Chapter 7 The Microscope

Physical properties of light

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

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

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

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

The Comparison Microscope

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

The Stereoscopic Microscope

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

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

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

Polarizing microscopy Polarized light

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

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

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

Polarizing microscope Polarizer Analyzer

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

Michel-Levy Chart of Birefringence

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

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

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

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

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

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

Plagioclase Feldspar showing albite twinning -used to determine Plagioclase composition