AQA Physics 3 - Medical Applications of Physics

Medical Applications of Physics
1. X-Rays
  1. High frequency & Short wavelength.
  2. Properties: - affect photographic paper the same way as light - absorbed by metal/bone - transmitted by soft, healthy tissue
  3. Often used in hospitals to check for fractures and dental problems.
  4. Some soft-tissue body organs can be filled with a contrast medium that absorbs x-rays.
  5. X-rays cause ionisation and damage living tissue when it passes through.
    1. Workers wear film badges so they know if they roughly how much they have been exposed to, or they can wear lead aprons.
    2. Even though x-rays can cause cancer, they can also treat cancerous tumours at or near the surface of the body.
  6. CT Scanners use X-Rays, they produce 3D images on a computer but are also much more expensive to make and run.
2. Ultrasound
  1. Human ear can detect between 50Hz and 20,000Hz
    1. Anything above 20,000Hz is called Ultrasound
  2. Can be used for diagnosis and treatment eg. Baby scanning
    1. Non-ionising, so it's safer than x-rays
  3. Ultrasound waves can be produced electronically for things such as fishing boats
    1. Part of the wave is reflected at a boundary between two different materials.
      1. Wave then travels back to a detector.
        The time it takes to reach the detector can be used to calculate how far away the boundary is.
        Using the equation: S = v x t. Where v is the speed of the ultrasound wave in s.
        Note that is may be double the correct distance as the wave will have travelled from the computer and back again.
3. Refractive Index
  1. The measure of how much a substance can refract a light ray.
    1. n= sin i / sin r
      1. n - refractive index
        sin i - sine angle of incidence sinr - sine angle of refraction
  2. Refraction takes place due to the change in speed of the wave as it crosses a boundary.
  3. A light ray travelling along the normal is not refracted
4. The Endoscope / Total Internal Reflection
  1. When a light ray crosses from glass to air, it refracts away from the normal and a partially refracted ray is seen.
    1. If the angle of incidence is increased, the angle of refraction increases until the light ray refracts along the boundary.
      1. The angle of incidence at which this happens is the critical angle.
  2. If the angle of incidence is increased above the critical angle, the light ray undergoes total internal reflection.
    1. When this occurs the angle of reflection is equal to the angle of incidence.
  3. n = 1 /sin c
    1. n = refractive index
    2. sinc = sine critical angle
  4. Endoscope contains many fibre optics (thin, flexible, glass wires). Used to see inside the body.
    1. Visible light rays can be sent along these by total internal reflection.
  5. ONLY HAPPENS WHEN RAY IS TRAVELLING FROM MORE DENSE TO LESS DENSE MATERIAL
5. Lenses
  1. Converging Lens
    1. Refracts two parallel rays to a point (the principal focus)
      1. Distance from principal focus to centre of the lens is the focal length.
    2. There is a principal focus on both sides of the lens.
    3. If the object is further from the lens than the principal focus it forms an - inverted - real image
      1. If the object is closer than the principal focus it forms an - upright - virtual image behind the object.
        This image is magnified.
        Magnification = image height / object height
  2. Diverging Lens
    1. Refracts two parallel rays away from a point (the principal focus)
      1. Centre of the lens to principal focus is the focal length.
    2. Image produces is ALWAYS virtual.
    3. Principal focus on both sides of the lens.
6. Using Lenses
  1. Use ray diagrams to find the image that different lenses produce with different objects and positions.
  2. Line 1 - Parallel to axis, refracted through F
    1. Line 2 - Passes straight through the centre of the lens
      1. Line 3 - Passes through F, refracted parallel to the axis.
7. The Eye
  1. Light enters the eye through the cornea.
    1. The cornea and the eye lens focus the light onto the retina.
      1. Iris adjusts the size of the pupil to control the amount of light entering.
    2. Ciliary muscles adjust the thickness of the lens to control focusing.
      1. Attached to the lens by suspensory ligaments.
  2. Normal human eye has a near point of 25cm and a far point of infinity. This is the range of vision.
  3. Lens Power
    1. P = 1/f
      1. Power = dioptres (D)
        Focal Length (m)
  4. Short Sightedness
    1. See close objects clearly but distance is blurred.
      1. Use diverging lens to correct sight.
      2. Because image is formed in front of the retina.
  5. Long Sightedness
    1. See distance objects but close up is blurred.
      1. Use converging lens to correct sight.
      2. Because image is formed behind the retina.
  6. The higher the refractive index used as glasses, the flatter and thinner the lens can be.

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AQA Physics 3 - Medical Applications of Physics

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