Week 3 - Microscopes Flashcards
How is light treated?
As a wave
How does light propagate in the wave model?
As electromagnetic waves that oscillate at right angles to the direction of propagation.
What causes colour effects in thin sections of minerals?
Absorption of specific wavelengths or dispersion of white light due to refraction or diffraction.
Define refraction.
The bending of light as it enters a new medium.
What is diffraction of light?
When light waves pass through an obstacle or a slit.
What is polarization of light?
When light waves vibrate in a defined direction.
What is plane polarized light?
Light consisting entirely of waves vibrating in a single direction (E-W)
What happens when two light waves interfere?
They can increase, decrease in intensity, or cancel each other out completely.
Why does light split into colours when passing through a prism?
Different colours bend differently, causing white light to separate.
What type of light is used to measure the refractive index?
Monochromatic light.
What distinguishes isotropic minerals?
Optical properties are identical in all directions, so in PPL they will be visible but in XPL they will be OPAQUE)
How many refractive indices do isotropic minerals have?
One
How do anisotropic minerals affect light?
They split light into two rays with different velocities and vibrations at right angles.
What is pleochroism?
The change in colour of a mineral when the stage is rotated
What is mineral habit?
The shape of a mineral (euhedral, subhedral, or anhedral).
Define cleavage in minerals.
The tendency of a mineral to break along flat, even planes.
What is relief in mineralogy?
The difference between the refractive index of a mineral and the surrounding resin.
Give examples of minerals with high relief.
Garnet and olivine.
What is alteration in minerals?
The reaction of a mineral to produce a new mineral, often due to water interaction.
How do isotropic minerals behave in plane polarized light (PPL)?
They show the same colour when the stage is rotated.
What causes interference colours in XPL?
Light splitting into two rays when passing through anisotropic minerals.
What is birefringence?
The difference between the refractive indices of the fast and slow rays
Define retardation in minerals.
The distance the fast ray travels before the slow ray emerges.
How is retardation calculated?
Thickness × birefringence.
What are extinction angles?
The angles at which an anisotropic mineral appears dark under XPL.
What is twinning in minerals?
Different extinction orientations within the same mineral grain.
What is zoning in minerals?
Compositional variations within a mineral as it grew.
What causes double refraction in anisotropic minerals?
Light splitting into fast and slow rays due to non-uniform crystal structure.
What is the difference between the fast ray and the slow ray?
The fast ray has a lower refractive index and higher velocity; the slow ray has a higher refractive index and lower velocity.
What do we need to describe in PPL of a mineral?
CRYSTAL SHAPE
SIZE
PERCENTAGE
TRANSPARENCY
COLOUR
PLEOCHROISM
CLEAVAGE?
FRACTURE?
INCLUSIONS?
RELIEF
REFRACTIVE INDEX
What do we need to describe in XPL of a mineral?
ISOTROPIC OR ANISOTROPIC?
MAX INTERFERENCE COLOUR
BIREFRINGENCE
EXTINCTION
TWINNING
ZONING
