P3.1 - Medical applications of physics
- Created by: Fionnuala Smith
- Created on: 31-01-13 14:14
View mindmap
- 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
- Two special cases involving refraction of light:
- 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
- Time taken for reflections how far away the boundary is
- 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
- Intense narrow beam of light
- 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
- increased radiation=more detailed image
- 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
- Eye defects
Comments
No comments have yet been made