medical applications of physics

x-ray are part of the electromagnetic spectrum. high frequency and short wavelength

properties of x-rays-

• affects photographic film
• absorbed by metal and bone
• transmitted by healthy tissue

charge-coupled devices can be used to form electronic images of x ray. CT scanners use x-rays to produce digital images of a cross section through the body.

x rays cause ionisation and can damage living tissue when they pass through it, therefore precautions must be taken when using them.

x rays may also be used for therapy, they can they can treat cancerous cells.

ultra sound are sound waves with a frequency higher than 20000Hz 

• when a wave meets a boundary between two different materials, part of the wave is reflected. 
• the wave travels back through the material to a detector. 
• the time it takes to reach the detector can be used to calculate how far away the boundary is .

s = v x t
s is the distance travel in metres
v is the speed of the ultra sound m/s
t is the time taken in seconds  

• it is safer than x-rays. 
• it can be used for scanning unborn babies 
• used to shatter kidney stones 

• refraction is the change of direction of light as it passes from one medium to another. 
• refraction takes place because aves change speed when it cross a boundary. 
• the refractive index of a substance is a measure of how much substance can refract a light ray 
• the top line is the angle of incidence- the bottom is the angle of refraction  

The minimum angle at which total internal reflection occurs is known as the critical angle. At any angle greater than the critical angle, total internal reflection occurs and the light ray obeys the normal rules for reflection (i.e. angle of incidence = angle of reflection).

 Optical fibres are used in medicine as a transmitting system for 'cold light' (light without the heat). They are used in an instrument called an endoscope or a fibroscope.

• laser light may be used:
  • as an energy source in a endoscope 
  • cutting, cauterising and burning.
  • the colour needs to be the same as the tissue for maximum absorption

A converging lens (or convex lens) is curved on both sides. This means the light rays coming out of it come together at a point – they converge.

• if the object is further away from the lens than the principal focus an inverted,real image is formed. the size of the image depends on the position of the object.
•  if the image is nearer  to the lens than the principle focus an upright, virtual image is formed behind the object. the image is magnified, this is a magnifying glass. 

• Cornea- 
• Front part of the tough outer coat, the sclera.
•  It is convex and transparent.refracts light - bends it as it enters the eye

• Iris-
• Pigmented - decides the colour of your eyes - so light cannot pass through. Its muscles contract and relax to alter the size of its central hole or pupil.
• controls how much light enters the pupil

• Lens-
• Transparent, bi-convex, flexible disc behind the iris attached by the suspensory ligaments to the ciliary muscles.
• focuses light onto the retina

• Retina- The lining of the back of eye containing two types of photoreceptor cells - 
  • rods - sensitive to dim light and black and white,
  •  cones - sensitive to colour. 
• A small area called the fovea in the middle of the retina has many more cones than rods.
• contains the light receptors

• Optic nerve-
• Bundle of sensory neurones at back of eye.
• carries impulses from the eye to the brain

                Myopia - short-sight                       Hypermetropia - long-sight
Someone with short-sightedness can see near objects clearly, but can't focus properly on distant objects. This is because the lens focuses the sharpest image in front of the retina, instead of on it.

Someone with long-sightedness can see distant objects clearly, but can't focus properly on near objects. This is because the lens focuses the sharpest image behind the retina, instead of on it.