Physics - P3.1 - Medical Applications of Physics - Revision Cards in GCSE Physics

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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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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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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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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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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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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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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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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