L6: special techniques in forensic microscopy of trace evidence Flashcards
what a low magnification microscope and what the magnification?
stereo microscope x45 3D
what a high magnification microscope and what the magnification?
compound microscope
what are the 2 versions of a compound mciroscope?
Polarised
Phase-contrast
when is a phased contrast microscope mainly used?
measurements on glass fragments.
when is a polairisng microscope mainly used?
birefringence in fibres and other optically active materials.
what lights does a stereo microscope have?
reflecting and transmission
when is a microspectrophotometer used?
quantifying the colour of fibres, paints, inks and other evidence at a spatial resolution of 10 m.
what determines refractive index?
electro density and bonding
material density
temperature
frequency of light wave
what does a refraction describe?
change of direction of a wave-front which may occur when the wave moves from one medium to another
synthetic fibres and glass have refractive indices between ?
1.3 - 2.3
what does snells law define the relationship between ?
the relative refractive index and the change in direction of the wave motion at a boundary:
what stem snells law equation?
n = sin I / sin R
why does polarisation happen?
vibrations between the negative electrons and the positive nucleus
how do light waves vibrate ?
in one direction or one mode, only.
How to measure the RI of glass?
- sample on a slide, drop of oil (of higher RI) on sample
- Phase-contrast microscope used with hot-stage and transmitted light
3.Oil RI reduces to that of the glass sample
4.white and then monochromatic light used
5.when rI’s equal, boundary invisible, oil temperature recorded.
6.value used to calculate the
exact RI of the oil
what are the results of glass rI usually ?
0.0001 with a typical precision of 0.00005
How to calculate RI of a fibre?
Tape lift or other extraction
Search for evidential fibres
Mount individually
what can you see of fibres with a compound microscope?
Fibre diameter and variation Fibre shape Surface features Internal detail Colour Fluorescent properties
what does it mean when a material is optically anisotropic?
The refractive index depends on direction in the material.
what is double refraction?
when light is split into two beams each polarised perpendicularly to each other.
what is a birefringent material?
when double refraction occurs and one has a constant n and the other varied due to the change in direction.
how Is birefringence determine numerically?
B= n par - n perp
B of acrylic ?
0.001 - 0.005
B of nylon 6 ?
0.049 – 0.061
B of rayon ?
0.020 –0.028
Oil immersion methods dissent always used to determine RI, why?
determine:
Birefringence using a polarising microscope
what is the Retardation method ?
fibre is examined in cross-polarisers, rotated till brightest colour. thickness known, Michel levy chart used.
what is Photoluminescence?
Emission of light by a substance when stimulated by incident ligh
light emission decays _____ over a short time
exponentially
what is Fluorescence
is whhen the emission stops when incident light is removed
what is Phosphorescence ?
when emission persists for longer
