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  • P3a
    • x-rays in medicine
      • x rays + photographicfilm
        • x rays interact with photophraphic film in the same way as visible light, the x ray image is formed by the varying intensities of x ray beam that are transmitted through to the film. where the rays are transmitted, more x rays react with the photographic plate causing it to turn dark. this leaves a white image of the denser areas
      • image formed electronically
        • a charge-coupled device can be used to form a high resolution image. A CCD is a grid of millions of X-ray sensitive pixels. each pixel generates an electric signal when it interacts with an x ray.
      • radiographers can wear lead aprons or stand behind a lead screen
      • to minimise patient's dose, minimise exposure time and avoid repeat x rays
    • ultrasound
      • When ultrasound waves reach a boundary between two substances with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through. A detector placed near the source of the ultrasound waves is able to detect the reflected waves. It can measure the time between an ultrasound wave leaving the source and reaching the detector. The further away the boundary, the longer the time between leaving the source and reaching the detector
      • generating ultrasounds of a particular frequency
        • electrical devices can be set to produce oscillations at a particular frequency, these can be converted into mechanical vibrations
    • refractive index = the ratio of the speed of light in a vacuum to the speed of light in that medium
    • lenses and images
      • to describe the nature of an image: size, real/virtual, inverted/upright
      • real: rays pass through object so can be projected, virtual, rays only appear to come from image
    • diverging lenses
      • always virtual, upright, smaller images
      • negative power
    • converging lenses
      • inverted and real - unless the object is at F or closer... gets larger as the object gets closer... gets further away as the object gets closer - unless the object is at F or closer
      • magnifying glasses
        • must be closer than F for image to be virtual and upright
      • positive power
    • correcting vision
      • short sightedness - eyeball too long, cornea and lens are too strong
    • laser- a narrow intense beam of light
      • cauterisation - burn and seal shut tissue (blood vessels)


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