microscopy chapter 6. objectives to be able to describe the light path through a simple lens to be...
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MicroscopyChapter 6
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Objectives• To be able to describe the light path
through a simple lens
• To be able to define a compound microscope and describe the light path through it
• To be able to name the parts of a compound microscope
• To be able to describe how a comparison microscope is constructed
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Objectives• To be able to describe how a stereo
microscope is constructed
• To be able to define plane polarized light
• To be able to describe how a polarized light microscope works
• To be able to describe how a scanning electron microscope works
• To be able to define and describe energy dispersive x-ray analysis
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Introduction
• The instruments you will encounter most often in a forensics lab is the microscope
• Most evidence is of the trace variety– There is a small amount of it so it must be
conserved
• Examination with a microscope does not destroy evidence
• Sometimes the only instrument needed is a microscope
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Types of Microscopes• The two major types of microscopes are
compound and electron microscopes
• Microscopes discussed in this chapter include:– Simple magnifier (magnifying glass)– Compound (basic, stereo, polarized light,
comparison, microspectrophotometer)– Electron
• Table 6.1 lists common evidence types and the microscopes that are used
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Lenses: How Objects are Magnified• The most simple type of all microscopes is
the simple convex lens
• Convex lenses bend (refract) light rays as they pass through the lens
• Light rays form a virtual image
• The shape of the lens determines the magnification
• The thicker the lens is in the middle, the higher the magnification– However, thickness causes distortion
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Lenses: How Objects are Magnified
• The practical limit of a single lens is 50x
• Improvements can be made by using two convex lenses– The first lens magnifies the object, the second
magnifies the virtual image
• The total magnification is the product of the magnification of each lens– 10x and 20x = 200x
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The Compound Microscope 1• A microscope made from two convex lenses
is called a compound microscope– Lens 1 = eyepiece, lens 2 = objective
• The evidence sits on the stage and a light source shines through the object
• The body tube is above the stage with the objective lenses mounted beneath it
• Most microscopes are parfocal - once an object is in focus, lenses can be changed and the object will remain in focus
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The Compound Microscope 2• At the top of the body tube is the eyepiece
(ocular lens)– Single lens = monocular– Two lenses (both eyes) = binocular– Three lenses (for photomicrographs) = trinocular
• A course and fine focus are used to focus the object
• The diaphragm is beneath the stage and controls the amount of light that reaches the object
• Filters can limit the wavelengths of light that reach the object
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The Compound Microscope 3
• Reflected light microscopes are used for opaque objects such as bullets– The light source is mounted above the stage
• The most important characteristics of compound microscopes are:– Magnification, resolution, field of view, and depth
of focus
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The Compound Microscope 4• Magnification: the product of the
magnification of the ocular and objective lens (up to 1000x)
• Resolution: The ability of a lens to separate details of an object into distinct images rather than one blurred image
• Field of view: How much of an object is visible at one time
• Depth of focus: how far inside the object the image will be in focus
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Microscopes Derived from CM’s• The compound microscope can be modified
in a number of useful ways to accommodate special circumstances
• Examples of modified compound microscopes include:– Comparison microscope– Stereo microscope– Polarized light microscope– Microspectrophotometer
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The Comparison Microscope
• Evidence often needs to be microscopically compared– Fired bullets, hairs, fibers, etc.
• The comparison microscope enables the examiner to view two objects, side-by-side, at the same time
• The comparison microscope consists of two compound microscopes connected with a comparison bridge
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The Stereo Microscope• Stereo microscopes typically have low
magnification (25-50x), the ability to manipulate the material, and the ability to see it in three dimensions
• It is the most versatile and commonly used in forensics labs
• It has a long working distance– Allows the examiner to get hands and tools under
the lenses
• Stereo microscopes consist of two monocular compound microscopes aligned with slightly different viewing angles to create a 3D image
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Scanning Electron Microscopy• Electrons microscopes use electrons
instead of light to magnify an image
• A scanning electron microscope (SEM) can magnify from 10 to 200,000 times
• A beam of electrons is aimed at the object and are backscattered
• Backscattered electrons are captured, amplified, and aimed at a cathode ray tube (TV) to create an image
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