P3- Topics 1 & 2

P3- Topics 1 & 2

1. Medical Physics and Ultrasound

   • X rays are used for medical imaging, they are high energy EM waves that are get absorbed by dense materials such as bone, they are mainly used to diagnose bone fractures. X rays are often used in CAT scans to produce a two-dimensional slice through the body, the X rays used are strongly ionising and can be used to look at soft tissue as well as bone.

2. Intensity of Radiation

   • Radiation: Energy that is omitted from a source The intensity of the radiation often depends on the distance away from the source. It can also change depending on what the radiation has to pass through, normally the more dense the medium the more radiation is absorbed and so there will be a lower intensity.

3. Lenses

   • Refraction occurs when a ray passes into a new medium of different density, if the ray slows down it bends towards the normal and if it speeds up it bends away from the normal.  The two main types of lenses are converging and diverging, converging lenses are convex and cause parallel rays of light to converge to a focus. Diverging lenses are concave so causes parallel rays to diverge.
   • Refraction in a converging lens: An incident ray parallel to the axis refracts through the lens and passes through the focal point on the other side, an incident rat passing through the focal point before entering the lens will refract through the lens and travel parallel to the axis. Refraction in a diverging lens: An incident ray that is parellel to the axis refracts through the lens and travels in line with the focal point, whereas an incident ray that enters through the focal point will refract through the lens parallel to the axis.

4. Power and the Lens Equation

   • Power of lens=1/Focal Length The focal length is related to the power, the more powerful the lens, the quicker the rays converge which makes the focal point shorter.

5. The Eye

   • The iris is the colored part of the eye, it controls the amount of light that is let into the eye. The cornea is the part of the eye that controls focusing. The retina is covered by light-sensitive cells that detect light and then send signals back to the brain. The eye also has both near and far points, if a person has perfect eyesight the furthest distance the eye can focus on is infinity and the closest point that the eye can focus on is 25cm. Short sighted people are unable to focus on objects that are far away whereas longsighted people are unable to focus on objects that are close up.

6. Correcting Vision Defects

   • To fix short sightedness you can use diverging lenses as they will have the focal point at the faulty far point making the objects that are far away in focus.  Long sightedness is often fixed using a converging lense as this makes the near point closer to 25cm allowing them to focus on things closer up. However the methods that we use to fix the vision problems varies from patient to patient. 

7. Snell's Law

   • When an incident ray passes into  a material the angle of incidence, angle of refraction and the refractive index are related by this formula: n=Sini/Sinr

8. Uses of Total Internal Refraction

   • Optical fibres can carry visible light and are often used in medical diagnosis and communications. They use total internal reflection as the light bounces off the insides of the inner core of the fibre and the wave is reflected off the sides until it reaches the other end. The fibre must be narrow so that the angles keep above the critical angle and the fibre can't be bent too sharply for the same reason.

9. Electron Beams

   • Electron beams are used in old style televisions to produce and image onto the screen. You can calculate the kinetic energy of each electron: KE=ElectronicChargeX acceleratingpotentialdifference You can also work out the size of the current produced: Current=NumberofparticlesX chargeoneachparticle

10. X Ray

    •  X Rays can harm the body's cells through ionisation as they can remove electrons from atoms. The thickness of the material affects X ray absorbtion as well as the distance from the source affecting the intensity. Fluroscopes use X rays to create moving images of patients' inside, the intensity of the X rays depends on what materials they have passed through and then the X rays hit a fluorescent screen which absorb the X rays and show a live image onto the screen.
    • CAT scans produce images of    2-D slices through the body, the X ray beam rotates around the body and is picked up by thousands of detectors and they use the X rays to produce a high quality 3 dimensional image. However using X rays comes with high risks so they are only used when necessary.

11. Electricity and the Body

    • Muscle cells generate potential difference, when the muscle cell is stimulated by an electrical signal the potential difference changes.

12. Pacemakers and Pulse Oximeters

    • A pacemaker is a device that is used to regulate heartbeat, it is given to patients when the heart is unable to control the heart rate of the patient itself. It can be implanted during minor surgery and many are able to monitor things like breathing and temperature and use this to adjust heart rate to match what your doing. Pulse oximeters use light to check the percentage of oxygen in the blood, it measures the amount of oxygen carried by the haemoglobin in a patients blood and is used for the patients health before and after surgery.