Physics - P3.1 - Medical Applications of Physics

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P3.1.1 - X-rays

  • X-rays - high frequency, short wavelength (wavelength ~size of atom)
  • X-ray properties: affect photographic film same way as light, absorbed by metal and bone, transmitted by healthy tissue - used to form images of bone on photogrphic film to look for fractures or dental problems
  • Charged-coupled devices (CCDs) - used to form electronic images of X-rays - CT scanners use X-rays to produce digital images of cross-section of body - some body organs made of soft tissue can be filled with X-ray absorbing contrast medium
  • X-rays cause ionisation and damage tissue when passing through it - precautions must be taken when used: workers wear film badges and sheild themselves with lead shield
  • X-rays can be used for therapy: treat cancerous tumours at or near body surface
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P3.1.2 - Ultrasound

  • Human ear detects sound 20-20000Hz - higher = ultrasound
  • Electronic systems used to produce ultrasound waves - when wave meets boundary between materials, part of the wave is reflected and travels back through material to detector - time taken to travel used to work out distance to boundary: results processed by computer to create image
  • Wave has travelled twice distance to boundary: there and back
  • Ultrasounds used in medicine for scanning - non-ionising: safer than X-rays - used to scan unborn babies and soft tissue eg. around eye
  • Can be used for therapy: used to shatter kidney stones into small pieces
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P3.1.3 - Refractive Index

  • Refractive index: change of direction of light from one transparent substance to another - takes place as waves change speed when crossing boundaries - change of speed = change of direction, unless travelling along normal
  • Light ray refracted towards normal when crossing from air to glass - refractive index measured by how much substance can refract light ray
  • n = sin(i) / sin(r)
    • n - refractive index of substance
    • i - angle of incidence
    • r - angle of refraction
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P3.1.4 - The Endoscope

  • Light ray refracted away from normal when travelling from glass to air - partially reflected ray seen
  • Angle of incidence increased = angle of refraction increases until refracted ray emerges along boundary - angle of incidence: critical angle
  • Angle of incidence > critical angle = total internal refraction: angle of incidence = angle of refraction
  • n = 1 / sin(c) - n: refractive index, c: critical angle
  • Endoscope: looks inside person - contains optical fibres: thin, flexible glass fibres - visible light sent down fibres by total internal reflection
  • Laser light used as energy source in endoscope to do cutting, cauterising and burning - colour matched to tissue type to produce maximum absorbtion - eye surgery performed by laser light passes through cornea but absorbed by retina
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P3.1.5 - Lenses

  • Coverging/Convex lense - parallel rays of light refacted and converge at point: principal focus/focal point - centre of lense to focal point = focal length - light can pass either direction: focal point on each side
  • Object further away than focal point: inverted real image - size depends on objects position: nearer to lense = bigger - object nearer than focal point: upright, virual image - image is magnified - magnification = image height / object height
  • Diverging/Concave image - parallel rays of light refracted and diverge away from point: principal focus - centre of lense to focal point = focal length - light can pass either direction: focal point on each side
  • Image produced is always virtual
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P3.1.6 - Using Lenses

  • Draw ray diagrams to find image different leses produce with objects in different positions - principal axis: line through centre and right angle to it
  • Ray diagrams use three construction rays from single point on object to locate corresponding point on image:
    • ray parallel to principal axis refracts through focal point
    • ray through centre of lens travels straight on without refraction
    • ray through focal point refracts parallel to principal axis
  • Camera uses converging lense to form real image of object on film or array of CCDs
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P3.1.7 - The Eye

  • Light enters eye through cornea (transparent layer protecting eye)
  • Cornea and eye lens focus light onto retina (light-senstive cells around inside of eye)
  • Iris (coloured ring of muscle) adjusts size of pupil (central hole) to control amont of light entering eye
  • Ciliary muscles (attached to lens by suspensory ligaments) alter thickness of lens to control fine focusing of eye
  • Blind spot - retina isn't sensitive to light: no light-sensitive cells present
  • Optic nerve - carries nerve impulses from retina to brain
  • Near point = 25cm, far point = infinity: range of vision = 25cm-infinity
  • Lens power - power of lens: P = 1 / f
    • P - power of lens - dioptres, D
    • f - focal length of lens - m
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P3.1.8 - More About the Eye

  • Short sight - close object clear, far object blurred - uncorrected image forms in front of retina - caused by eyeball being too long or the eye lens too powerful - corrected by diverging lens
  • Long sight - far object clear, close object blurred - uncorrected image forms behind retina - caused by eyeball being too short or eye lens too weak - corrected by converging lens
  • Focal length of lens affected by refractive index of material from which lens is made and curvature of two lens surfaces
  • For lens of given focal length: greater refactive index of lens material = flatter and thinner lens can be manufactured
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