Physics P3

Physics P3

1. Medical Physics

   • Ionising
     • Can cause damage to cells, gives better images and can be better to diagnose a patient
       • Compromises have to be made, minimise the amount of damage the radiation can do
     • x-rays
       • absorbed by dense materials like bone, not by soft tissues
       • Make x-ray photographs used to diagnose bone damage
       • CAT scans, create 2d images of slices of the body using strongly ionising x-rays
         • Can be used to image soft tissues as well as bones
   • Non Ionising
     • Doesn't cause damage to patient
     • Endoscopes, bounce visible light along an optical fibres, used to shine light inside the body - avoids any type of surgery
       • Can be used to help with keyhole surgery
     • Ultrasound (above 20khz)
       • Breaking kidney stones that block urinary tract, sends concentrated, high energy beams that turns them to sand like particles, painless and without surgery
       • pre-natal scanning, ultrasound waves are reflected off soft tissues of the foetus, the time it takes to reflect is processed by a computer to create a image
       • Measures the speed of blood as it works in real time, identify blockages and problems especially in liver and heart

2. Intensity of Radiation

   • Energy that originates from a source
   • Depends on distance from source, the further away from source, the less energy received
   • Depends on what its passing through, is always absorbed by something (unless in vacuum), denser materials absorb more radiation
   • Intensity = Power / Area
     • Greater intensity, the more energy it carries per second (power)
     • A bigger area affects the amount of energy absorbed

3. Lenses

   • Refraction, light slows down in denser mediums, when it slows down bends towards the normal, speeds up bends away from the normal. Most light is refracted, some is reflected
   • The further the object is from the lens, the more diminished the image is
   • Power
     • The shorter the focal length, the more powerful the lens. The thicker the lens, the more powerful it is.
     • Real image = positive power, virtual image = negative power
   • Eye
     • Pupil allows more/less light to enter, controlled by the coloured iris, made up of muscles to change the size
     • Cornea is convex - focuses most of the light and has a fixed shape (power). Lens can change shape to change focal point
       • Lens is controlled by ciliary muscles, when they contract lens gets bigger (more powerful), relax and the lens gets thinner (less powerful)
     • Retina, lots of light sensitive cells that interpret the light and sends signals to brain via optic nerve
     • Near point - closest distance the eye can focus on (25cm is average), Far point - the furthest distance the eye can focus on (usually infinity)
       • Short-sighted, can't focus on far away objects, image forms before the retina because of powerful lens, damaged ciliary muscles etc.
         • Fixed by using concave (diverging) lenses, they spread out light rays so eye focuses light directly on the retina
       • Long-sighted, can't focus on close objects, image forms behind retina, weak lens, damaged ciliary muscles
         • Fixed by using convex (converging) lenses, focuses more light so image forms directly on retina
         • Older people become long-sighted as ciliary muscles become weaker
       • Eyesight correction
         • Contact lenses, sit on the cornea - convenient, lightweight, relatively cheap, can be uncomfortable, could cause eye infections if misused
         • Laser-eye surgery, change the shape of cornea to make it more/less powerful, expensive and can have risks, eyes will still deteriorate so may not be worth it
   • Total internal reflection
     • Refractive index, higher index means less refraction (light slows down more)
       • n = sin i / sin r
     • Critical angle, angle of incidence where light is refracted along boundary . TIR happens when angle of i is above critical angle
       • Sin C = n(r) / n(i)
         • n(r) is the refractive index of the medium the light is going to
         • n(i) is the refractive index of the medium the light is coming from
     • Optical fibres TIR light

4. Thermionic Emission

   • Used to create x-rays from heat
   • takes place in an evacuated tube to stop the electrons colliding with gas particles
     • A filament acts as a cathode, it heats up to emit its electrons
       • The electrons are accelerated by a potential difference towards an anode
         • The electrons will collide with the anode, causing some of their kinetic energy to be converted to x-rays
           • A lead casing with one hole means that the x-rays only exit the tube in one place (so they can be aimed at a specific place)
     • Stops the electrons from losing energy
   • Kinetic energy = electron charge x potential difference
     • Charge of an electron is -1.6x10^-19
   • Current = number of particles per second x electron charge
     • A current is the rate of flow of charge so it is produced by the beam of electrons