1-IMAGING Flashcards
What is a wave-front?
A line of disturbance moving through a material or space .
What does the word ‘reciprocal’ mean?
1 / set number
For a circle, how do you calculate the curvature of the circumference?
1 / Radius
If the radius of a circle is 0.5 metres, what is the curvature of the circumference?
2D
What is curvature measured in?
Dioptres
If the radius of a circle is 2m, what is the curvature of the circumference?
0.5D
What happens when light rays come from a distant source? (Curvature)
The rays are straight (not curved) and parallel to each other.
When light rays leave a small source, are the rays straight or curved?
Straight
What is a converging lens?
A lens that focuses light behind it
How do you calculate the power of the lens?
1 / Focal length
What is the focal length measured in?
metres
What is the power of the lens measured in?
Dioptres
What does a lens add to rays?
Curvature
What is the relationship between the power of the lens and the size of the focal length?
The smaller the focal length is, the more powerful the lens is
If the focal length is 0.1m, what is the power of the lens?
10D
If the focal length is 0.05m, what is the power of the lens?
20D
What does the brain do so you can see the world the right way up?
It flips the image
What is the curvature of waves leaving the lens =?
Curvature of waves entering the lens + Curvature added by the lens
What is the curvature added by the lens also called?
The power of the lens
What is 1/v?
The curvature of waves leaving the lens
What is 1/u?
The curvature of waves entering the lens
Why is 1/u negative?
Because u is a negative distance from the object to the lens.
What is 1/f?
The power of the lens
If the light rays come from a very distant object, what is the value of 1/u?
Virtually 0
If the object is at the focus, what happens?
Once the light rays pass through the lens they become parallel.
What is magnification?(1)
Image height / Object height
What is magnification?(2)
Image distance/ Object distance
What does ‘CCD’ stand for?
Charge-coupled device
What is a charge-coupled device?
A light sensitive microchip.
What are charge-coupled devices covered by?
Millions of tiny picture elements
What are tiny picture elements also called?
Pixels
When does each pixel store electric charge?
When light falls on it
What happens when there is a very bright light falling on the pixel?
There is a great charge stored in it.
After light falls on the pixels, what does the image become?
An array of numbers.
What can the array of numbers be used for?
Manipulating the image
What are the 2 values in binary system?
1 and 0
What 2 shades do pixels not record?
Dark and bright
How many bits is in 0010?
4
How mant bits is required to store 256 alternatives?
8 Bits
What is a byte?
A group of 8 bits
What is 5 in binary?
101
What is 16 in binary?
10000
How can you calulate the number of arrangements of bits?
N=2 to the power of b
What is b?
The number of bits available.
What does log squared x N =?
The number of bits
What is the resolution of an image?
The scale of the smallest detail that can be distinguished
What is the equation of resolution?
Width of object in image / Number of pixels across object.
What is the amount of information in an image?
Number of pixels x bits per pixel
What is the amount of information in an image recorded in?
Bits or Bytes
How many bits is 1 byte?
8
Calculate the storage required for a 1 megapixel camera that uses 3 bytes to encode colour information for each pixel:
3 Million Bytes or 24 Million bits
In an 8 bit pixel, what will its darkest value be represented by?
0
In an 8 bit pixel, what will its brightest value be represented by?
255
How do you increase the brightness of an 8 bit image?
By increasing the value on each pixel by the same amount until the brightest pixel in the image is coded at 255.
What is the ‘noise’ in an image?
The random speckles across an image.
What are the 2 ways that you can remove noise from an image?
Replacing the pixel with the median/mean of its value and those around it (usually 8 pixels around it)
How do you enhance the edges in an image?
Find the average value of the pixels neighbors and subtract it from each pixel.
What does enhancing the edges of an image do?
Removes uniform areas of brightness and picks out areas where the brightness changes abruptly.
What will an image with little contrast not use?
A full range of values (usually only between 75-150)
How do you change the contrast of an image?
The pixel range is stretched so 75 becomes 0 and 150 becomes 255.
What do some snow goggles and sunglasses have?
Polarizing lenses or filters
What do polarising lenses do that makes them useful in snow goggles?
They cut down the blare in bright environments.
What is speed measured in?
m/s
What is frequency measured in?
Hz
What is wavelength measured in?
m
What is frequency equal to?
1/time
If the time is 2 seconds, what is the frequency?
0.5Hz
If the time is 0.25s, what is the frequency?
4Hz
What is 1 oscillation?
1 complete wave
What is wave speed equal to?
Frequency x wavelength
What type of wave are all electromagnetic waves?
Transverse waves
Do electromagnetic waves travel at the same speed in an empty space?
Yes
What speed do EM waves travel at in an empty space?
3 X 10*8 m/s
Which EM wave has the highest wavelength?
Radio waves
What is the wavelength range of radio waves?
106 - 10-1 metres
What is the order of electromagnetic waves in reducing wavelength?
Radio, Microwaves, Infrared, VISIBLE, Ultraviolet, X-rays, Gamma rays.
What is the wavelength range of microwaves?
10-1 - 10-3 metres
What is the wavelength range of infrared waves?
10-3 - 7 x 10-7
What is the wavelength range of visible waves?
700nm - 400nm
What is polarisation a property of?
Transverse waves
When are transverse waves polarised?
If they vibrate in one plane only.
How do unpolarised waves vibrate?
In a randomly changing plane.
What is the relationship between the electric field and the magnetic field in electromagnetic fields?
They are at right angles to one another
Can some electromagnetic waves not be polarised?
No
If a wave is polarised, what happens to the direction of oscillation?
It remains fixed
When is unpolarised light polarised?
When is passes through a polarising filter
If vertically polarised light passes through a horizontal filter, what would happen?
No light would be transmitted
How can you detect whether light is polarised?
By observing it through a single polarising filter and rotating the filter.
What happens if the intensity of the light remains constant whilst you rotate the polarising filter?
The light source is emitting unpolarised light
What happens if the intensity of the light varies as the polarising filter rotates?
The source is emitting polarised light
What experiment can you do to observe the polarisation of light?
You can shine a narrow beam of light through a tank of water that contains a few droplets of milk
If you look at the slightly cloudy tank of water with a polarising lens, and rotate the filter slowly, what will happen?
The light observed will vary in intensity, showing that the light has been polarised
How can you observe the polarisation of microwaves?
Place a metal grille between a transmitter of polarised waves and a receiver.
What will happen if you rotate the metal grill in front of the transmitter of polarised waves?
The signal at the detector will change.
When viewing a source of light through two polarisers, rotating one of the polarisers through 90 degrees reduces the transmitted intensity to zero. Rotation through a further 90 degrees results in transmission of light again, why does this happen?
Light from the source is unpolarised. The first polariser transmits light with one polarisation, say vertical. If the second polariser is also aligned to transmit vertically polarised light, then light from the source will be visible through the two polarisers. Rotating either polariser through 90 degrees means that the light transmitted by the first polariser will not be transmitted by the second, so no light is seen.
Why does a microwave (or radio) receiver aerial needs to be aligned vertically in order to detect vertically polarised microwave radiation?
If the microwaves are, say, vertically polarised, then the aerial must also be aligned vertically so that the oscillating electric field of the microwaves can cause electrons to be accelerated up and down the aerial wire. This corresponds to absorption.
Polaroid lenses on ski goggles / sunglasses should be oriented to admit vertically polarised light to cut glare from snow / wet road reflections. Why?
Unpolarised light strikes snow / wet road. The light that is reflected will be horizontally polarised, as there cannot be any vertically polarised light reflected. If vertically polarised light was to be reflected, the electric field of the light would be partly oscillating in the same direction as that of wave travel, which is impossible for a transverse wave.
