Chapter 4: Microscopy

Astigmatism

a.k.a. spherical aberration -- results from a lens not being properly spherical.

Infinity Corrected Lens

Produce very high- quality images and allow for the addition of a variety of analytical components to the microscope.

Birefringence

The result of this division of light into at least two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material, depending on the polarization of the light.

Numerical Aperture

An angular measure of the lenss light- gathering ability and, ultimately, its resolving quality.

SEMs

are used in forensic laboratories to analyze a wide variety of samples, including paint, particles, fractures, toolmarks, and gunshot residue.

Objective Lens

The most important part of the microscope.

BSEDs

assist analysts in detecting materials of interest, like gunshot residue particles.

Apochromats

Contain several internal lenses that have different thicknesses and curvatures in a specific configuration unique to .

Lens

A translucent material that bends light in a known and predictable manner.

Analyzer

Located above the objectives, it can be manually slid into or out of the light path.

It is a device used to determine whether the light is plane polarized or not.

Auguste Michel Lévy

a French geologist, was born in Paris and became inspector-general of mines and director of the Geological Survey of France.

Compound microscope

Employs a magnification system that exceeds the limits imposed by simple lenses.

Critical Illumination

Concentrates the light on the specimen with the condenser lens; this produces an intense lighting that highlights edges but may be uneven.

Regular achromats

lack correction for flatness of field, but recently most manufacturers have started offering flat- field corrections for achromat objectives, called plan achromats.

microscope stage

The is the platform where the specimen sits during viewing.

simple lens

A focuses a flat specimen on a microscope slide onto the lens, a rounded surface.

transmission electron microscope

In , the electron beam passes through a specimen that has been very thinly sectioned and projects the beam onto a specially treated plate that transmits the image to a monitor.

Refractive Index

The relative speed at which light moves through a material with respect to its speed in a vacuum.

Wavelength dispersive spectrometer

Measures the wavelength of the emitted radiation using a tightly spaced crystal lattice.

Focal Length

The distance between the two points of focus on either side of the lens.

Simple magnification system

a single lens used to form an enlarged image of an object

Compound magnification system

Where magnification occurs in two stages and the total magnification is the product of the magnification of the first lens and the second lens

Tube length

The distance from the lowest part of the objective to the upper edge of the eyepiece.

Condenser

Used to obtain a bright, even field of view and improve image resolution

Field diaphragm

A control that allows more or less light into the lens system of the microscope

Köhler illumination

Sets the light rays parallel throughout the lens system, allowing them to evenly illuminate the specimen

Polarizing light microscope

Exploits optical properties of materials to discover details about the structure and composition of materials, and these lead to its identification and characterization

Isotropic Materials

Demonstrate the same optical properties in all directions, such as gases, liquids, and certain glasses and crystals

Interference Colors

The wavelengths being added and subtracted through interference; these colors are caused by the interference of the two rays of light split by the anisotropic material interfering destructively with each other

Michel-Levy Chart

a chart of diameter, birefringence, and retardation

Back-scattered electron detector

Detects elastically scattered electrons.

Energy-dispersive spectrometer

A used technique for elemental analysis and chemical composition determination, associated with electron microscopy.

Virtual Image

The observer looks at the first image with a lens that produces an enlarged image called a _.

Real image

the one that could be seen on the screen—that is, projected onto the screen.

Lens

a translucent material that bends light in a known and predictable manner.

Eyepiece / ocular

the lens that the observer looks into when viewing an object microscopically.

Chromatic Aberration

where white light from the specimen is broken out into multiple colored images at various distances from the lens.

curvature of the field

a rounded surface wherein a simple lens focuses a flat specimen on a microscope slide onto the lens.

plain achromats

Regular achromats lack correction for flatness of field, but recently most manufacturers have started offering flat-field corrections for achromat objectives, called _.

Refraction

occurs as light passes from one medium to another when there is a difference in the index of refraction between the two materials, and it is responsible for a variety of familiar phenomena such as the apparent distortion of objects partially submerged in water.

Polarizing light microscope

exploits optical properties of materials to discover details about the structure and composition of materials, and these lead to its identification and characterization.

Polarizer

a special filter used when the light passes through it, the only light that passes is that which vibrates in that “preferred” direction.

Phosphorescence

Long-lived emission.

Fluroscence

This is where the emission stops when the excitations stops.

Electron microscopy

employs a particle beam of electrons focused on magnetic lenses. It does have a much higher resolving power and greater depth of field than light microscopes and can magnify a specimen hundred of thousands of times.