# P3 1 - Medical applications of physics

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• Created by: Natalia
• Created on: 11-03-13 20:04
• P3 1 - Medical applications of physics
• X-rays
• Part of the electomagnetic spectrum.
• Very short wavelength.
• Cause ionisation.
• They affect a photographic film in the same way as light.
• Absorbed by metal and bone.
• Transmitted by healthy tissue.
• Their wavelength is of the same order of magnitude as light.
• Can be used to diagnose and treat some medical conditions.
• Using CT scans
• Such as bone fractures and dental problems.
• To kill cancer cells.
• The use of charge-coupled devices (CCDs) allows images to be formed electronically.
• Precautions must be taken when X-ray machines and CT scanners are in use.
• Ultrasound
• Can be produced by electronic systems.
• 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.
• Are partially reflected when they meet a boundary between two different media.
• The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is.
• Calculation of the distance between interfaces in various media: s = v X t  where 's is distance in metres, m'.     'v is speed in metres per second, m/s'.  't is time in seconds, s'.
• Can be used in medicine.
• Pre-natal scanning.
• Removal of kidney stones.
• Lenses
• Refraction of light
• Refraction is the change of direction of light as it passes from one medium to another.
• A lens forms an image by refracting light.
• Refractive index = (sin i) / (sin r). Where i is the angle of incidence and r is the angle of refraction.
• Converging lenses
• Thickest at the centre.
• A converging lens can be used as a magnifying glass.
• The image produced by the lens is real (on the other side of the lens), inverted and smaller (for distant objects) or magnified.
• Concave lenses
• The image produced is virtual (on the same side as the object) and upright (not inverted).
• The magnification caused by a lens can be calculated using the equation: magnification= (image height) / (object height)
• The eye
• The structure of the eye
• The retina contains the light receptors.
• The lens focuses right onto the retina and provides further refraction.
• The cornea refracts light - it bends it as it enters the eye.
• The iris controls how much light enters the pupil.
• The pupil adjusts the light intensity.
• The ciliary muscles control the shape of the eye lens. This allows light from objects at different distance to be brought into focus.
• The suspensory ligaments hold the lens in position and alter its shape in conjunction with the ciliary muscles.
• Correction of image to produce an image on the retina:
• Long sighted, caused by the eyeball being too short or the lens being unable to focus.
• Short sighted, caused by the eyeball being too long, or the lens being unable to focus.
• Calculation of the distance between interfaces in various media: s = v X t  where 's is distance in metres, m'.     'v is speed in metres per second, m/s'.  't is time in seconds, s'.
• A lens forms an image by refracting light.