omi term 2 polarisation Flashcards
what is light
transverse electromagnetic wave
transverse vibration so within same beam of light vibrations in all directions exist
what is unpolarised light
light that vibrates in all directions
eg; naturally occuring
what is polarisation
the process of transforming unpolarised light into polarised state
wha happens when light is polarised
the waves in whcih vibrations occur are in a single plane
what does a polarised filter do
only let a certain direction of unpolarised light through to form polarised light
how does light become polarised
transmission, reflection,refraction
1st method of transmission (polaroid filter)
polaroid material serves as device that filters out one half of vibration
when unpolarised light is trasnmitted through polarised filter it emerges as polarised light and causes 50% reduction in light transmission.
2nd method of transmission (crystals of quinine sulphate pero)
crystals are mechanically alligned by stretching them along their axis causing them to become linearly polarised light
what does crystals mean
dichroic meaning light polarised parallell to it is transmitted
method 3 trasnmission (pva impregnated with iodine)
modern polarising filters made from stretched sheet of pva plastic impregnated with iodine which serves as electron donor
stretched hydrocarbon chains are though of like wires
wave oscillations set up an oscillation in electrons with wires which absorb wave energy in direction of wire
chain molecules are spaced at distance comparable to wavelenght of light
method 4 transmision (crossed polarisers
two filters with their axes of polaristaion at 90 degrees to each other
if 2nd polariser was less than 90 degrees from other axis of polarision then some light transmission which would increase as angle between polarising axis decreases
reflection polarisation
unpolarised light can be polarised by reflection off non metallic surfaces
metallic surfaces reflect light in variety of directions
extent of polaristion occuring is dependant on angle at whihc light approaches the surface as well as material that surface is made from
surfaces like roads snow and sea reflect light with high conc of vibrations in the plane parallel to reflecting surface
causes reflected light to be predominantly polarised
a light ray incident on refractive boundary will have small amount of light reflected and some refracted
when refracted and reflected ray are 90 degrees the reflected light will be linearly polarised
direction o fpolarised is parallell to interface and angle of incidence which causes this to occur is called brewster angle
what types of glare can eye suffer from
disability glare
acts as secondary light which scatters in eye producing a veiling glare over object of attention make its viewing more difficult
discomfort glare
brightness of glare causes light scattering in the eye which is uncomfortable for viewer
refraction polarisation
most material isotropic, same optical properties in all directions
some materials have degree of symmetry which means natural frequency of material can be different depending on axis
when refraction happens 2 linearly polarised rays are produces, this double refraction is birefringernce meaning two images seen
one polarised beam is in a direction parallel to surface and other perpindicular to surface
to block one of these images u can use polarising filter
birefringence in retina
retinal nerve fibre layer contains highly ordered axon bundles
in axons there re micro tubles which are parallel to each other
this makes rnfl birefringent-polar materials that can split into 2 components travelling at diff velocities
retardation is the diff in velcoities that creates shift in phase of rays
retardation proportional to thickness of rnfl
