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Applied Science > Physics Unit 1 - Waves in communication > Flashcards

Physics Unit 1 - Waves in communication Flashcards

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1
Q

what is a wave?

A

-a movement of energy
-the medium the wave moves through doesn’t move, but energy does

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2
Q

what is a medium?

A

-a substance or material which contains particles

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3
Q

what is meant by periodic time / time period ( T )?

A

-the time it takes to complete one full wave
-measured in seconds ( s )

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4
Q

what is meant by frequency ( f )?

A

-the number of complete waves that pass a point per second
-measured in Hertz ( Hz )

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5
Q

what is the formula linking time period and frequency?

A

-T = 1 / f
-time period ( T ) in s = 1 / frequency ( f ) in Hz

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6
Q

what is meant by wavelength ( λ )?

A

-the distance between a point on one wave and the same point on the next wave
-measured in metres ( m )

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7
Q

what is meant by wave speed ( v )?

A

-the distance the wave travels in a given amount of time
-measured in metres per second ( m/s )

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8
Q

how is wave speed calculated?

A

-v = fλ
-wave speed ( v ) in m/s = frequency ( f ) in Hz x wavelength ( λ ) in m

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9
Q

what is meant by amplitude?

A

-the height of the peak / crest, or the depth of the trough, from the centre of the wave ( i.e. the maximum displacement )

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10
Q

what is meant by oscillation?

A

-going back and forth repeatedly between two positions or states

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11
Q

what is meant by a longitudinal wave?

A

-the particles oscillate parallel to the direction of propagation ( i.e. energy transfer )
-a series of compressions and rarefactions

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12
Q

what are some examples of longitudinal waves?

A

-sound waves
-ultrasound waves
-seismic P ( primary ) waves

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13
Q

what is the difference between compressions and rarefactions?

A

-compressions are where particles are closer together ( at the peak )
-rarefactions are where particles are further apart ( at the trough )

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14
Q

how is a longitudinal wave produced on a slinky spring?

A

-by stretching out the slinky and releasing one end

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15
Q

what is meant by a transverse wave?

A

-the particles oscillate 90° to the direction of propagation

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16
Q

what are some examples of transverse waves?

A

-water waves
-electromagnetic waves
-seismic S ( secondary ) waves

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17
Q

how is a transverse wave produced on a slinky spring?

A

-by holding one end of the stretched slinky and moving your hand up and down, perpendicular to the slinky’s direction

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18
Q

what is light made up of?

A

-transverse waves that oscillate at different angles relative to the direction of propagation

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19
Q

what is a polariser and what does it do?

A

-an optical filter which removes any oscillations that are at 90° to the filter

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20
Q

why can longitudinal waves not be polarised?

A

-their particles oscillate parallel to the direction of propagation, so there is only one oscillation direction

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21
Q

if unpolarised light passes through a horizontal polariser, will the light be polarised vertically or horizontally and why?

A

-horizontally
-the polariser removes the oscillations that are at 90° to the filter

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22
Q

what is Giga ( G ) in standard form?

A

-x10^9

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23
Q

what is Mega ( M ) in standard form?

A

-x10^6

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24
Q

what is kilo ( k ) in standard form?

A

-x10^3

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25
Q

what is milli ( m ) in standard form?

A

-x10^-3

26
Q

what is micro ( µ ) in standard form?

A

-x10^-6

27
Q

what is nano ( n ) in standard form?

A

-x10^-9

28
Q

what is meant by phase difference?

A

-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

29
Q

what is a wave the same as?

A

-a circle
-this means that 1 wavelength = 1 circle, and 1/2 λ = 1/2 circle etc.

30
Q

how many degrees is one wavelength?

A

-360°
-this means that each quarter ( each middle to peak / trough and vice versa ) is 90°

31
Q

what is the radian ( rad )?

A

-a unit of angle which is more accurate than degrees

32
Q

how are degrees converted to radians?

A

-rad = number of quarters x π / 2
-e.g. 180° = 2π / 2 rad

33
Q

how is the phase difference between two waves calculated?

A

-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

34
Q

what is meant by a constant / fixed phase relationship?

A

-the phase difference between the waves never changes

35
Q

what is meant by coherent waves?

A

-waves with the same frequency and a constant phase difference

36
Q

what is diffraction?

A

-the spreading of waves as they pass through a gap or go past an obstacle

37
Q

what are wavefronts?

A

-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

38
Q

how does gap size affect diffraction?

A

-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 )

39
Q

can wavefronts pass through a gap that is much smaller than their wavelength?

A

-no, they can’t fit through

40
Q

what is superposition?

A

-when two or more waves occupy the same space ( which is possible because they aren’t charged )

41
Q

what are some examples of interference / superposition?

A

-large and small water ripples
-light and dark fringes ( visible light )
-loud and soft ( sound )
-good and poor reception ( radio / phone )

42
Q

when are waves in phase?

A

-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

43
Q

when are waves in antiphase?

A

-when their phase difference is π rad / 180° ( when they are half a wavelength apart )
-they are aligned peak to trough

44
Q

what is constructive interference?

A

-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

45
Q

what is destructive interference?

A

-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

46
Q

what is meant by path difference?

A

-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

47
Q

what is a diffraction grating?

A

-a slide that has a large number of parallel, closely spaced slits
-the light passing through each slit is diffracted

48
Q

how are bright and dark interference fringes produced by superposition?

A

-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

49
Q

how does the wavelength of light affect diffraction?

A

-the larger the wavelength, the larger the angle at which the light spreads out / is diffracted

50
Q

what happens when white light is diffracted?

A

-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

51
Q

what is the order of diffraction?

A

-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

52
Q

what is fibre optics?

A

-a technology that uses thin, flexible cylinders of glass or plastic to transmit data as pulses of light over long distances

53
Q

what is the formula for refractive index?

A

-n = c / v = sin i / sin r

54
Q

what is total internal reflection?

A

-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

55
Q

what happens when the angle of incidence is equal to the critical angle at a glass-air boundary?

A

-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

56
Q

how does light travel through an optical fibre?

A

-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

57
Q

what is the purpose of the structure of an optical fibre?

A

-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 )

58
Q

what are the applications of fibre optics?

A

-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

59
Q

what are the two types of fibre optic cables?

A

-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

60
Q

what are the advantages of using fibre optics in communication?

A

-provide greater bandwidth which allows them to carry a higher data rate than traditional copper cables due to their higher frequencies
-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
-resistent to EM interference which improves the quality of the transmitted signals

61
Q

compare the transmission of signals using optical fibres and satellites

A

-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

Applied Science (3 decks)

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