Medical applications of physics

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Electromagnetic waves at the short-wavelength end of the spectrum (about width of an atom)

X-rays pass through soft tissue but are absorbed by bones, teeth and metal objects. The parts of the film that the x-rays reach become darker. An organ that consists of soft tissue can be filled with a contrast medium which absorbs X-rays easily enabling the internal surfaces of the organ to be seen on the radiograph.

Sensors in a CCD are covered in a layer that convert x-rays into light which then create electronic signals which are sent to a computer to display a digital x-ray image.

X-radiation is dangerous because it ionises substances it passes through. High doses kill living cells or low doses can cause cell mutation and cancer. Workers must wear a special film badge and if the badge is overexposed to x-radiation, the wearer is stopped from working with the equipment.

X-ray therapy kills cancerous tumours - these waves are shorter than those for imaging.

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CT scanner and Ultrasound

A CT scanner produces a digital image of any cross-section through the body or to construct a 3D image of an organ. A CT scanner can distinguish between different soft tissues, but also has a much higher dose than an ordinary x-ray machine (and costs more).

Ultrasound waves are sound waves of frequency above 20,000 Hz. 

They are used in medicine for ultrasonic scanning and the destruction of kidney stones.

Ultrasound waves are partly reflected at a boundary between two different types of body tissue.

An ultrasound scan is non-ionising so it is safer than an x-ray.

distance travelled by pulse = spped of waves in tissue x transit time

depth of boundary below surface = 1/2 x speed of waves  transit time

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Refractive Index

Refraction of the light ray takes place at the boundary between air and the transparent substance.

Angle of refraction = angle from normal to ray (in substance)

Angle of incidence = angle from normal to incident ray (in air)

Angle of refraction is always less that the angle of incidence (from air into glass) and the greater the angle of incidence, the greater the angle of refraction.

Snell's law sin i/sin r is always the same and gives the refractive index of the block tested.

light ray is always refracted away from the normal (glass into air) and the larger the angle of incidence, the larger the angle of refraction.

sine(angle in air) = n x sine(angle in glass)

The critical angle is the angle of incidence when the angle of refraction just reaches the border (90degrees from normal)

Total internal reflection's when the angle of incidence is increased past the critical angle.

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Real and Virtual images

Real image

  • When the object is far away, the image is formed at the principal focus because all the light rays from the object are effectively parallel to eachother when they reach the lens.
  • If the object is moved nearer the lens (towards it's principal focus), the screen must be moved further away to see a clear image. The nearer the object is to tlens, the larger the image is.

Virtual image

  • When the object is nearer the lens than the prinicipal focus, a maginified image is formed. It is called a virtual image because it is formed where the rays appear to be coming from.
  • You can only see the image if you look into the lens on the opposite side from where the lens is coming from.
  • Maginification = image height/object height.
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The eye

The normal human eye has a range vision from 25cm to infinity. It has a near point of 25cm and a far point of infinity.

To see a nearby object clearly, the lens has to be thicker than if the object is far away.

Power of lens (dioptre) = 1/focal length (in metres)

Converging lenses have a positive value of power, diverging lenses have a negative value.

Short sight is when either an eye is too long or a lens is too powerful so the person can't focus objects that are far away

Long sight is when the lens can't be made thick enough to form a clear image and the image is made behind the retina. This mean the person can't see things close up.

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