# Physics P3.1

• 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