microbio ch 2 microscopes

condenser

focuses light on specimen

resolution

minimum distance between two objects when those objects can still be observed as separate entities.

fine detail

refraction

the bending of light rays that occurs when light passes from one substance to another of a different refractive index

Refractive index

a measure of the relative velocity of light as it passes through a medium.

Note: oil and glass have almost the same refractive index.

Therefore, oil is used when viewing specimens at a high power to capture the greatest amount of light possible.

compound microscope

two lenses, the objective lens and the ocular lens

—The total magnification of the sample is determined by multiplying the powers of the two lenses.

• —Example: Ocular lens= 10X

•                 Objective lens= 4X

•                 Total Magnification= 10X ● 4X= 40X

Bright-field(compound light microscope)

light is used to illuminate specimen. Staining is required to provide contrast. Most common type of microscope.

cells are dead

Phase-contrast (light microscope)

amplified differences in refractive index to create contrast.

• unstained cells are visible.

• Living cells are made visible.

Interference (light microscope)

two light beams pass through the specimen and then recombine.

• This produces a 3D image.

• Living cells are made visible, unstained

• 2 beams-inter

Dark-field (light microscope)

light is directed toward specimen at an angle. The result is an unstained organism appears bright against a dark background.

• Living organisms are made visible.

• (Best for viewing Treponema pallidum- syphilis)

Fluorescence(light microscope)

UV light is projected through a specimen that is fluorescent (converts UV light into longer wavelength).

The fluorescent cells appear illuminated against a dark background. Note: this technique can be used to identify a specific organism.

Cells may be tagged by specific antibodies that fluoresce, distinguishing them from other cells in a sample.

Confocal (light microscope)

a laser beam passes through planes of a specimen. A computer constructs an image from data gathered.

• A 3D image is produced.

• This is useful for viewing layers of cells.

• multiple layers useful for slime layers, biofilms

Scanning electron

a beam of electrons is passed over the surface of metal covered cells.

This produces a 3D image of the surface of the cells.

Cells are dead

Transmission electron

cells are “thin sectioned”, a process in which the cell is preserved, embedded into a plastic resin, then sliced.

The slices are then exposed to a beam of electrons and an image is produced by the scattering of the electrons.

This is used to view fine details of cell structure (internal structures!)

Atomic Force Microscopy (electron)

produces a detailed image of the surface of cells with greater resolution than electron microscopes.

Does not require preparation of cells to be viewed.