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36 Cards in this Set

  • Front
  • Back
Focal point
Light rays are focused at a specific place
Focal Length
Distance between the center of lens and the focal point
Short focal length?
More magnification
Refractive index
A measure of how greatly a substance slows the velocity of light.

When light passes from air to glass, it is bent and slowed
4 Major types of light microscopes?
1. Brightfield
2. Darkfield
3. Phase-contrast
4. Fluorescent
Microscope remains in focus when objectives are changed
Resolving power
Ability of a lens to distinguish small objects that are close together
Working distance
Distance betwee the front surface of lens and the surface of cover glass or specimen. (amount you can move the lens up and down)
Total magnification
Product of the ocular lens X the objective lens
What two factors determine the resolving power of a light microscope?
1. Wavelength of light (shorter=greater)
2. Numerical aperature
Which color of light would provide the best resolving power, blue or red?
What are the basic features of a darkfield microscope?
Produces a bright image of the object against a dark background. Better contrast between dark & light. Able to determine motility.
Is the resolving power of a darkfield microscope better than that of a brightfield microscope?
Yes, darkfield has better contrast
What are the basic features of a phase contrast microscope?
Enhances the contrast between intracellular structures having slight differences in refractive index. Excellent way to observe living cells. Able to determine motility.
Describe a fluorescence microscope.
Specimens usually stained with fluorochromes. Shows a bright image of the object resulting from the fluorescent light emitted by the specimen.
What kind of light is used in a fluorescent microscope?
Ultraviolet, violet, or blue light.
Why are microbiology specimens stained before viewing under a microscope?
-Increases visibility of specimen
-Accentuates specific morphological features (flagella so narrow you cant see them without staining)
-Preserves specimens
Heat fixing
Used routinely with procaryotes. Preserves overall morphology but not internal structures.
What is a differential stain?
Divides microorganisms into groups based on their staining properties.
Why is a differential stain used?
Important becase it allows you to distinguish between major groups of Bacteria.
Gram stain
Most widely used differential staining procedure. Divides Bacteria into two groups based on differences in cell wall strucutre. Gram staining is less useful for Archaea (many different cell wall structures).
Acid-fast Stain
Used to identifiy mycobacteria
Why is the acid-fast stain necessary?
Mycobacteria contains lipids within and so it doesn't take up other stains
Mycobacterium tuberculosis-causes tuberculosis

Mycobacterium leprae-causes leprosy
Negative staining
Used to visualize capsules surrounding bacteria. Capsules are colorless against a stained background.
Endospore staining
Double staining technique. Bacterial endospore is one color and vegetative cell is a different color.
Flagella staining
Mordant applied to increase thickness of flagella
Transmission Electron Microscope
Transmitted electrons (those that do not scatter) are used to produce an image. Denser regions in specimen, scatter more electrons and appear darker.
Why is the resolving power of the electron microscope greater than that of a light microscope?
Wavelength of electron beam is much shorter than light, resulting in much higher resolution.
How are specimens prepared for electron microscopy?
For transmission, specimens must be dried, embedded in plastic, cut very thin.
Specimens are chemically fixed and stained with electron dense material.
Coating specimen with a thin film of heavy metal
Freeze specimen then fracture along lines of greatest weakness
Scanning Electron Microscope
Uses electrons reflected from the surface of a specimen to create a 3D image.
Confocal Microscope
Confocal scanning laser microscope. Laser beam used to illuminate spots on specimen. Computer complies images created from each point to generate a 3D image.
Advantage of confocal microscopy?
Have extremely high resolution and can be used to observe individual atoms. 3D.