7.2 Notes

The Microscope continued

Objectives

Appreciate how a polarizing microscope is designed to characterize polarized light

Appreciate how a microspectrophotometer can be used to examine trace physical evidence

Compare and contrast the image formation mechanism of a light microscope to that of a scanning electron microscope

Outline some forensic applications of the scanning electron microscope

Microspectrophotometer

Combination of computerized spectrophotometer and microscope

Allows for viewing a small particle under the microscope

Allows for the a beam of light directed at the particle to obtain an absorption spectrum

Microspectro. cont.

Visual inspection of color is essential in examining paint, fiber, ink evidenceUsed to distinguish counterfeit and authentic currency by comparing spectral patterns of inked lines on currencyUseful in determining fiber and paint chip chemicals

Scanning Electron Microscope (SEM)

A beam of electrons on the specimen from a tungsten filamentElectrons are focused by electromagnetsThe upper layers of the specimen rebound electrons Rebounded electrons collected by a cathode ray tube or the TV

SEM cont.Emitted electrons are converted into an imageHigh magnification, high resolution, great depth of focus10x to 100,000 x

Can be used to determine elemental composition of a specimen by coupling with an X-rayCan be used to tell if a suspect has recently fired a gun