imaging + signalling Flashcards
how is most information transferred?
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by waves
what are some examples of information being transferred by waves?
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some examples are:
• scientific imaging
• remote sensing (infrared)
• heat cameras (infrared)
• communications
• data streaming
where does the wave transfer energy to?
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wave transfers energy away from its source
what does a progressive wave do?
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a progressive (moving) wave carries energy (+ usually information) from one place to another without transferring material
how can you tell if a wave is carrying energy?
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ways to tell if a wave is carrying energy:
• em waves can cause things to heat up
• x-rays + gamma rays knock electrons out of their orbits, causing ionisation
• loud sounds make things vibrate
• wave power can be used to generate electricity
what happens when waves carry energy away from their source?
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since waves carry energy away from their source, source of wave loses energy
what parts of a wave do you need to know?
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you have to know these parts of wave:
• displacement, x
• amplitude, A
• wavelength, λ
• time period, T
• frequency, f
• phase difference
what is definition + units for displacement, x
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displacement is how far point on wave has moved from equilibrium/resting point
units: metres, m
what is definition + units for amplitude, A
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amplitude is maximum displacement of wave from its equilibrium/resting position
units: metres,m
what is definition + units for wavelength, λ
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wavelength is distance from one point on wave to equivalent point on next consecutive wave
units: metres, m
what is the definition + units for time period, T
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time period is the time taken for one whole oscillation (of a wave)
units: seconds, s
what is the definition + units for frequency, f
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frequency is the number of whole oscillations passing a given point per second
units: hertz, Hz or per second, s^-1
what is the definition + units for phase difference
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phase difference is the amount by which one wave lags behind another wave
units: degrees or radians
what is the relationship between the frequency + time period?
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frequency is inverse of time period
f = 1/T
what is equation for frequency?
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f = 1/T
therefore 1 Hz = 1 s^-1
what equation links wave speed, frequency + wavelength
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the wave equation
what is the wave equation?
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wave speed = frequency x wavelength
v = f x λ
what is a feature of all electromagnetic waves?
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all electromagnetic waves are transverse waves
what is a transverse wave?
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a transverse wave is a wave where the direction of oscillation is perpendicular to the wave’s direction of travel
what are the two main ways of drawing traverse waves?
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the two main ways of drawing transverse waves are:
1) displacement against time graphs
2) displacement against distance graphs
what are the features of a displacement against distance graph for a transverse wave?
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what are the features of a displacement against time graph for a transverse wave?
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why should you be careful when looking at displacement-distance and displacement-time graphs for transverse waves?
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both displacement-distance and displacement-time graphs give the same shape, so check label on x-axis to figure out which one it is
are all waves transverse?
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NO
eg) sound is a longitudinal wave
what are longitudinal waves?
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longitudinal waves are waves where the oscillations are parallel to the direction of the waves travel
what are the definition + units for intensity?
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intensity is the rate of flow of energy per unit area perpendicular to the direction of travel of the wave
units: Watts per square metre, W m^-2
what is the equation for intensity?
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intensity = power/area
I = P/A
what would intensity be a good measure for in real life?
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the amount of light energy that hits your retina per second
what is a simple definition for intensity?
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intensity is the measure of how much energy a wave is carrying
when does refraction happen?
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refraction happens when a wave changes speed at a boundary between two mediums
what exactly happens to a wave’s/ray’s energy when it meets a boundary between two mediums?
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when a wave/ray meets a boundary between two mediums, some of it’s energy is reflected back into the first medium and the rest of the energy is transmitted through into the second medium
what happens when light meets a boundary between two mediums at an angle to the normal?
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when light meets the boundary at an angle, the TRANSMITTED light is bent - “refracted” - as it travels at a different speed in each medium
what happens if light is transmitted through a very optically dense material?
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• the more optically dense a material is, the more slowly light travels in it
• so if light travels through a very optically dense material, light will slow down A LOT
what does the amount of refraction depend on and what does this mean?
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• the amount of refraction depends on the λ of the light
• this means that the focal length for a given lens will change depending on the λ of light passing through it
what does a converging lens do?
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a converging lens changes the curvature of wavefronts
how do lenses change the curvature of wavefronts?
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lenses change the curvature of wavefronts by adding curvature to waves via refraction as they pass through the lens
(converging lens adds positive curvature, convex lens adds negative curvature)
what happens to waves that are initially uncurved (+ are also parallel to the lens axis) as they pass through a converging lens?
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if a wave has zero curvature before passing through a converging lens (and the wavefronts are also parallel to the lens axis), the wave will be given a spherical curvature, centred on the focal point of the lens
how does a converging lens curve the wavefronts passing through it?
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a converging lens curves the wavefronts by slowing down the light travelling through the middle of the lens more than the light travelling through the lens’ edges
how long does it take for points on a wave to reach the focus point of a lens?
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all points on a wave take the same amount of time to get to the focus point of the lens
what is the focal length of a lens?
the focal length is the distance between the lens [axis] and the focus of the lens
what are the different ways of showing lens diagrams?
you can show lens diagrams in the following two ways:
• lens diagram using wavefronts
or
• lens diagram using rays (aka ray diagram)
example of lens diagram using wavefronts:
example of ray diagram
what happens as the power of the lens increases?
the more powerful (thicker) the lens, the more it will curve wavefronts that travel through it, so the shorter the lens’ focal length
what is the focus [aka focal point] of a converging lens?
the focus of a converging lens is the point where all the wavefronts that have passed through the lens converge
what is the equation for lens power?
lens power = 1/focal length
P = 1/f
when working out the power of a lens, the focal length HAS to be in METRES
what is the power of a lens + the units for it?
the power of a lens is the amount of curvature added by the lens to a wave passing through the lens
units: dioptres
what is the equation for the curvature of a wave?
curvature of a wave = 1/radius of wave’s curvature
curvature = 1/r
what is the relationship between the curvature added to a wave by a lens and the thickness of the lens?
the thicker the lens, the more curved its sides and so the more curvature the lens adds to the wave
what can you use the lens equation for?
you can use the lens equation to find where an image will be formed
what does the lens equation link together?
the distances between the lens, the image and the source of light
what is the lens equation?
the lens equation:
1/v = 1/u + 1/f
where:
* v = image distance (the distance between a lens and the image)
* u = object distance (the distance between a lens and the object (object == “light source”)
* f = focal length (the distance between a lens and it’s focal point)
showing the lens equation using a ray diagram:
what does the lens equation assume?
the lens equation assumes that the lens is thin
what do you have to remember when measuring distances for the lens equation?
- you always measure distances from the lens axis (from the lens basically) and count distances to the right of the lens axis as positive and distances to the left of the lens axis as negative - just like the negative and positive x-axis when you draw graphs
- so due to cartesian graphs, u is ALWAYS measured as NEGATIVE and must be used as NEGATIVE in calculations
what does the lens equation tell you about the curvature of the wave passing through it?
1/v = 1/u + 1/f
is the same as
curvature of wavefront after lens = curvature before lens + curvature added by lens
what do the waves coming towards a lens from a “distant” [i.e. almost infinite] light source look like?
- if you have a distant light source, the wavefronts approaching the lens will be [almost] flat/parallel
- this is bc when u = ∞ (signifying very distant light source), 1/u (curvature before lens) = 1/∞
- 1/∞ approaches 0, so the wave has [basically] zero curvature
what are flat wavefronts called?
flat wavefronts are called plane wavefronts
what curvature does a converging lens give plane wavefronts
a converging lens gives plane wavefronts a curvature of exactly 1/f
what happens to wavefront passing through the lens, when the source is at the focus of the lens?
⋅ when the source is at the focus of the lens (there is a focus to both left + right side of the lens obvs bc you can shine light from both directions), wavefronts will start off with negative curvature (bc u is measured as negative)
⋅ this negative curvature is then cancelled out by positive curvature added by converging lens
⋅ so wave fronts will be made flat
what happens to wavefronts passing through a converging lens, where the source is between the focus and the lens?
⋅ waves coming from a source between the focus and the lens will have curved wavefronts before and after they pass through the lens
⋅ the difference between curvature before and after is 1/f
what can you use instead of wavefronts to demonstrate the lens equation?
⋅ you can use ray diagrams bc the lens equation work for both models
⋅ the light rays are just “bent” by the lens instead of being curved like with wavefronts
⋅ [INSERT RAY DIAGRAM EG]
EG of using the lens equation
what can lenses also do, aside from forming images?
a lens can produce a magnified image
what is the equation for linear magnification?
linear magnification = image height/object height
m = v/u
how can you find the power of a converging lens? (simple)
you can find the power of a converging lens by focusing an image on a screen
what is the method to find the power of a converging lens?
1) set up equipment as shown, connecting bulb to low-voltage power supply
2) place bulb exactly 0.200 m away from lens (i.e. u = 0.200 m) + turn on power supply. move screen until you can see clear picture of filament on screen
3) measure DISTANCE between LENS + SCREEN - this is your value for v. record u and v in table, as shown
4) repeat experiment 5 or more times, for range of different values of u (however, don’t increase u so much that you can no longer see image of filament though)
5) WORK OUT 1/u and 1/v for each of your readings. ADD them together to get 1/f, which is POWER of lens
6) find AVERAGE of your values for 1/f + DIVIDE 1 by your answer to find FOCAL LENGTH of your lens
practice Qs
what is a bit?
a bit is a single binary digit
how many bits is one byte?
8 bits = 1 byte
what is the binary system used for in computers?
⋅ the binary system is used to store data in the computer memory
⋅ computers store files as a string of bits
what is the number of alternatives that a string can represent determined by?
the number of bits (b) in a string determines how many alternatives that string can represent
what is the equation for the number of alternatives?
number of alternatives = 2^(number of bits)
N = 2^b
EG) 1 bit only has 2 alternatives (0 and 1), while 1 byte (8 bits) has 256 alternatives - number of alternatives double with each additional bit - further explanation: bc for 1 bit, 2^1 = 2 alternatives bc the bit can only be either 1 or 0 - whereas for 1 byte (8 bits), 2^8 = 256 alternatives
what does the number of bits you need depend on?
the number of bits you need depends on how many alternatives you want
what is the equation for the number of bits you need?
what are images stored as?
images are stored as arrays of binary number
what is a pixel?
a pixel is the smallest single digital unit that can make up an image on a digital screen
what happens when an image is stored?
⋅ when an image is stored (eg, on a computer or memory card), each pixel is represented by a binary number
⋅ the binary numbers are then stored in an array
⋅ the image is then stored as arrays of binary numbers
why is the location of each binary number in an array important?
⋅ binary numbers are stored in an array
⋅ this array is a grid of numbers where the location of each number in the grid also matches the location of a pixel in a photo
(not definition for array, just how it’s used to store photos)
why is the value of the binary number - in an array used to store an image - important?
the value of the binary number maps to the colour of the corresponding pixel
in coloured images, how many binary numbers can each pixel be described by?
three binary numbers, 1 for each of the primary colours (R, G + B)
what does the length of a binary number used to store an image depend on?
the length of the binary number depends on how many colours are needed
EG) on typical PC display, each of numbers for red, green, + blue are 8 bits (1 byte) long, giving 256^3 = 16.8 million possible colours
can image resolution mean different things, and if so, what?
yes, image resolution can mean different things:
1) image resolution refers to how many bits (length of binary number) are represented in each pixel
(eg, if object of 1.0 m is represented by 200 pixels in image, resolution is 1/200 = 0.005 metres per pixel)
OR
2) resolution of an image can also mean the number of pixels in an image, in the format width x height (eg, 1920 x 1080) -> this type of resolution can also be given as just the total number of pixels in image without formating (eg, 1920 x 1080 = 2 073 600 pixels, so resolution is 2.1 megapixels)
what does the amount of information in an image depend on?
⋅ the amount of information in an image depends on the number of bits per pixel
⋅ the more bits per pixel, the more information that is held by each pixel
⋅ so therefore, the more pixels there are in an image, the more information is held by an image also
what is the equation for the total amount of information in an image?
total amount of information in an image = number of pixels x bits per pixel
what does multiplying the values of binary numbers making up an image do?
multiplying the values of the binary numbers by number larger than 1 increases the contrast (by stretching the range of the values of the pixels)
what can adding false colour to image do?
⋅ adding false colour can highlight important features by making them very bright colours
⋅ adding false colours is done by changing what colour the values of the binary numbers map to
⋅ EG) thermal imaging cameras show heat in colour instead of visible light
how can you reduce noise in an image?
⋅ you can reduce/get rid of the noise in an image by replacing pixels with the median value of themselves and the 8 pixels surrounding it pixels
⋅ the result is that any ‘odd’ (i.e. very high or very low) values are removed and the image is made smoother
how do you detect edges in an image?
⋅ you use the ‘Laplace rule’:
1) multiply the value of the pixel by four and then subtract the values of the pixels immediately above, below, to the left and to the right of the pixel
2) if the answer is negative, the pixel is treated as if its value is zero (and so the value of the pixel maps to black)
⋅ the result is that any pixel not on an edge goes white - so you’re just left with the edges
practice Qs
what is a feature of polarised waves?
polarised waves only oscillate in one direction
can EM radiation be polarised?
yes
what waves make up EM radiation?
EM radiation is made up of two transverse waves (the electric and magnetic fields) oscillating perpendicular to each other
what happens after most types of light pass through a polarising filter?
they only oscillate in one direction
what happens if light tries to pass through two polarising filters that are perpendicular to each other?
if light tries to pass through two polarising filters that are perpendicular to each other, no light will get through as all directions of oscillation have been blocked
what type of waves can be polarised?
only transverse waves can be polarised, you cannot polarise longitudinal waves like sound
(the fact that you can polarise visible light is an indication that it is a transverse wave)
how can you investigate the polarisation of light?
you can observe the polarisation of light by shining unpolarised light through 2 polarising filters
method for investigating the polarisation of visible light
1) align transmission axes of 2 polarising filters so they are both vertical. shine unpolarised light on first filter. keep position of first filter fixed + rotate second one
2) light that passes through first filter will always be vertically polarised
3) when transmission axes of 2 filters are aligned, all of light that passes through first filter passes through second
4) as you rotate second filter, amount of light that passes through second filter varies
(just like vectors, you can think of transmission axis of rotating filter as having vertical + horizontal component. larger the vertical component, the more vertically polarised light will pass through filter)
5) as second filter is rotated, less light will get through it as vertical component of second filter’s transmission axis decreases. this means intensity of light getting through second filter will gradually decrease
6) when two transmission axes are 45 degrees to each other, intensity will be half that getting through first filter. when they’re perpendicular to each other, no light will pass through - intensity is 0
7) as you continue turning, intensity should then begin to increase once again
8) when two axes realign (after 180 degree rotation), all light will be able to pass through second filter again
what are some examples of uses for polarising filters?
⋅ 3D films use polarised light to create depth - filters in each lens are perpendicular to each other so each eye gets a slightly different picture
⋅ polaroid sunglasses also use polarising filters - reflected light is partially polarised so polaroid sunglasses block this out to help prevent glare
why can’t you polarise microwaves with polarising filters + what do you use instead?
you cannot use polarising filters to polarise microwaves bc their λ is too long, so instead you use a metal grille
what bare minimum equipment do you need to investigate the polarisation of microwaves?
you can investigate the polarisation of microwaves using a metal grille, a microwave transmitter and a microwave receiver connected to a voltmeter
what is the method for investigating the polarisation of microwaves?
1) place metal GRILLE between microwave TRANSMITTER + microwave RECEIVER, as shown below
2) intensity of microwaves passing through grille is at MAXIMUM when direction of oscillation of microwaves + wires on grille are PERPENDICULAR to each other
3) as you rotate grille, INTENSITY of polarised microwaves able to pass through metal grille DECREASES, so reading on voltmeter DECREASES
4) when wires of metal grille are ALIGNED with direction of oscillation of polarised microwaves, NO SIGNAL will be shown on voltmeter
why does the intensity shown on the voltmeter drop to zero when the wires of the metal grille are aligned with the direction of oscillation of the microwaves? (simple explanation)
the intensity shown on the voltmeter drops to zero when the wires of the metal grille are aligned with the direction of oscillation of the microwaves bc the electrons in the wires of the grille are absorbing the microwave’s energy
why do you only need 1 metal grille when investigating the polarisation of microwaves?
microwave transmitters already transmit polarised microwaves, so you only need 1 grille
why does the intensity shown on the voltmeter drop to zero when the wires of the metal grille are aligned with the direction of oscillation of the microwaves? (longer explanation)
1) vibrating electric field of microwave EXCITES electrons in metal grille
2) energy of incoming microwave is ABSORBED by electrons in grille + RE-EMITTED in ALL DIRECTIONS
3) only few of those re-emitted waves are vibrating in DIRECTION of microwave receiver
4) microwave RECIEVER only receives microwaves in ONE PLANE, so even if RE-EMITTED wave travels towards receiver, it might not be picked up
5) when wires and oscillations are ALIGNED, MORE electrons are excited than when they’re perpendicular to each other - all energy is absorbed + INTENSITY reading drops to ZERO
6) when wires + oscillations are PERPENDICULAR to each other, some electrons in grille are still excited + so there is still SMALL DROP in INTENSITY
how do analogue signals vary?
analogue signals vary continuously and are not limited in the values they can take
what are digital signals represented by?
digital signals are represented by binary numbers
can digital signals vary continuously?
no
are digital signals limited in the values that they can take?
yes, the values that a digital signal can take depends on the number of bits used for the signal
are digital signals more resistant to the effects of noise than analogue signals?
yes
why is it good that digital signals have a high resistance to noise?
⋅ when you transmit an electronic signal, it will pick up noise (interference) from electrical disturbances or other signals
⋅ so the receiver needs to be able to reconstruct the original signal from the noisy signal if they’re to get an accurate representation of what was sent
⋅ this is much easier with digital signals than analogue signals bc the number of values a digital signal can take is limited
can analogue signals be digitised, and if so, what is the process called?
yes, analogue signals can be digitsed and the process is called digitising
digitising an analogue signal means converting an analogue signal into a digital signal
what is the process for digitising an analogue signal?
1) take VALUE of analogue signal at REGULAR TIME INTERVALS, then find NEAREST DIGITAL VALUE
2) each DIGITAL VALUE is represented by BINARY NUMBER, so you can CONVERT ANALOGUE values to BINARY numbers
what is an inevitable downside to digitising?
the DIGITAL SIGNAL you end up with won’t be EXACTLY the same as the original ANALOGUE SIGNAL, bc some information is always lost (but it’s usually quite CLOSE!)
what two factors do the quality of a digitised signal and how quickly a digitised signal matches the original analogue signal depend on? (simple answer)
1) the resolution (the number of possible quantisation levels)
2) the sampling frequency
what two factors do the quality of a digitised signal and how quickly a digitised signal matches the original analogue signal depend on? (longer answer)
1) the DIFFERENCE between possible DIGITAL VALUES [aka quantisation levels] (this is just RESOLUTION)
⋅ bc if signal is digitised using only FEW, WIDELY SPACED digital values, it’s likely that lot of analogue signal will be FAR from NEAREST DIGITAL VALUE
⋅ but, if LARGE number of CLOSELY SPACED digital values are used, most of analogue values will be VERY CLOSE to digital value, so will only change SLIGHTLY
⋅ this means that HIGHER THE RESOLUTION (i.e. MORE POSSIBLE DIGITAL VALUES there are), MORE CLOSELY the digital signal will MATCH original
2) the TIME from one SAMPLE to the next (SAMPLING FREQUENCY aka sampling rate)
what is resolution determined by in digital signals?
the RESOLUTION in digital signals is determined by the NUMBER OF BITS in BINARY NUMBERS representing digital values - so the GREATER the number of BITS in the binary numbers, the GREATER the RESOLUTION
how can you lower the rate of transmission used to send information in real time?
you can lower the rate of transmission used to send information in real-time by using a lower resolution and a lower sampling frequency
what are some of the advantages of digital signals over analogue signals?
⋅ digital signals can often be sent, received and reproduced more easily than analogue signals bc they can only take a limited number of values
⋅ digital files can be compressed to reduce their size + manipulated easily for artistic effect
⋅ digital signals are more resistant to the effect of noise than analogue signals
⋅ digital signals can be used to represent different kinds of information in the same way (eg, images + sounds can both be represented as a string of bits)
⋅ computers can be used to easily process digital signals since they are digital devices too
what are some of the disadvantages of digital signals?
⋅ digital signals can never reproduce analogue signals exactly - some information will always be lost
⋅ bc digital signals can be copied more easily, digital information can then easily be reproduced illegally an unlimited number of times (so confidential information may be stolen + copied without the owner’s knowledge or consent more easily)
are signals always only made up of one frequency?
⋅ no, a signal can be made up of lots of different frequencies
⋅ in fact, most signals are made up of several waves (not just one), all with different frequencies, added together
what happens to amplitude when waves are added together?
when waves are added together, the amplitude of the final signal is the sum of the amplitudes of the individual waves at each point in time
what is the fundamental frequency?
the fundamental frequency is the lowest frequency wave that can make up a sound wave
how do you calculate the fundamental frequency of a sound wave?
you can calculate the fundamental frequency of a sound wave by finding the shortest repeating part of that sound wave and calculating the inverse of its period
what does noise limit when digitising signals?
noise limits the number of bits that can be used when digitising signals
what does a high resolution mean when digitising signals?
the higher the resolution used to sample a signal, the better a digitised signal matches the original
when is a high resolution disadvantageous instead?
if the original signal contains noise, then a really fine resolution will reproduce the variation caused by the noise to higher degree of detail also (which is not useful)
what is resolution limited by when digitising signals?
resolution is limited by the ratio of the total variation in the signal to the variation caused by noise
what is the equation for finding out the maximum number of [useful] bits?
maximum number of [useful] bits = log[2] (total variation/noise signal)
b = log[2] (Vtotal/Vnoise)
what does Nyquist’s rule state?
when you digitise a signal, the minimum sampling frequency must be twice the maximum frequency in the original signal
why must the sampling frequency be twice the highest frequency in the original signal, according to Nyquist’s rule?
the sampling frequency must be twice the maximum frequency of the og signal so that the sampling frequency is high enough to record all the high-frequency detail of the og signal
what can happen if the sampling frequency is too low, when digitising a signal?
⋅ detail can be lost if the sampling rate is too low
⋅ a low sampling frequency can create false low-frequency signals (called aliases) that weren’t in the original signal at all
what is the equation to find the rate of transmission?
rate of transmission = samples per second x bits per sample
OR
rate of transmission = sampling frequency x bits per sample
units: bits per second
what two factors does rate of transmission depend on?
rate of transmission depends on these two factors:
1) sampling frequency - sampling frequency must be at least twice the highest frequency in the original signal to ensure that all frequencies within the spectrum are transmitted correctly
2) the number of bits per sample - the number of bits per sample must be high enough that the transmitted signal closely matches the original signal, but not so high that signal is negatively affected by noise
what is the equation to find the time taken to transmit a signal?
time taken to transmit a signal = number of bits to transmit / rate of transmission
