# P3.1 - Medical applications of physics

• P3.1 - Medical applications of physics
• Eye defects
• Long sighted -  eye ball too short can't focus on near objects
• Short sighted - eye ball too long & unable to focus on distant objects
• Glasses adjust light before it enters eye to allow eye to focus it properly
• Refraction of Light
• Interface  ( boundary between two transparent media of different densities)
• When light crosses an interface it changes direction
• No refraction occurs when light enters interface at a 90 degree angle (normal line)
• Refractive Index = sin (angle of incidence) / sin (angle of refraction)
• Camera and magnifying glass
• Unlike an eye they have fixed converging lenses with definite focal lengths
• Magnifying glass enlarges object size if distance between lens & object is less than focal length
• The image is: Virtual, upright & enlarged
• Camera film and CCD (charged coupled device) = retina in eye
• Convex lens magnifies is between focal point and centre of lens
• Reflection Of Light
• Normal line = perpendicular to reflecting surface at point of incidence
• Angle of incidence = Angle of reflection
• Power of a Lens
• Power in Dioptres = 1 / focal length in metres
• The power of a converging lens is positive (real focal point)
• The power of a diverging lens is negative (virtual focal point)
• Converging and Diverging Lenses
• A lens refracts light
• Converging Lens (convex lens) - thickest at centre
• Light's refracted inwards toward the focal point on opposite side of lens to the object and is real
• Diverging lens (concave lens) - thinnest in the centre
• focus appears to come from a point on same side as object and isn't real i.e. virtual
• Normal Line  in ray diagram = principal axis
• Distance between centre of lens and focus point = focal length
• For parallel rays of light the focal point lies on the principal axis
• The Critical Angle
• Two special cases involving refraction of light:
• Critical angle can be used to find refractive index: Refractive Index = 1 / sin (critical angle)
• 2) Angle of incidence is greater than critical angle. In this case total internal reflection occurs.
• No refraction occurs so no light escapes glass
• Total Internal Refraction: Angle of incidence = Angle of refraction
• 1) Angle of refraction = 90 degrees & light ray travels on boundary between air & glass. Then angle of incidence is called the critical angle
• Medical endoscopes use the principle of total internal reflection to see inside the body
• Visible light - optical fibres - operations & the light is returned to an eye peice or camera
• = when angle of refraction is 90 degrees from normal line
• Ultrasound
• Ultrasound=sound waves of frequencies > 20, 000 Hz = beyond limit of human hearing
• non-ionising
• As ultrasonic waves pass from one medium to another they partially reflect at the boundary
• Time taken for reflections how far away the boundary is
• distance in meters = speed in meters per second x half the time taken for the pulse to leave the source and return to the detector
• electrical oscillations are used to generate ultrasonic waves
• Uses: Pre-natal scanning, imaging damaged ligaments & muscles & kidneys and destruction of kidney stones
• The structure of the eye
• information about light is sent to brain via optic nerve
• Cornea refracts most of light
• Pupil (opening in iris) adjusts light intensity
• Lens provides further refraction before image forms on light - sensitive retina
• Ciliary muscle controls shape of lens. This allows light from different distances to be focused
• Near point -  aprox. 25 cm Far point - Infinity
• Images produced by lenses
• Size of image produced by convex or concave lens depends on distance of object from lens
• Convex lens: real, inverted, smaller for distant objects and magnified for objects between 'F' and '2F'
• Concave lens: Virtual (on same side as object) and upright ( not inverted)
• Lasers
• Intense narrow beam of light
• by stimulated emission of radiation
• Can be made from solids liquid & gases
• Used in eye surgery to: repair damaged retina's, remove diseased / damaged cells by cutting & burning tissue
• Optical fibres used to guide laser beams
• X-rays
• short wavelengths the same order of magnitude as diameter of atoms
• causes ionization when it hits an atom
• to see bone fractures, dental problems and computerised tomography (CT Scan)
• affect photographic film as light does
• Can be detected using charge-coupled devices (CCDs) to form image electronically
• advantages: transmitted by healthy tissue, absorbed by metal and bone to produce shadow pictures
• Personal radiation detectors, protective screening & protective clothing used to limit & monitor exposure
• Magnification and Focal Length
• Magnification = Image height / Object height
• A converging lens can be used as a magnifying glass
• Focal length of a lens is determined by: refractive index of material from which the lens is made and curvature of two surfaces of lens
• for a given focal length, the greater the refractive index, the flatter the lens