LECTURE 14: MICROSCOPY // CHAPTER 4 //

Question 1

Physical evidence

Answer 1

• volatiles – fire accelerants, explosives, pepper sprays, cosmetic-lubricants
• very small particles – gunshot residues
• non-volatile trace evidence – fibres, paint, rubber, plastic, metal, glass, soil

Question 2

Light microscopy

Answer 2

• light source
• sample holder
• magnifier/ lenses
• viewer
• series of lenses
• movement or switching changes magnification
• image viewed using eye pieces or cameria set-up
• different set-ups for different samples
• transmission
• reflectance
• brightfield
• darkfield
• phase contrast

Question 3

Some microscope terms

Answer 3

• depth of field
• field of view
• focal length
• working distance
• magnification
• resolution

Question 4

Stereomicroscope (dissecting scope)

Answer 4

Used in trace evidence for

• routine searching of items for trace evidence
• viewing layer sequence in paint chips
• manipulating and preparing small sample for subsequent analysis

Question 5

Compound microscope

Answer 5

-	higher mags than stereo
•	up to 1000x magnification 
-	very narrow depth of field 
•	limited to thin or narrow samples
-	good for fine detail
-	majority of use in PE is fibres
•	e.g. scales on wool fibre
•	delusterant on synthetic fibres

Question 6

Comparison microscope

Answer 6

• 2 compound microscopes
• viewing bridge adjustable
• allows for comparison ‘side by side’
• up to 1000x
• major of use in PE is fibres
  • comparison of recovered and control fibres side by. side

Question 7

Polarised light microscope (PLM or petrological microscope)

Answer 7

• use two polarising filters
• regularly-spaced, patterned or crystalline samples rotate the light
• some of this rotated light passes through the second polarising filter, so these regularly spaced areas show up coloured against a black background
• up to 500x mag
• difference in the way materials interact with the polarised light give changes in appearance
• in PE used for:
  • mineral ID
  • ID of fibre types
  • confirmation of glass fragments

Question 8

Scanning electron microscope (SEM) with energy dispersive X-ray spectrometry (EDS)

Answer 8

• Higher resolution •Higher magnification
  • up to 500,000x
• Secondary e’s -> topography
• Backscattered e’s -> relative elemental distribution
• X-rays characteristic of
• Grey scale images- NO colour
• More complex and expensive instrument than light microscopes
• Used for many substrates
  • Must be dry (no solvent or water)
  • Stable (most of our work done under high vacuum)
  • EDS- elements higher than elements

Question 9

compound magnification

Answer 9

magnification occurs in 2 stages & the total magnification is the product of the magnification of the 1st lens & the 2nd lens

Question 10

lens

Answer 10

is a translucent material that bends light in a known & predictable manner.

Question 11

The size & position of an image produced by a lens can be determined through geometry based on the

Answer 11

focal length

Question 12

focal length

Answer 12

the distance between the 2 points of focus on either side of the lens & determines much of the image quality.

Question 13

Refraction

Answer 13

occurs as light passes from 1 medium to another when there is a difference in the index of refraction between the 2 materials.

Question 14

Refractive index (RI) =

Answer 14

the relative speed at which light moves through a material with respect to its speed in a vacuum.