Physics Further

= X-Rays are high frequency, short wavelength electromagnetic waves.

- They pass through healthy tissue but are absorbed by denser materials like bothes and metal.

- X-Ray photographs can be used to diagnose many medical conditions
   → Bone fractures, dental problems

- X-Ray images can be formed electronically using charge-coupled deviced (CCDs)
   → CCDs are silicon chips about the size of a postage stamp, divided up into a grid of millions of identical pixels. 

- CCDs detect X-Rays and produce electronic signals
    → They form high resolution images. ( same tech used to take photographs in digital cameras)

- CT scans use X-Rays to produce high resolution images of soft and hard tissue. 

- Patient placed inside cylinder scanner
   → X-Ray beam fired through th ebody from an X-Ray tube 
       → Picked up by detectors on the opposite side.

- The X-Ray tube and detectors are rotated during the scan. 
   → A computer interprets the signals from the detectors to form an imagine of a 2D slice through        the body.

- Multiple 2D CT stans can be put together to make a 3D image of the inside of the body.

- X-Rays can cause ionisation.
    → Ionisation = High doses of X-Rays kill living cells

-They can therefore be used to treat cancers.
   → The X-rays have to be carefully focuses and at just the right dosage to kill the cancer cells without damaging too many normal cells. 

TO TREAT CANCER:
1) The X-Rays are focused on the tumour using a wide beam.
2) This beam is rotated round the patient with the tumour at the centre. 
3) This minimises the exposure of normal cells to radiation
     → So it reduces the chances of damaging the rest of the body.

- Prolonged exposure to ionising radiation can be very dangerous to your health.

1) Radiographers who work with X-Ray machines or CT scanners need to take precautions to minimse their X-ray dose.

2) They wear lead aprons, stand behind a lead screen, or leave the room while scans are being         done. 

3) Lead is used to shield areas of the patient's body that aren't being scanned
   →  the exposure time to the X-Rays is always kept to an absolute minimum. 

- Ultrasound is sound with a higher frequency that we can hear

- Electrical systems can be made which produce electrical oscillations of any frequency.

- These can easily be converted into mechanical vibrations to produce sound waves of a higher frequency than the upper limit of human hearing. 
    → This is called ultrasound. 

- Ultrasound waves get partially reflected at a boundary between media.

1) Partial reflection - When a wave passes from one medium into another, some of the wave is reflected off the boundary between the two media, and some is transmitted (and refracted). 

2) This means that you can point a pulse of ultrasound at an object
    → Wherever there are boundaries between one substance and another
        → some of the ultrasound gets relfected back.

3) The time it takes for the reflections to reach a detector can be used to measure how far away the boundary is. 
     → This is how ultrasound imaging works

- The oscilloscope trace below shows an ultrasound pulse relfecting off two separate boundaries.

- Given the "Seconds per division" setting of the oscilloscope
   → you can work out the time between the pulses by measuring on the screen.

- If you know the speed of the sound in the medium
   → you can work out the distance between the boundaries using the formula below: 

S = V x T

S is distance in metres, m.
V is speed in metres per second, m/s.
T is time in seconds, s.

Breaking down kidney stones
- Kidney stones are hard masses that can block the urinary tract
- An ultrasound beam concentrates high-energy waves at the kidney stone
    → Turns it into sand-like particles.
        → Particles pass out of body through urine.
            → Good method because patient doesn't need surgery, and painless.

Pre-Natal scanning of a fetus
- Ultrasound waves can pass through the body
    → But when they reach boundary between two different media 
        → Example: fluid in the womb and the skin of the fetus)
             → Wave gets reflected back and defected

- The exact timing and distriution of these echos are processed by a computer to produce a video image of a fetus. 

1) Ultrasound waves are non-ionising and safe.

2) X-Rays are ionising. They can cause cancer if you're exposed to too high a dose
    → Not safe to use on developing babies

3) CT scans use alot more X-Ray radiation than standard X-Ray photographs
     → Patient exposed to even more ionising radiation. 

Generally CT scans aren't takenunless they're really needed because of the increased radiation dose. 

1) Ultrasound images are typically fuzzy
    → This can make it harder to diagnose some conditions using these images. 

2) X-Ray photographs produce clear images of bones and metal, but not a lot else.

3) CT scans produce detailed images and can be used to diagnose complicated illnesses
     → As the high resolution imahes can make it easier to work out the problem. 

High quality 3D images can also be used in the planning of complicated surgery.

- Refraction is when waves change direction as they enter a different medium. 
   → This is caused by the change in density from one medium to the other 
        → Which changes the speed of the waves.

1) When waves slow down they bend towards the normal.

2) When light enters glass or plastic it slows down 
     → To about 2/3 of its speed in air.

3) If a wave hits a boundary at 90 degrees ( ie along the normal)
    → It will not change direction - but it'll still slow down

4) When light hits a different medium (e.g plastic or glass)
    → some of the light will pass through the new medium 
        → but some will be reflected, depending on the angle of incidence ( angle it hits medium)

- Refractive index of a medium is the ratio of speed of light in a vacuum to speed of light in that medium. 

- Angle of incidence, i, angle of refraction, r, and refractive index, n, are all linked.

- When an incident ray passes from air into another material
   → The angle of refraction of the ray depends on the refractive index of the material 

Refractive index (n) = Sin i / Sin r

- Lenses form images by refracting light and changing its direction.
- There are two main types of lens - converging and diverging. 
   → They have different shapes and opposite effects on light rays. 

- A converging lens is convex (outwards)
  → Causes parallel rays of light to converge (move together) at the principle focus.

- A diverging lens is concave (inwards)
  → Causes parallel rays of light to spread out.

- The axis of a lens is a line passing through the middle of the lens
- The principal focus of a converging lens is where rays hitting the lens parallel to the axis all meet
- The principal focus of a diverging lens is the point where rays hitting the lens parallel to the axis appear to all come from. 
    → You can trace them back until they all appear to meet up at a point behind the lens.

- There is a principal focus on each side of the lens.
  → The distance from the centre of the lens to the principal focus is called the focal length. 

1) An incident ray parallel to the axis refracts through the lens
    → and passes through the principal focus on the other side.

2) An incident ray passing through the principal focus refracts though the lens
   → and then travels parallel to the axis

3) An incident ray passing through the centre of the lens carries on in the same direction. 

1) An incident ray parallel to the axis refracts through the lens
  → travels inline with the principal focus 
      → So it appears to have come from the principal focus

2) An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis.

3) An incident ray passing through the centre of the lens carries on in the same direction.

1) A real image is where the light from an object comes together to form an imagine on a "screen"

2) A virtual image is when the rays are diverging
  → So the light from the object appears to be coming from a completely different place.

3) When you look in a mirror you see a virtual image of your face 
   → because the object appears to be behind the mirror.

4) You can get a virtual image when looking at an object through a magnifying lens 
   → The virtual image looks bigger than the object actually is. 

To describe an image properly, you need to say 3 things:
1) How big it is compared to the object
2) Whether it's upright or inverted ( upside down)
3) Whether it's real or virtual