Physics

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  • Created by: Chesca
  • Created on: 14-05-13 18:54

X-Rays

  • They are Electromagnetic Waves
  • Produced when FAST MOVING ELECTRONS hit a target
  • To take X-Ray, X-Ray directed at the patient, flat panel detector placed on other side of patient
  • When tube ON, X-Ray passes through the patients body
  • Pass through Soft tissue, X-Ray Absorbed by BONES, TEETH and METAL OBJECTS
  • Bones = LIGHTERcolour, ray hasn't passed through
  • Tissue = DARKER, ray passed through
  • Contrast medium = absorbs X-Rays easily
  • Allows INTERNAL VIEW
  • Radiograph of stomach = BARIUM taken by patient
  • Lead absorber plates = between patient and tube, stop rays reaching other parts of body
  • Flat Panel Detector: small screen, contains CCD(charge-coupled device) converts X-Rays to light, so can be seen!
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x-rays

Danger!!!

X-radiation + Gamma radiation = ionise cells

High doses kill cells

Low doses cause cancerous tumours

People working with X-Radiation wear badge

X-RAY THERAPY!

  • Destroys cancerous tumours in body
  • Thick plates between x-ray tube and body stop x-rays reaching healthy body cells
  • Gap allows x-rays to reach tumour
  • 
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CT Scanner

Computerised Tomography Scanner

Digital image of cross-section of body

3-D image

Patient lies stationary in ring of detectors

X-Ray tube moves around ring

At each position, X-Rays pass through patient to ring detector

Electronic signals from detectors are recorded

Computer displays image

Detector signal depends on tissue type and distance travelled through tissue.

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Ultrasound Scanner

  • Human hearing range = 20 - 20,000 hertz
  • Anything higher = ULTRASOUND WAVE
  • Used for pre-natal scan of baby in womb, see organs
  • Transducer = produces/detects pulses of ultrasound waves
  • Each pulse = reflected from different tissue boundries, returns as sequence of reflected pulses
  • Build up image on screen
  • Advantages = NON-IONISING, can SCAN ORGANS and SOFT TISSUE
  • Oscilloscope = measure transit time of each pulse
  • Distance travelled by pulse = speed of ultrasound waves in body tissue X transit time
  • Depth of tissue = 0.5 X speed X transit time
  • Kidney stones removed via ultrasound.

                     

 

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Refractive Index

Refraction = change of direction of a light ray  when it passes across a boundry between two substances.

  • If incident ray at right angles to the surface (along normal) no change of direction will happen.
  • From glass to air  = angle of refraction is less than incidence
  • The GREATER the angle of INCIDENCE, the GREATER the angle of REFRACTION.
  • SIN I / SIN R
  • Relationsip between ^^ called SNELL'S LAW
  • When light ray travels from transparent surface into air at non-zero angle of incidence: light ray refracted away from the normal......larger the angle of incidence.....larger the angle of refraction.
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Lenses

Lenses: change direction of light passing through

Converging (convex) lens = makes parallel rays, that converge to a focus. Used as magnifying glass and in camera, to form clear images of distant objects

Focal point: where parallel rays meet

Diverging (concave) lens =makes parallel rays diverge (spread out). Point where rays appear to come from, forms principal focus

Focal Length: distance from centre of the lens to the principal focus

 

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Converging lens

Real image = formed on screen where light rays meet

To see object, beyond focal point, adjust till visibly clear

When object far away, image formed at principal focus

  • rays on object are parallel to each other when reach lens

Object moved near lens, towards PF, screen need to be moved away for clear image.

  • Nearer the object to lens = LARGER image

When object nearer to lens than FP, magnified image formed.

  • Image = VIRTUAL = formed where rays appear to come from (normally diverging lens)

 

 

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Magnification

  • MAGNIFICATION = IMAGE HEIGHT
  •                                       OBJECT HEIGHT

 

If image larger than object = mag greater than 1

If image smaller than object = mag less than 1

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Using lenses

Position + nature of image formed of lens depends on:

· Focal length

· Distance from object to lens

  • To form real image using converging lens, object = beyond principal focus.
  • Principal axis = straight line passing along normal at centre of lens
  • Image = real, inverted and smaller than object

The camera:

  • In camera converging lens produces real image of object on film, (or array of pixels).
  • Position adjusted to focus on image on film.
  • Distant object = distance from lens to film, must equal focal length of lens
  • Nearer object is to lens, greater distance from lens to film.  
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The Camera

The camera:

In camera converging lens produces real image of object on film, (or array of pixels).

Position adjusted to focus on image on film.

Distant object = distance from lens to film, must equal focal length of lens

Nearer object is to lens, greater distance from lens to film.  

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Virtual image

Converging lens = object between lens and principal focus.

  • image formed on same side as object
  • Image = virtual, upright, larger than object
  • Seen by looking through lens (magnifying glass)

 

Diverging lens = virtual, upright, smaller than object!

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The Eye

Cornea = light enters through (transparent layer, protects eye, focuses light onto retina)

Retina = light-sensitive cells at back of eye. (image inverted)

Iris = controls amount of light entering eye (adjusts size of pupil)

Eye lens = focuses light to give sharp image on retina

Ciliary muscles = changes thickness of eye lens

 

 

 

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How the eye works

  • Object far away:
  • Ciliary muslces alter thickness of eye lens
  • Attached by suspensory ligaments
  • Fibres of ciliary are parallel to circular edge of eye lens
  • When contract, shorten and squeeze eye lens, making thicker

Eye = range of vision from (near point) 25cm - (far point) infinity!!

Power of lens = 1/ focal length (m)

 

 

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Short sighted

When eye CAN'T focus on distant object

Image formed INFRONT of retina = eyeball too long / eye lens too powerful

Eye lens not thin (too thick!) enough

Corrected by diverging lens in front of eye

Diverging lens = counteracts 'excess' focusing power of eye lens

 

 

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Long sighted

Eye can't focus on nearby object

Image formed BEHIND the retina

Eye lens not thick enough to focus image on retina

Corrected by placing converging lens in front of eye

Makes rays diverge less

Adds to focusing power.

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Eye VS Camera

Eye = variable focus converging lens

Camera = fixed focus converging lens

Eye = Ciliary musucles alter thickness

Camera = Adjustment of lens position

Both = Real, inverted image, magnification less than 1

Eye = Detects image using retina

Camera = Photographic film

 

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Lenses at work

Used for contact lenses and glasses

Focal length depends on refractive index of lens material

Larger refractive index = greater power of lens

Greater refractive index = flatter and thinner lens

  • Surface less curved
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