1: Imaging Flashcards
Define Frequency
The number of whole vibrations per second passing a given point
Define Period
The time taken for a whole vibration
Define Phase Difference
The amount by which one wave lags behind another wave
What do lenses do?
They change the curvature of the incident wavefront
Explain how a converging lens curves wavefronts by changing the speed of the wave
It slows down the part of the wave travelling through the middle of the lens more than parts of wave at the lens edges. All points on a 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
lens power equation (D)
lens power D = 1 / f (focal length in metres)
curvature of wave / circle equation
1 / r (radius / distance in metres)
lensmaker equation words and fractions
curvature of waves leaving lens = curvature of waves before entering lens + curvature added by lens 1/v = 1/u + 1/f v = image distance (lens to image formed u = object distance (lens to source)
why is object distance (1/u) negative
lens to image formed is usually the positive direction so object distance is considered negative
Describe the wavefronts of a distant light source. What curvature will a converging lens give them
Flat. The converging lens will give them a curvature of 1/f
Describe the wavefronts if the source is at the focus of the lens
The wavefronts will start off curved w/ negative curvature . This -ve curvature is then cancelled out by the +ve curvature added by the converging lens - so the wavefronts will be made flat
linear magnification equations for height and distance
linear magnification =
image height (m) / object height (m)
image distance (m) / object distance (m)
What is a bit?
A single binary digit
What is a byte?
A group of 8 bits
what does a pixel do
stores electric charge when light falls on it, the brighter the light, the greater the charge stored on it.
equation for number of arrangements of bits(b) N
N = 2^b
how to calculate number of bits b from number of arrangements N
b = log2N
equation to calculate image resolution
resolution = width of image / number of pixels across object
equation to calculate amount of information in an image
number of pixels * bits per pixel
What does adding a fixed number to each value of a pixel do?
Increases the brightness (if the fixed number is +ve) - each pixel has a higher number therefore has a lighter colour
how do you increase the brightness of an image what is the effect of this
add a fixed number value to each pixel produces a lighter image
What does multiplying each value of a pixel, by a fixed number, do?
Increase/Improves the contrast (if the fixed number is greater than 1)
how do you increase the contrast of an image effect?
multiply the value of each pixel with a number greater than one, this will affect bigger numbers more than smaller numbers, increasing the contrast and so making the image more vivid
what is noise in the context of images
random unwanted speckles across the image
Describe how to remove noise on an image what is better, median or mean for this?
Replace each pixel with the mean or median of itself and the 8 pixels surrounding it. The result is that any ‘odd’ (very high/low values are removed and the image is made smoother) - you can also do this replacing each pixel by the mean of itself and surrounding pixels. This isn’t as good because the ‘odd’ value affects the new value
how do you carry out edge detection
subtract the average value of a pixel’s neighbours from the pixel, removing uniform areas of brightness, leaving the areas where the brightness changes abruptyl such as at an edge
equation for wave velocity how to calculate frequency
wave velocity = frequency * wavelength frequency = 1 / time period
Describe a longitudinal wave, give an example
The vibrations are along the wave’s direction of travel Sound
Describe a transverse wave
The vibration is at a right angles to the waves direction of travel
What is an electromagnetic wave made of
It is made up of 2 transverse waves, vibrating in different planes (directions) - electric and magnetic field.
property of electromagnetic waves
they can be polarised
What is the defining characteristic of a polarised wave? give an example
a transverse wave that oscillates (vibrates) in only one fixed direction
a transverse wave can be polarised
What is an indication that light is a transverse wave?
The fact it can be polarised - only transverse waves can be polarised
How do you investigate polarising microwaves?
1) Put a metal grille between the microwave transmitter and receiver
2) The intensity of the microwaves is at a maximum when the direction of the vibration of the microwaves and the wires are at right angles to each other
3) As you rotate the grille, the intensity decreases, so the reading on the voltmeter (connected to receiver) decreases
4) When the wires of the grille are aligned with the direction of the polarised waves, no signal will the shown on the voltmeter
Explain what happens when microwaves pass through a grille
1) The vibrating electric field of the microwave excites electrons in the metal grille
2) The energy of the incoming microwaves is absorbed by the grille and re-emitted in all directions
3) Only a few of the re-emitted waves are vibrating in the direction of the receiver
4) The receiver only receives waves in one plane, so even if the re-emitted wave travels toward the receiver, it might not be picked up
Why do you only need one filter when investigating the polarisation of microwaves?
Microwave transmitter transmits polarised waves so you only need 1 filter
Explain why how and why the intensity of the microwaves changes when the metal grille is rotated.
1) When wires and vibrations are aligned, more electrons are excited than when they’re at right angles to each other - all the energy is absorbed and the intensity reading drops to 0 2) When the wires and vibrations are at right angles, some electrons in the grille are still excited and so there is still a small drop in intensity
Why does the intensity drop to 0 when the wires are aligned to the direction of polarisation of the microwaves?
The grille is absorbing their energy
Why can you not polarise microwaves using a polarising filter? How do you polarise them?
Their wavelength is too long. Use metal grilles
State 2 examples of polarising filters
1) 3D films use polarised light to create depth 2) Polaroid sunglasses block out some light to reduce glare
What happens if you try to pass light through 2 polarising filters at right angles to each other?
No light will get though, all directions of vibration will be blocked
Describe how you would investigate the polarisation of light using 2 polarising filters
1) Align the transmission axes of 2 polarising filters so they are both vertical. Shine unpolarised light on the first filter. Keep the position of the first filter fixed and rotate the second one 2) Light that passes through the first filter will be vertically polarised 3) As you rotate the 2nd filter, the amount of light that passes through the second filter varies
approximate value for radiowave wavelength
106 to 10-1
approx value for microwave wavelength
10-1 to 10-3
approx value for infrared wavelength
10-3 to 7 * 10-7
approx value for visible light wavelength
7*10-7 to 4*10-7
approx value for uv light wavelength
4 * 10-7 to 10-8
approx value for x-ray wavelength
10-8 to 10-13
approx value for gamma ray wavelength
10-10 to 10-16
how to remember EM spectrum
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why is 1/u the negative of 1/v
when 1/v = 0, 1/f = -1/u + 0 -> 1/f = -1/u
when 1/u = 0, 1/f = 1/v + 0 -> 1/f = 1/v
how to find number of grains in a picture given length and width and spacing between grains
number of grains = area (length * width) / spacing²
Intensity of a wave equation
Intensity of a wave
I=P/A
I = intensity (W/m^2)
P = Power (W)
A = Area (m^2)
one way to calculate image size
image size = object size / distance from camera
