L6: special techniques in forensic microscopy of trace evidence

Question 1

what a low magnification microscope and what the magnification?

Answer 1

stereo microscope x45 3D

Question 2

what a high magnification microscope and what the magnification?

Answer 2

compound microscope

Question 3

what are the 2 versions of a compound mciroscope?

Answer 3

Polarised

Phase-contrast

Question 4

when is a phased contrast microscope mainly used?

Answer 4

measurements on glass fragments.

Question 5

when is a polairisng microscope mainly used?

Answer 5

birefringence in fibres and other optically active materials.

Question 6

what lights does a stereo microscope have?

Answer 6

reflecting and transmission

Question 7

when is a microspectrophotometer used?

Answer 7

quantifying the colour of fibres, paints, inks and other evidence at a spatial resolution of 10 m.

Question 8

what determines refractive index?

Answer 8

electro density and bonding
material density
temperature
frequency of light wave

Question 9

what does a refraction describe?

Answer 9

change of direction of a wave-front which may occur when the wave moves from one medium to another

Question 10

synthetic fibres and glass have refractive indices between ?

Answer 10

1.3 - 2.3

Question 11

what does snells law define the relationship between ?

Answer 11

the relative refractive index and the change in direction of the wave motion at a boundary:

Question 12

what stem snells law equation?

Answer 12

n = sin I / sin R

Question 13

why does polarisation happen?

Answer 13

vibrations between the negative electrons and the positive nucleus

Question 14

how do light waves vibrate ?

Answer 14

in one direction or one mode, only.

Question 15

How to measure the RI of glass?

Answer 15

1. sample on a slide, drop of oil (of higher RI) on sample
2. 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

Question 16

what are the results of glass rI usually ?

Answer 16

0.0001 with a typical precision of 0.00005

Question 17

How to calculate RI of a fibre?

Answer 17

Tape lift or other extraction
Search for evidential fibres
Mount individually

Question 18

what can you see of fibres with a compound microscope?

Answer 18

Fibre diameter and variation
Fibre shape
Surface features
Internal detail
Colour
Fluorescent properties

Question 19

what does it mean when a material is optically anisotropic?

Answer 19

The refractive index depends on direction in the material.

Question 20

what is double refraction?

Answer 20

when light is split into two beams each polarised perpendicularly to each other.

Question 21

what is a birefringent material?

Answer 21

when double refraction occurs and one has a constant n and the other varied due to the change in direction.

Question 22

how Is birefringence determine numerically?

Answer 22

B= n par - n perp

Question 23

B of acrylic ?

Answer 23

0.001 - 0.005

Question 24

B of nylon 6 ?

Answer 24

0.049 – 0.061

Question 25

B of rayon ?

Answer 25

0.020 –0.028

Question 26

Oil immersion methods dissent always used to determine RI, why?

Answer 26

determine:

Birefringence using a polarising microscope

Question 27

what is the Retardation method ?

Answer 27

fibre is examined in cross-polarisers, rotated till brightest colour. thickness known, Michel levy chart used.

Question 28

what is Photoluminescence?

Answer 28

Emission of light by a substance when stimulated by incident ligh

Question 29

light emission decays _____ over a short time

Answer 29

exponentially

Question 30

what is Fluorescence

Answer 30

is whhen the emission stops when incident light is removed

Question 31

what is Phosphorescence ?

Answer 31

when emission persists for longer