Physics 3

Physics 3

1. X RAYS IN MEDICINE

   • X rays are high frequency, short wavelength electromagnetic waves
   • They pass through healthy tissue, but are absorbed by denser materials like bones and metals
   • CT SCANS
     • Scans used to produce high resolution images of soft and hard tissue.
     • An x ray beam is fired through the body and is picked up on the other side by detectors. This rotates throughout to create a 3D image of the patient's head.
   • CANCER
     • x rays can cause IONISATION (high doses of x rays will kill living cells)
     • 1)The x rays are focused on the tumour using a wide beam 2)The beam is rotated round the patient with the tumour at the centre 3)This minimises the exposure of the normal cells to radiation, and reduces the chances of damaging the rest of the body.

2. ULTRASOUND

   • Electronic systems can be used to produce ultrasound waves, which have a frequency higher than the upper limit of hearing for humans. ◦ The range of human hearing is about 20 Hz to 20 000 Hz.
   • Ultrasound waves are partially reflected when they meet a boundary between two different media. ◦ Know that the time taken for the reflections to reach a detector can be used to determine how far away such a boundary is.
   • To calculate the distance between interfaces in various media you use the equation ◦ s = v x t ◦ s is distance in metres, m ◦ v is speed in metres per second, m/s ◦ t is time in seconds, s

3. LENSES

   • Refraction is the change of direction of light as it passes from one medium to another
   • A lens forms an image by refracting light.
   • in a convex or converging lens, parallel rays of light are brought to a focus at the principal focus.
   • The distance from the lens to the principal focus is called the focal length
   • ◾ refractive index = sin i / sin r ◾ i is the angle of incidence, r is the angle of refraction
   • The nature of an image is defined by its size relative to the object, whether it is upright or inverted relative to the object and whether it is real or virtual.
   • The nature of the image produced by a converging lens for an object placed at different distances from the lens
   • magnification = image height / object height

4. THE EYE

   • RETINA:The retina is responsible for collecting light to create an image LENS: the transparent structure behind the pupil that changes shape to help focus images CORNEA: is the part of the eye that projects the eye and bends light to provide focus PUPIL: the adjustable opening in the center of the eye through which light enters IRIS: a ring of muscle tissue that forms the colored portion of the eye around the pupil and controls the size of the pupil opening CILIARY MUSCLE: adjust the shape of the lenses in order to focus the eyes.
   • -->long sight - caused by the eyeball being too short, or the eye lens being unable to focus --> short sight - caused by the eyeball being too long, or the eye lens being unable to focus.
   • The near point is approximately 25 cm and the far point is infinity
   • The power of a converging lens is positive and the power of a diverging lens is negative
   • For a given focal length, the greater the refractive index of the glass, the flatter the lens

5. OTHER APPLICATIONS USING LIGHT

   • refractive index = 1 / sin c c is the critical angle
   • Visible light can be sent along optical fibres because of total internal reflection
   • The cornea of the eye is reshaped using a laser with a flap of tissue being cut as part of the procedure. In LASEK laser eye surgery, the epithelium, or outer layer of the cornea, is cut.