Unit 1: Section 1 Imaging and Sensing

A summary of chapters 1 and 4 from OCR Advancing Physics AS

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  • Created by: R_Hall
  • Created on: 05-04-13 12:54

The Nature of Waves

  • Waves are used in imaging (medical imaging, scientific imaging, seeing, heat cameras) and signalling (communications)
  • A progressive wave carries energy and information without transferring any material. Waves carry energy as they heat things, x-rays and gamma ionise, sound waves cause vibration, wave power generates electricity. As waves carry energy away, the source loses energy
  • Displacement, X (m)- how far a point on the wave has moved from an undisturbed position
  • Amplitude, a (m)- maximum displacement
  • Wavelengthλ (m)- length of one whole wave (crest-crest or trough-trough)
  • Period, T (s)- time for one whole vibration
  • Frequency, f (Hz)- number of vibrations per s, passing a point
  • Phase difference- amount by which one wave is behind another (in degrees or radians)
  • Reflection- the wave is bounced back when an obstacle is reached
  • Refraction- the wave changes direction when it enters a different medium
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Transverse Waves and Polarisation

  • Transverse wave- vibration at 90° to direction of travel- electromagnetic waves are transverse, whereas sound is longitudinal (vibrations along direction of travel
  • Electromagnetic radiation- two transverse waves
  • Polarisation is used to distinguish between transverse and longitudinal waves- only transverse waves are be polarised. A polarising filter acts as a fence, it filters out any waves which are not in the right direction to the slit in the filter
  • Light that has passed through a polarising filter will only be vibrating in one direction
  • If two polarising filters are at right angles to each other, no light will get through
  • Rotating a polarising filter in a beam of light shows the fraction of light vibrating in each direction. 
  • When you direct a beam of unpolarised at a reflective surface and view the reflected ray through a polarising filter, the intensity changes with the orientation of the filter. This is because the light is partially polarised when reflected
  • Polarised light has many uses-
  • 1. Communication satellites uses different polarisations for signals in the same frequency band to reduce interference
  • 2. Polaroid sunglasses (skiers and anglers) reduce glare. Polaroid lenses for photography remove unwanted reflection
  • 3. CD players have a crystal to transmit p. light in one direction, and reflect in other
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Forming Images with Lenses

  • When a ray of light meets a boundary, some energy is reflected into the 1st medium and some is transmitted
  • Light meets a boundary at an angle to the normal- ray is refracted (bent) as it travels at a different speed. More optically dense, more slowly light travels
  • Lenses change the curvature of wave fronts through refraction. A lenses adds curvature to a passing wave- the slow down the light in the middle of the lens more than the edges. All points take the same amount of time to reach the focal point (or focus)
  • Focal length, f, distance between lens axis and focus
  • More powerful lens- wavefront curved more strongly- shorter focal length
  • Power of a lens is measured in dioptres
  • Distances are measured from the lens axis- to the right is +, to the left is -
  • If a light source is very far away, wave fronts will be flat. If source is at the focal point, you'll have a negatively curved wavefront before the light reaches the lens, which will be made flat
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Information in Images

  • The binary system is a system using two digits (1 and 0). A single binary digit is a bit, and 8 bits make up a byte
  • The number of bits in a sequence determines how many alternatives can be coded for
  • The pixels in an image are individually represented by a binary number. The value of the binary number gives the colour of the corresponding pixel
  • Black and white images are made up of 256 shades of grey, each one represented by a eight-digit binary number
  • In colour, each pixel is described by three binary numbers, one for each primary colour (red, blue and green)
  • Changing binary values changes the image as a whole-
  • 1. Adding false colour highlights features
  • 2. Replacing pixels with the median of neighbours reduces noise (unwanted interference- bright/dark spots on an image). Any odd values are removed, and the picture is smoother
  • 3. The Laplace rule is used to find edges (working out if there is anything in an image- not just noise). You multiply a pixel by four, and subtract the value of the pixels immediately surrounding the pixel. Any pixel not on an edge goes black- left with the edges
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Sampling

  • Digital signals are represented by binary numbers, so can only take one of a set number of values. 
  • Analogue signals are not limited in the values they can take- very continuously
  • When an electronic signal is transmitted, it will pick up noise from electrical disturbances or other signals.The receiver must reconstruct original signals from the noisy signal to get a representation of what was sent. Easier with digital signals as the number of values is limited
  • Analogue signals can be digitised- take the value of the signal at regular time intervals then find the nearest digital value (represented by a binary number). Wont be exactly the same, but usually close
  • How well they match depends on the resolution (difference between possible digital values) and sampling rate
  • The higher the resolution and greater the sampling rate, the better the copy. However, a fine resolution will reproduce all the wiggles caused by noise
  • Resolution is determined by the number of bits in the binary representing the digital values- more bits= better resolution
  • The sampling rate needs to be high enough to record all high frequency detail. A low sampling rate can create aliases (low frequency signals not present in the original signal). Sampling rate must be 2x the highest frequency in the signal
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Advantages of Digital Signals

  • 1. Digital signals can be sent, received and reproduced easier than analogue, they can only take a limited number of values
  • 2. They are resistant to the effects of noise
  • 3. Can be used to represent different kinds of information in the same way (eg. images and sounds)
  • 4. Easy to process using computers
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Signal Spectra and Bandwidth

  • Most signals are made up of several sine curves added together, all with different frequencies. The frequencies that make up a signal are called the spectrum
  • All frequencies are needed to reconstruct a signal, as they all carry information
  • The range that a signal's spectrum covers is the bandwidth (found by subtracting lowest from highest frequency), In communications systems, the bandwidth of a signal determines how many signals can be sent at the same time
  • Communication signals are transmitted using carrier waves. An audio signal is converted into an electronic signal, then mixed with a carrier wave before transmission
  • All radio stations are given a particular carrier frequency to broadcast on. All local stations have a different frequency to prevent interference
  • There must also be a gap between frequencies used. The larger the bandwidth, the larger the gap needed to stop signals overlapping. However, the gaps limits the number of signals that can be transmitted
  • Rate of transmission of a digital signal depends on-
  • 1. The number of samples per second (at least x2 the highest frequency)
  • 2. The number of bits per sample (must be high enough to match original, but no so high that it is negatively affected by noise)
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