Physics Unit 3

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  • Created by: Chris
  • Created on: 16-05-13 09:48

X-rays in medicine

X-rays pass through healthy tissue, but are absorbed by things such as bones and metals. The X-ray photographs can be used to diagnose many medical conditions such as bone fractures or dental problems.

CT scans produce high resolution images of soft and hard tissue.

X-rays can be used to treat cancer:

1)The X-ray is focused on the tumour using a wide beam

2)This beam is rotated around the patient with the tumour in the centre 

3)This minimises the exposure of normal cells to radiation, and so reduces the chances of damaging the rest of the body.

Precautions have to be taken before this procedure such as wearing lead aprons and standing behind a screen or leave the room. Lead can also be used on a patients body as a shield to protect the other areas which are not being exposed.

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Electrical systems can be made which produce oscillation of any frequency. Higher frequency than the upper limit of human hearing = ultrasound

When an ultrasound wave is passed through a medium some of the wave is reflected and some is refracted. The time it takes for the reflections to travel back and reach a detector we can measure how far the boundries are away. This is called ULTRASOUND IMAGING.

We can measure ultrasound imaging by using this equation: DISTANCE = SPEED x TIME

Ultrasound has several uses:

Breaking Kidney Stones - kidney stones are hard masses that can block the urinary tract. Ultrasound waves are high-energy waves which can break the stones into sand-like particles. It does NOT need surgery and is PAINLESS.

Pre-natal Scanning - ultrasound waves can pass through the body, but whenever they reach a boundry and goes through a different medium part of the wave is partially reflected - and this can be detected. These 'echoes' can can be processed and create an image of the baby.

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Properties of the Waves

Are they safe?

Ultrasound - non-ionising and completely safe

X-ray - ionising and can cause cancer if exposed in high doses. Definitly should not be used to scan babies

CT scan - even more ionisinig than X-rays and should definitly NOT be used unless in extreme circumstances

Picture Quality?

Ultrasound - fuzzy images so is hard to diagnose a medical condition from them 

X-ray - clear images of the bone and metal but not a lot else as they transmit through soft tissues

CT scan - detailed images which can be used to diagnose complicated illnesses - as they are very high resolution. 3D imagese can also be created

The use of charge-coupled devices (CCDs) allow images to be formed electronically

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Refractive Index

Refraction is when waves change direction as they enter a different medium. This is caused by the change of density from ne medium to another.

When waves SLOW DOWN they bend TOWARDS the normal

When waves SPEED UP they bend AWAY from the normal 

You can calculate the REFRACTIVE INDEX by using the equation:

Refractive Index = Sin i(ncedent ray)/Sin r(efracted ray)

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Different Lenses

There are two types of lenses:

Convex/Converging lense - bulges outwards, causes rays of light to move together at a principle focus.

Concave/Diverging lense - caves inwards and causes rays of light to spread outwards. The principle focus is where the rays of light appear to be coming from.

When describing an image there are three things that you must state:

  • How big it is compared to the original image
  • Whether it is upright or inverted
  • Whether the image is real or virtual. A real image can be captured on a screen

The distance of the object from the focal point will change how the image is constructed.

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Magnifying Glasses work by the object being closer to the lens than the focal length. Since the image produced is virtual then the light rays don't actually come from where they are meant to be. Remember you can't project a virtual image onto a screen.

The magnification formula is:

Magnification = image height/object height 

A powerful lense has a short focal length, they length of the focal lenght is determined by:



To calcualte the power you use the formula Power (D) = 1/Focal Length


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Turning Forces and the Centre of Mass

Moment = Force x perpendicular distance from the line of action of the force to the pivot


To get the maximum moment you need to push at right angles - at any other angle it would be smaller so you would get a smaller moment.


The centre of mass is always vertically inder the point of SUSPENSION

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Parts of the eye

Cornea - The cornea is a convex shape with a high refractive index, this does the most of the eyes focusing.

Iris - The iris is the coloured part of the eye which controls the size of the pupil, so it controls how much light is let in.

Lens - The lens changes shape to focus light from varying distances. The lens is connected to the Ciliary Muscle. When the ciliray muscle contracts the lens takes on a fat spherical shape. When the ciliary muscles relax the suspensory ligaments pull the lens to a more thin flat shape.

Retina - The retina is where the images are formed

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