1. Chapter 7 The Microscope

2. Physical properties of light

3. The Compound Microscope
Schematic diagram of stereoscopic microscope
Two separate monocular microscopes
Each has own set of lenses except for the lowest
Objective lens common to both microscopes

4. The Compound Microscope
Condenser: lens system under the microscope stage that focuses light onto the specimen
THE MICROSCOPE

5. The Comparison Microscope
Two independent objective lenses joined together by an optical bridge to a common eyepiece lens.
The objects under investigation are observed side-by-side in a circular field that is equally divided into two parts
Modern firearms examination
Began with the introduction of the comparison microscope
Ability to give the firearms examiner a side-by-side magnified view of bullets
THE MICROSCOPE

6. Comparison Microscopes
Are used to compare two specimens
Consist of two compound microscopes connected by an optical bridge
Provide a single eyepiece through which the examiner sees both images side by side
Can be lighted from below the stage or via a vertical or reflected illumination system

7. The Comparison Microscope

8. The Stereoscopic Microscope
Two monocular compound microscopes
Three-dimensional image of a specimen
Use both eyepiece lenses
Evidence not requiring very high magnification (10x–125x)
Large working distance
Microscopic examination of big, bulky items
THE MICROSCOPE

9. The Stereoscopic Microscope

10. Double-refraction in Calcite:
-the “secret” key to PLM

11. Polarizing microscopy
Birefringence
Calcite produces two images when it is placed over the blue pencil. One of the images appears normally as when viewing an object through clear glass. The other pencil image appears displaced, due to the nature of doubly-refracted light. When anisotropic crystals refract light, the resulting rays are polarized and travel at different velocities. One of the rays travels with the same velocity in every direction through the crystal and is termed the ordinary ray. The other ray travels with a velocity that is dependent upon the structural properties of the crystal

12. f = V/w
f-frequency
V-velocity of light
w-wavelength

13. Polarizing microscopy
Polarized light

14. Polarizing Microscopy
Plane-polarized
Light that is confined to a single plane of vibration
Polarizing microscope
Examination of the interaction of plane-polarized light with matter
THE MICROSCOPE

15. Polarizing Microscopy
Wide applications for the study of birefringent materials
Materials that split a beam of light in two
Each with its own refractive index value
Determination of these refractive index data
Provides information that helps to identify minerals present in a soil sample or the identity of a man-made fiber.
THE MICROSCOPE

16. Polarizing Microscopes
Include two polarizing filters, a polarizer lens (fixed below the specimen), and an analyzer lens (fixed above the specimen)
The stage with the sample is rotated to determine how the polarized light interacts with the sample

17. Polarizing microscope
Polarizer
Analyzer

19. Polarizing Microscopes
This can provide information about the shape, color, and size of minerals and it is used to identify hair, human-made fibers and paint.
Hair Sample:
Polarized Light
Hair Sample:
Natural Light

20. Michel-Levy Chart of Birefringence

21. f = V/w f-frequency V-velocity of light w-wavelength
f is nearly always constant, therefore is V changes, w must change as well.
n (Refractive Index) = c/v
c-speed of light in vacuum

23. When light crosses boundary of 2 materials with
different refractive indices [n], it is bent

24. Polarizing microscopy
Analyzer out of path = plane polarized light; gives specimen’s “true color”
Analyzer in path = crossed polars; gives specimen’s interference color
Polyester,
plane-polarized light
Polyester,
under crossed polars

25. Polarizing microscopy
Improves specimen contrast
Stained plant fibers, left – stained; right – polarized light

26. Polarizing microscopy
Many crystalline substances are birefringent
Forensic analysis of soil components
Many synthetic fibers are birefringent

28. Refractive Index by the Becke Line Method
Note: when refractive index of
mineral & liquid are close,
relief if low.

29. Plagioclase Feldspar showing albite twinning
-used to determine Plagioclase composition