AQA Physics 3 - Medical Applications of Physics

AQA Physics 3 - Medical Applications of Physics

1. Medical Applications of Physics

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

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