Physics P3.1

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  • Physics P3.1
    • X-Rays
      • Used in hospitals
        • To make images and CT scans
        • To destroy tumours at or near the body surface
      • Can damage living tissues as they pass through
      • Are absorbed more by bones than soft tissue
    • Ultrasound
      • Are sound waves of frequency above 20,000Hz
      • Used for diagnosis & treatment
      • Ultrasound waves are partly reflected at a boundary between two different types of body tissue
      • An ultrasound scan is non-ionising so it is easier than an X-ray
    • Refractive index (n)
      • Refraction is the change of direction of light as it passes from one transparent substance to another
        • Refraction happens because waves change speed when they cross a boundary
      • Is a measure of how much a substance can refract a light ray
      • n=sin i / sin r
    • The endoscope
      • The critical ange is the angle of incidence of a light ray in a transparent substance which produces refraction along the boundary
      • n= 1 / sin c
      • Total internal reflection occurs when the angle of incidence of a light ray in a transparent substance is greater than the critical angle
      • An endoscope is used to see inside the body directly
    • Lenses
      • A converging lens focuses parallel rays to a point call the principle focus
      • A diverging lens makes parallel rays spread out as if they came from a point called the principle focus
      • A real image is formed by a converging lens if the object is further away than the principle focus
      • Magnification = Image height / object height
    • Using lenses
      • A ray diagram can be drawn to find the position and nature of an image formed by a lens
      • A camera contains a converging lens that is used to form a real image of an object
      • A magnifying glass is a converging lens that is used to form a virtual image of an object
    • The eye
      • Light is focused on to the retina by the cornea and the eye lens, which is a variable focus lens
      • The normal human eye has a range of vision from 25cm to infinity
      • P= 1 / f
      • Short sighted
        • See near objects
          • Use diverging (Concave) lens to correct it
      • Long sighted
        • See distant objects
          • Use converging (Convex) lens to correct it


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