Imaging and signalling Flashcards
How is information transferred?
Waves
How does medical scanning use waves?
Ultrasounds scans build up an image of a foetus by detecting reflected ultrasound waves
What is a progressive wave?
A wave that carries energy (and usually information) from one place to another without transferring any material
How can you tell a wave carries energy?
EM waves cause things to heat up
X-rays and gamma rays knock electrons out of their orbits causing ionisation
Sound waves make things vibrate
What happens to the source of waves?
Loses energy as the waves are carrying energy
What is the displacement of a wave?
How far a point on the wave has moved from its undisturbed position
What is the amplitude of a wave?
The max. displacement of a wave from its undisturbed position
What is wavelength?
The length of a whole wave (distance from one point on a wave to its identical point on the adjacent wave)
What is the period of a wave?
The time taken for a whole vibration
What is frequency?
The no. of waves passing a given point per second
What is phase difference and what is it measured in?
The amount by which one wave lags behind another wave
degrees or radians
What is the wave equation?
v=fλ
What are electromagnetic waves?
Transverse waves which consist of two perpendicular planes, a magnetic and an electric field
What is a transverse wave?
A wave where the vibration is at right angles to the wave’s direction of travel
What is a longitudinal wave?
A wave where the vibrations are along the wave’s direction of travel
What is intensity?
The rate of flow of energy per unit area at right angles to the direction of travel of the wave
(measure of how much energy a wave is carrying)
Intensity equation (waves) and unit?
intensity = power/area
Wm-2
What is a polarised wave?
A wave that only oscillates in one direction
What do all electromagnetic waves have in common?
Transverse waves
Can be polarised
Travel at the speed of light in a vacuum
What is a polarising filter?
Something that only transmits waves oscillating in one direction
What happens when you shine light through two polarising filters at right angles?
No light will be transmitted as all directions of oscillation will be blocked
How do you know if a wave is transverse?
If it can be polarised
What happens when two polarising filters are at 45°?
Intensity getting through the second filter will be exactly half of that transmitted through the first
What are some examples of polarising filters?
3D filed use polarised light to create depth, the filters in each lens are at right angles to each other so each eye gets a slightly different picture
polaroid sunglasses use polarising filter to help prevent glare
Why don’t polarising filters work on microwaves and what do we used instead?
wavelengths too long so use a metal grille
Why do you only need one metal grille when investigating polarisation of microwaves?
Microwave transmitters transmit polarised microwaves
What equipment do you need to investigate polarisation of microwaves?
You need a metal grille between a microwave transmitter and a microwave receiver which is connected to a voltmeter
When is the intensity of microwaves passing through a metal grille at a maximum?
When the direction of the vibration of the microwaves and the wires on the grille are at right angles
What happens when the wires of the metal grille are aligned with the direction of polarised microwaves?
No signal will be shown on the voltmeter
Why is no signal detected by the microwave receiver when the metal grille wires are aligned with the direction of polarisation?
Because the grille is absorbing the energy
The vibrating electric field of the microwave excites electrons in the metal grille so the grille is absorbing the energy and then re-emitting it in all directions, only a few of those re-emitted waves are vibrating in the same direction of the microwave receiver
Why is there still a drop in intensity when the wires of the metal grille are at right angles to the oscillations of the microwaves?
Some electrons in the grille are still excited so there is still a small drop
Why doesn’t the microwave receiver pick up the re-emitted microwaves?
Only receives microwaves in one plane
What is refraction?
When a wave changes speed at a boundary
Why does a wave change speed at a boundary?
Some of its energy is reflected back into the first medium and the rest is transmitted through into the second medium
How does density of a material affect speed?
The more optically dense a material is the more slowly light travels in it
What does the amount of refraction depend on?
The wavelength of light
So the focal length for a given lens will change depending on the wavelength
What happens when light travels from a less dense material to a more dense material?
Slows down so bends towards the normal
What do converging lenses do and how do they do it?
Change the curvature of wavefronts by refraction
What does a lens do to a wave as it passes through it?
Adds curvature
How does a converging lens curve the wavefronts?
It slows down the light travelling through the middle of the lens more than the light at the edges of the lens - all points of the wavefront take the same amount of time to get to the focus point
What is the focal length?
Distance between the lens axis and the focus
What does a more powerful lens means?
It is thicker so will curve the wavefronts travelling through it more, meaning it has a shorter focal length
How do you find the power of a lens and units?
1/f
dioptres
How do you find the curvature of a wave?
curvature = 1/radius of curvature
What is the lens equation?
1/v = 1/u + 1/f
curvature after = curvature before + curvature added by lens
where u is negative as objects displacement from the lens
How do you calculate linear magnification?
image height/object height
v/u
What is a bit?
A single binary digit
How many digits does the binary system use?
2 (0 and 1)
How many bits in a byte?
8
How is the binary system used?
To store data in computer memory
How do you find the number of alternatives?
N=2^b
b is the no. of bits
How do you find the no. bits given the no. of alternatives?
b=log2 N
always round up to the nearest bit
How are images stored?
As arrays of binary numbers
In coloured images how can each pixel be described?
three binary numbers (one for each primary colour of light)
Typically each no. has 8 bits
What is image resolution?
The length represented by each pixel
The no pixels in the formate widthxheight
Total no. pixels, sometimes given in megapixels
What does the amount of information in an image depend on?
no. pixels and no. bits per pixel
How do you calculate total amount of information?
no. pixels x bits per pixel
How do you improve the contrast of an image?
multiply each binary no. by a fixed value
How do you make an image lighter?
Add a fixed value to each binary value
makes the photo lighter but won’t increase contrast
What does adding false colour to an image do?
Highlights features
How do you reduce the noise of an image and what does it do?
Replace the pixels with the median of itself and its neighbouring 8 pixels
Any ‘odd’ values are removed and the image is made smoother
What is noise?
Unwanted interference affecting a signal
What does edge detection tell you?
If there is something in your image
How do you carry out edge detection?
Multiply the value of a pixel by four and then subtract the value of the pixels immediately above, below, to the left and the right.
If the answer is negative, the pixel is treated as if its 0
This means any pixel not on an edge becomes 0
What must there be in order for an edge to be detected?
A steep change in brightness
What are digital signals?
Signals represented by binary numbers so can only take a fixed no. of values
What are analogue signals? give an example
Signals that are not limited in the values they can take.
Speech, sound waves produced vary continuously
What is more noise resistant, digital or analogue, and why?
Digital is noise resistance because the no. values a digital signal can take is limited
Why does an electronic signal pick up noise?
Electrical disturbances or other signals
What is digitising a signal?
Turning an analogue signal into a digital one
How do you digitise a signal?
Take the value of a signal at regular time intervals and then find the nearest digital value
What does the quality of a digitised signal depend on?
Its resolution (difference between the possible digital values)
sampling rate
What is the correlation between resolution and the quality of a digitised signal?
Higher the resolution the more accurate the digitised signal
What uses high resolution when digitising and what uses a low resolution?
When music is digitised to CDs high resolution is used
Low resolution is used in telephone lines as top quality audio isn’t essential, needs to be audible but doesn’t need an accurate reproduction of the callers voice
What are the advantages of digital signals?
Sent, received and reproduced more easily as they can only take a limited no. of values
Noise resistant
They can be compressed to reduce their size and manipulated
A digital signal can be used to represent different kinds of information in the same way, for example images and sounds can both be represented as a string of bits
What are the disadvantages of digital signals?
Can never reproduce analogue signals exactly as some information will always be lost
More easily copied meaning digital information can like films and music can be reproduced illegally, unlimited times
Confidential information such as persona data and photographs may be stolen and copied without the owners knowledge or consent
What are signals made up of?
Several waves with different frequencies added together
What is the amplitude of the final signal?
The sum of the amplitudes of the individual waves at each point in time
What is the fundamental frequency and how you spot it?
The lowest frequency wave that makes up a sound wave
You can spot it by finding the shortest repeating part of the sound wave and calculating the inverse of its period
What is the problem with using high resolution to sample?
If the original sample contains noise then a really high resolution will reproduce all the little wiggles caused by noise
What limits the no. bits used for sampling?
Noise
How do you calculate the maximum number of bits when digitising a sample?
b = log2 (total variation / noise variation)
What is the minimum sampling rate?
Twice the maximum frequency
What happens when the sampling rate is too low?
Detail can be lost
Can also create low frequency signals, aliases, that weren’t in the original signal at all
Why does music need a high sampling rate?
Needs to be able to accurately reproduce the original sounds
What is the sampling rate of CDs and why?
44100 Hz because the max frequency is about 20000Hz
What is the sampling rate of video DVDs, digital TV broadcast and Blu-ray Discs?
DVDs and TV 48000Hz
Blue-ray 192000Hz
How do you calculate the rate of transmission?
samples per second x bits per second
What factors affect rate transmission?
no. samples per second
no. bits per sample
How do you calculate the time taken to transmit a signal?
no. bits / rate of transmission
How do you speed up the time taken to transmit a signal?
Compress the information you’re sending
What are the advantages of analogue signals?
more detailed
What are the disadvantages of analogue signals?
Cannot be easily amplified
How do modern cameras store images?
Using CCDs (charged couple devices) which consists of a screen covered in pixels which store charge when light falls on them. The charge they store is proportional to the light incident upon it.
What is meant by the term ‘real image’?
A real image is one that can be projected onto a screen
What are plane wavefronts?
Wavefronts that are parallel to each other and dont appear to have any curvature
What is meant by the resolution of an image?
Smallest distance between which two points can be distinguished
What is the equation for resolution?
resolution = width of image/ no. pixels
Explain the difference between analogue and digital signals?
Analogue signals can continuously vary between values whereas digital signals can only take discrete values
What problem can arise when carrying out sampling?
Sampling can produce quantisation errors, which is where there is a difference between the actual level and the quantisation level
How do you calculate the maximum number of useful quantisation levels?
total noisy signal variation / noise variation
What is the equation linking quantisation levels used and the image resolution?
resolution = p.d range of signals / no. of quantisation levels
What is a pixel?
A picture element
What is a virtual image?
An image that appears to come from behind the lens
Describe how Global Positioning System (GPS) is used to locate the position of a car on the Earth’s surface
- (Several) satellites used
- Distance from (each) satellite is determined
- Position / distance is determined using c / speed
of e.m waves / radio waves / microwaves and delay time - Trilateration is used to locate the position of the car
Or position of car is where circles / spheres cross
Why does the sampling rate have to be twice the highest frequency?
There must be at least two samples per cycle to pick up any variation
