Optical minerology 2 - cross polarization and birefringence Flashcards
What is the effect of single and double refraction on an image?
single refraction will show a clear image behind
Double refraction- blurred double image
How will light react with isometric crystals?
light pass as single might refract regardless of orientation
Uniformity= isotropy/ isotropic
What are anisotropic crystals?
variability in crystal structure (non-cubic)
Non-uniform all directions
What can you call the uniformity or lack of in anisotropic crystals?
anisotropy
What will happen to light which passes through anisotropic crystals?
split into perpendicular lines (ordinary and extra-ordinary)
Why does twinkling occur in anisotropic crystals?
due to rapid change in relief as ordinary and extraordinary only permitted to vibrate in in E-W and N-S directions
What is the analyser on a microscope?
second polarizer at 90* to east west
What will be seen if an isotropic material or no material is placed between the 2 polarizers on a PPL/ XPL?
blackness
What is extinction when analysing anisotropic crystals?
This is when at a specific orientation when rotating (if it matches lines of ordinary or extraordinary) where crystals will be black/ reflect no light
How many times will extinction occur in a 360*?
4 times roughly every 90*
What is the general description of birefringence?
anisotropic highly coloured between extinctions
What will happen to wavelength if frequency reduces?
if will increase to maintain frequency
How will colours be presented for birefringence?
orders with each order containing a different band of colours
What does a bigger offset of wavelength/ velocity between ordinary and extraordinary mean?
brighter colours when they combine again
What is the name for the offset of wavelengths?
interference
What is destructive interference?
if offset by 1/2 when peaks and troughs line up (dark)
What will a constructive interference be like?
if offset when peak and trough line up (bright)
What creates the interference colours?
sum of colours that come about when white light passes through a mineral and creates a pattern of light and dark interference
What is retardation with with birefringence?
distance offset between the two rays
What is pleochroism?
the change in colour of a mineral seen in Plane polarized light when the mineral is rotated in regard to plane of polarization
What does PPL and XPL stand for?
PPL= plane polarized light
XPL= Crossed polarized light
