher the accuracy and quality of the signal
how can an analogue signal be converted into a digital signal?
-by using an analogue to digital converter / ADC
-voltages of the analogue input signal are sampled at discrete intervals ( i.e. sampling rate and sensitivity are selected )
-the sampled voltages are converted to a binary code and transmitted as a digital signal
what is the electromagnetic ( EM ) spectrum?
-a continuous spectrum of the range of all types of EM radiation, organised by frequency or wavelength
-all EM waves transfer energy, are transverse and travel at the same speed ( 3 x 10^8 m/s ) in a vacuum
what is the inverse square law in relation to the intensity of a wave?
-waves that radiate in all directions from an antenna have spherical wavefronts, so they lose intensity with radius from the source according to the equation: I = k / r^2
-intensity ( I ) in W/m^2 = intensity 1 m from the centre of the source ( k ) in W / radius^2 ( r^2 ) in m^2
what is the formula for comparing the intensity of two sources?
-initial intensity / final intensity = ( final radius / initial radius )^2
what is the EM spectrum in order of increasing frequency and what are the uses of each region?
-radio waves = TV and radio transmission
-microwaves = satellite communication, Wi-Fi and cooking ( microwave ovens )
-infrared = thermal imaging cameras, fibre-optic cables and TV remotes
-visible light = allows us to see as can be detected by the human eye, fibre-optic cables, photography and illumination
-ultraviolet = sterilisation, detecting forged bank notes and visual navigation by bees and butterflies
-X-rays = medical imaging and treating cancer
-gamma rays = sterilisation, detecting cancer and treating cancer
what are the frequency ranges of the EM waves?
-radio waves = 3 Hz to 300 GHz
-microwaves = 300 GHz ( 0.3 THz ) to 300 THz
-infrared = 300 GHz to 430 THz
-visible light = 430 THz to 790 THz
-ultraviolet = 750 THz to 30 PHz
-X-rays = 30 PHz to 30 EHz
-gamma rays = more than 30 EHz
what types of EM waves are used in communication?
-radio waves
-microwaves
-infrared
-visible light
what is the ionosphere?
-a layer of charged particles in the upper atmosphere which reflects frequencies below 40 MHz ( e.g. some radio waves )
what are the advantages of using radio waves in communication?
-non-ionising
-don’t require line of sight so can pass through obstacles, allowing for reliable communication
-aren’t affected by most weather conditions so are subject to less interference
-can travel long distances so are suitable for broadcasting and communication over large areas
-are reflected by the ionosphere ( aren’t absorbed ) so can be used for terrestrial communication to receivers beyond the horizon and communication with low orbit satellites
-can be transmitted underwater
what are the disadvantages of using radio waves in communication?
-short frequency so can carry less information per second
-lower bandwidth which can lead to congestion and interference
-subject to interference by other radio signals which affects quality
what are the advantages of using microwaves in communication?
-non-ionising
-can pass through the ionosphere so can travel long distances ( to the satellite )
-less energy is lost as aren’t absorbed by the ionosphere
-higher bandwidth and frequency than radio waves so can carry more information
-good quality and clear communication between devices as can transmit a lot of information
what are the disadvantages of using microwaves in communication?
-require line of sight so signal is affected by obstacles
-subject to interference by wet weather ( e.g. rain, fog, mist ) as microwaves are strongly absorbed by water which affects quality
-can’t communicate with vessels underwater
what are the advantages of using infrared waves in communication?
-don’t interfere with other devices
-high frequency and bandwidth so can carry more information
-good quality and clear communication between devices as can transmit a lot of information
what are the disadvantages of using infrared waves in communication?
-require line of sight so signal is affected by obstacles and can only be used for short distance communication
-most are absorbed by the ionosphere
-subject to interference by sunlight which affects quality, but longer wavelength band can be used to reduce this
what are the advantages of using visible light waves in communication?
-visible ( i.e. can be detected by the human eye )
-high frequency and bandwidth so can carry more information
-can pass through transparent objects and partially through translucent objects
what are the disadvantages of using visible light waves in communication?
-can affect eyes if exposed to them for long periods of time
-are absorbed by the ionosphere
-subject to interference by sunlight and other illumination which affects quality
-can’t pass through opaque objects
how does satellite communication work?
-EM waves with high enough energy to penetrate the earth’s atmosphere ( e.g. radio waves / microwaves ) are sent from a ground station to a satellite
-the satellite amplifies the signal and retransmits it back to different locations on earth
-this makes global broadcasting and networking possible, providing services such as GPS, TV broadcasting and internet connectivity
why might a mobile phone not receive a signal when in range of a transmitter?
-destructive interference may be occurring due to the reflection of signals off large buildings or hills / overlapping signals
-network overload due to too few frequencies being avaliable
-the signal may not be avaliable at ground level due to being too close to the transmitter
-attenuation / reduction in intensity of signal due to wet weather, storms, being below ground level, large buildings etc.
what is Bluetooth and what are some useful applications of it?
-a technology standard used to enable short-range wireless communication between electronic devices which operates on radio frequencies
-useful applications include tethering a mobile phone for hands free use in a car, wireless devices e.g. headphones, speakers, mice and keyboards, file / data transfer, opening a car / garage doors / gates
what is Wi-Fi?
-a wireless networking technology that uses radio waves to provide wireless high-speed internet access using a router and enable devices to communicate with one another wirelessly
why does Wi-Fi not interfere with Bluetooth despite operating in the same frequency band ( i.e. short wave radio )?
-Bluetooth devices frequency-hop across a range of channels many times a second to limit their interference with Wi-Fi
-time division multiplexing ( TDM )
why do upload and download signals have different frequencies?
-upload needs less power so has a lower frequency
-interference needs to be avoided because if the signals were to interfere, it would be difficult to differentiate between them
why do upload signals need to be amplified?
-received upload is weak / has less power so needs amplifying
-download signal needs to be made stronger / have more power for transmission
