P3.1 Medical applications of Physics

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X-rays are part of the electromagnetic spectrum. Their wavelength is about the same size as that of an atom.

X-rays are used in hospitals to make images and CT scans. They can also be used to destroy tumours at the surface of the body. X-Rays damage living tissues when they pass through it. They are absorbed by bones and teeth. CT scans can distinguish between different types of soft tissues as well as between bone and soft tissue.

When the X-ray tube is switched on, x-rays from the tube pass through the part of the patients body being examined in front of photographic film. X-rays pass through soft tissue but are absorbed by bones, teeth and thick metals. The parts of the film/detector that the x-rays reach become darker. Bones appear lighter than surrounding tissue. The radiograph shows a negative image of the bones

When an X-ray is needed for an organ e.g in the digestive system, the patient is given barium sulfate, this outlines the organ as it is a dense metal. This is known as a contrast medium. A lead absorber is used between the tube and patient to stop x-rays reaching other parts of the body.

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Most humans can hear frequencies between 20Hz and 20000Hz. Any frequency over 20000Hz is known as ultrasound. Ultrasound can be created by using an electronic system which is designed to vibrate and produce sound waves at any frequency above 20000Hz.

They are used in medicine for ultrasonic scanning, measuring the speed of blood flow in the body and the destruction of kidney stones. The ultrasound waves used to image babies and organs have a small amplitude so therefore a low energy. This makes it safer for the patient.

Ultrasound waves are partially reflected by body organs because they have a different density which makes a boundary that reflects it. To work out the distance you can use d = s x t. An ultrasound scanner is better than x-rays when looking at a body organ as they are non ionising. They are often used instead of x-rays for scanning for a baby as the babies cells are still developing and the ioninsing x-rays may disturb this.

Speed describes the amount of time taken for an object to cover a distance (s = d/t). This can be rearranged to find the distance (d = s x t). Using this we can use ultrasound when finding the distance from the bottom of a boat to rocks (sonar) and scanning for babies.

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Uses of Ultrasound

  • Medical scannning - Parental scans of a baby in the womb, used to see organs in the body. Scanner = transductor + control system and display screen. Transductor produces pulses of ultrasound reflected by dfferent tissues, Gel is used to exclude air and is non ionising.
  • Depth finding - Waves are beamed into the sea, these bounce back from any objects as the wave echo would be quicker returned if hitting something closer to the boat.
  • Removing kidney stones - Powerful ultrasound waves can be used to break a kidney stone into tiny bits. The fragments are small engough to leave the kidney naturally. The transmitter is used in an A scan system so the waves can be aimed precisely.
  • Cleaning - Generates waves at 40Hz creating air bubbles that pop and blast dirt away. Less cleaning equipment is used and no strong chemicals are needed.
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Refraction is the change in direction of a wave due to a change in material. A line perpendicular to the new surface is the normal. From the normal, the angle of incidence and refracted wave can be measured. Refraction is caused by the change in properties (density) of the new material. The wave is slowed causing it to change direction as part of the new wave reaches the new material first.

When light enters ray boxes it bends towards the nomal and as it emerges it bends away from the normal. The speed of light inside the glass is slower due to the denisty of the glass. The more light refracts the bigger a materials refractive index;

refractive index = speed of light in a vacuum/ speed of light in medium

White light splits in the prism to give the colours of the spectrum (red, orange, yellow, green, blue, indigo and violet). This effect is known as dispersion. This is because the different colours have different refractive indices due to their different wave lengths.

refractive index =  sin i / sin r

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Lenses have a range of uses and can be found in the eyes, glasses, microscopes, telescopes and cameras. The work by refracting light. There are two types of lenses which are commonly used convex and concave.

In concave lens the rays of light are refracted inwards and meet at the focal point. The image is always real.

In diverging lenses (convex) however the rays of light are refracted outwards and a virtual image is fomed.

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A real image is an image formed when light rays meet, these can be projected onto a screen.

A virtual image is an image seen in a lens or mirror from which light rays appear to come after being refracted by the lens/mirror.

An enlarged image is one which is larger than the original

A smaller image is smaller than the original.

An upright image is the same way up as the original

An inverted image is one which is the opposite way up to an object (upside down)

Magnification = image height / object height

Magnification is usually written with an 'x' in front of the number. The units for the image and the object height must be the same.

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The eye

  • Iris - coloured ring of muscle that controls the amount of light entering the eye
  • Eye lens - focuses light onto the back of the retina
  • Vitreous humour - transparent jelly like substance that supports the back of the eye
  • Retina - the light sensitive cells around the eye
  • Blind spot - region where the retina is not sensitive to light (no cells pesent)
  • Optic nerve - carries nerve impulses from the retina to the brain
  • Eye muscles - move eye in the socket
  • Ciliary muscles -attatced to the lens by suspensory ligaments, these change the thickness of the eye lens
  • Pupil - central hole formed by the iris, light enters through this
  • Cornea - transparent layer that protects the eye and helps to focus light onto the retina
  • Aqueous humour - transparent watery liquid that supports the front of the eye
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Correcting vision

Short sight is corrected by diverging lenses. Short-sighted people cannot focus on distant objects. Short sight can be caused by the eyeball being too long or by the cornea and lens system being too powerful, this means the eye lens can't produce a focused image on the retina where it is supposed to. Images of distant object are brought into focus in front of the retina. To correct short sight a diverging lens is needed. This diverges light before it enters the eye so the lens can focus it onto the retina.

Long sight is corrected with converging lenses. Long sighted people can't focus clearly on near object, this happens when the cornea and lens are too weak or the eyeball is too short. Images of near objects are therefore brought into focus behind the retina. A converging lens can be used to correct long sight as the light is refrated and starts t converge before it enters the eye.

Lasers can be used to surgically correct eye problems, these are narrow, intense beams of light. The light waves that cime from a laser all have the same wavelength, these can be used in surgery to cut through body tissue instead of using a scalpel. Lasers cauterise small blood vessels as they cut through the tissue, this means they burn and seal shut. This reduces the amount of blood lost and prevents inection. A laser can be used to vaporise some of the cornea to change its shape changing the focusing ability.

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Total internal reflection

When a light ray travels from air into glass and changes direction, it undergoes refraction. When a light ray inside glass reaches the surface with air and stays in the glass, it undergoes total internal reflection. When a light ray passes from glass to air some of the light undergoes partial reflection. When a light ray inside glass reaches the surface with air, it undergoes refraction and reflecion if the angle of incidence is less than the critical angle. The critical angle is the angle of incidence beyond which rays of light no longer refracted but totally reflected.

refractive index = 1 / sin critical angle

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Optical fibres

Optical fibres are used in endoscopes, they are long, thin, transparent rods made out of glass. Light is internally reflected from one end to the other, making it possible to send large chunks of information.

Optical fibres can be used for communications by sending electrical signals through the cable. The main advantage of this is a reduced signal loss and endoscopes use this principle.

Doctors use an endoscope to observe and treat the inside of a patients stomach. An endoscope consists of two bundles of optical fibres running alongside each other. One of which is to carry light into the body, the other is to carry light reflected off internal body surfaces back out to form an image.

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