P3.1 - Medical applications of physics

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  • P3.1 - Medical applications of physics
    • Eye defects
      • Long sighted -  eye ball too short can't focus on near objects
      • Short sighted - eye ball too long & unable to focus on distant objects
      • Glasses adjust light before it enters eye to allow eye to focus it properly
    • Refraction of Light
      • Interface  ( boundary between two transparent media of different densities)
      • When light crosses an interface it changes direction
      • No refraction occurs when light enters interface at a 90 degree angle (normal line)
      • Refractive Index = sin (angle of incidence) / sin (angle of refraction)
    • Camera and magnifying glass
      • Unlike an eye they have fixed converging lenses with definite focal lengths
      • Magnifying glass enlarges object size if distance between lens & object is less than focal length
        • The image is: Virtual, upright & enlarged
      • Camera film and CCD (charged coupled device) = retina in eye
      • Convex lens magnifies is between focal point and centre of lens
    • Reflection Of Light
      • Normal line = perpendicular to reflecting surface at point of incidence
      • Angle of incidence = Angle of reflection
    • Power of a Lens
      • Power in Dioptres = 1 / focal length in metres
      • The power of a converging lens is positive (real focal point)
      • The power of a diverging lens is negative (virtual focal point)
    • Converging and Diverging Lenses
      • A lens refracts light
      • Converging Lens (convex lens) - thickest at centre
        • Light's refracted inwards toward the focal point on opposite side of lens to the object and is real
      • Diverging lens (concave lens) - thinnest in the centre
        • focus appears to come from a point on same side as object and isn't real i.e. virtual
      • Normal Line  in ray diagram = principal axis
      • Distance between centre of lens and focus point = focal length
        • For parallel rays of light the focal point lies on the principal axis
    • The Critical Angle
      • Two special cases involving refraction of light:
        • Critical angle can be used to find refractive index: Refractive Index = 1 / sin (critical angle)
        • 2) Angle of incidence is greater than critical angle. In this case total internal reflection occurs.
          • No refraction occurs so no light escapes glass
          • Total Internal Refraction: Angle of incidence = Angle of refraction
        • 1) Angle of refraction = 90 degrees & light ray travels on boundary between air & glass. Then angle of incidence is called the critical angle
      • Medical endoscopes use the principle of total internal reflection to see inside the body
        • Visible light - optical fibres - operations & the light is returned to an eye peice or camera
      • = when angle of refraction is 90 degrees from normal line
    • Ultrasound
      • Ultrasound=sound waves of frequencies > 20, 000 Hz = beyond limit of human hearing
      • non-ionising
      • As ultrasonic waves pass from one medium to another they partially reflect at the boundary
        • Time taken for reflections how far away the boundary is
          • distance in meters = speed in meters per second x half the time taken for the pulse to leave the source and return to the detector
      • electrical oscillations are used to generate ultrasonic waves
      • Uses: Pre-natal scanning, imaging damaged ligaments & muscles & kidneys and destruction of kidney stones
    • The structure of the eye
      • information about light is sent to brain via optic nerve
      • Cornea refracts most of light
      • Pupil (opening in iris) adjusts light intensity
      • Lens provides further refraction before image forms on light - sensitive retina
      • Ciliary muscle controls shape of lens. This allows light from different distances to be focused
      • Near point -  aprox. 25 cm Far point - Infinity
    • Images produced by lenses
      • Size of image produced by convex or concave lens depends on distance of object from lens
      • Convex lens: real, inverted, smaller for distant objects and magnified for objects between 'F' and '2F'
      • Concave lens: Virtual (on same side as object) and upright ( not inverted)
    • Lasers
      • Intense narrow beam of light
        • by stimulated emission of radiation
      • Can be made from solids liquid & gases
      • Used in eye surgery to: repair damaged retina's, remove diseased / damaged cells by cutting & burning tissue
      • Optical fibres used to guide laser beams
    • X-rays
      • short wavelengths the same order of magnitude as diameter of atoms
      • causes ionization when it hits an atom
      • to see bone fractures, dental problems and computerised tomography (CT Scan)
      • affect photographic film as light does
      • Can be detected using charge-coupled devices (CCDs) to form image electronically
      • advantages: transmitted by healthy tissue, absorbed by metal and bone to produce shadow pictures
      • increased radiation=more detailed image
        • Personal radiation detectors, protective screening & protective clothing used to limit & monitor exposure
    • Magnification and Focal Length
      • Magnification = Image height / Object height
      • A converging lens can be used as a magnifying glass
      • Focal length of a lens is determined by: refractive index of material from which the lens is made and curvature of two surfaces of lens
      • for a given focal length, the greater the refractive index, the flatter the lens

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