Chapter 7 THE MICROSCOPE

Introduction A ____________is an optical instrument that uses a lens or a combination of lenses to magnify and resolve the fine details of an object. The earliest methods for examining physical evidence relied solely on the microscope. Magnifying glass- earliest and simplest microscope (single lens) How it works- the light rays are refracted(bent) in passing from the air to the glass and back to the air magnifies the image Various types of microscopes are used to analyze forensic specimens.

Basics of the Microscope ______________: The magnified image seen by looking through a lens; image cannot be seen directly ______________: whereas an image viewed directly by the naked eye The object to be magnified is placed under the lower lens, called the objective and viewed through the upper lens, called the eyepiece. Various types of microscopes are used to analyze forensic specimens. The compound microscope, the comparison microscope, the stereoscopic microcope, the polarizing microscope,the microspectrophotometer, and the scannin elctron micoscope

The Compound Microscope In the basic compound microscope, the object to be magnified is placed under the lower lens (objective lens) and the magnified image is viewed through the upper lens (eyepiece lens). The magnification of the image can be calculated by multiplying the magnifying power of the objective lens times the magnifying power of the eyepiece lens. The microscope is composed of a mechanical system which supports the microscope, and an optical system which illuminates the object under investigation and passes light through a series of lens to form an image of the specimen.

The Compound Microscope The _____________ System Base: the support. Arm: the C-shaped upright structure. Stage: the plate on which the specimens are placed. Body Tube: the hollow tube on which the objectives and eyepiece lenses are mounted. Coarse Adjustment: the knob used to focus the microscope lenses by moving the body tube. Fine Adjustment: the knob also used to focus the lenses by moving the body tube, but by a much smaller magnitude.

The Compound Microscope The ______________ System Illuminator: artificial light, usually supplied by a lightbulb, to illuminate the specimen. Transmitted Illumination: when the light is directed up through the specimen from the base. When specimen is _________________ Vertical or Reflected Illumination: when the light comes from above and reflects off the specimen. When specimen is ________________ Condenser: lens system under the microscope stage that focuses light onto the specimen.

The Compound Microscope The Optical System (continued) Objective Lens: the lens closest to the specimen; usually several objectives are mounted on a revolving nosepiece. Parafocal: when the microscope is focused with one objective in place, another objective can be rotated into place and the specimen remains very nearly in correct focus. Eyepiece or Ocular Lens: the lens closest to the eye. Monocular: a microscope having only one eyepiece. Binocular: a microscope having two eyepieces.

The Compound Microscope Magnification and Resolution 4x, 10x, 20x, 45x and combined with 10x Best first to select a low magnification to get a good overall view and then increase magnification to get more detail Field of view- the area of the specimen that can be seen after it is magnified Depth of focus- the thickness of a specimen that is entirely under a microscope http://micro.magnet.fsu.edu/primer/virtual/magnifying/index.html

The Comparison Microscope The comparison microscope consists of two independent objective lenses joined together by an optical bridge to a _______________eyepiece lens. When a viewer looks through the eyepiece lens of the comparison microscope, 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, with its ability to give the firearms examiner a side-by-side magnified view of bullets.

The Stereoscopic Microscope The stereoscopic microscope is actually two monocular compound microscopes properly spaced and aligned to present a three- dimensional image of a specimen to the viewer, who looks through both eyepiece lenses. It is particularly useful for evidence not requiring very high magnification (10x-125x). The most frequently used and versatile microscope in the crime lab Wide field of view and depth of focus- ideal for locating trace evidence in debris, garments, weapons, and tools Large working distance- ideal for examming big/bulky items Can be fitting with vertical illumination for diverse physical evidence- paint, soil, gunpowder residue, drugs, etc.

Polarizing Microscope Plane- polarized: Light that is confined to a single plane of vibration The examination of the interaction of plane-polarized light with matter is made possible with the polarizing microscope. Polarizing microscopy has found wide applications for the study of birefringent materials (materials that split a beam of light in two, each with its own refractive index value). The 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 Microspectrophotometer The microspectrophotometer is a spectrophotometer coupled with a light microscope. The examiner studying a specimen under a microscope can simultaneously obtain the visible absorption spectrum or IR spectrum of the material being observed. This instrument is especially useful in the examination of __________________________________________________ ________________________________

The Scanning Electron Microscope Finally, the scanning electron microscope (SEM) bombards a specimen with a beam of electrons instead of light to produce a highly magnified image from 100x to 100,0000x. Its depth of focus is some 300 times better than optical systems at similar magnification. The bombardment of the specimen’s surface with electrons normally produces X-ray emissions that can be used to characterize elements present in the material under investigation.

The Scanning Electron Microscope How it can be used An SEM image of a vehicle’s headlight filaments may reveal whether the headlights were on or off at the time of a collision