Chapter 3 - Methods of Microbiology Flashcards
What microscope type is most commonly used for observing microbes?
A compound light microscope
In the compound microscope, the objective lens forms a magnified _____ _________ inside the tube of the microscope.
Real image
What lens of the compound microscope forms a magnified real image?
The objective lens
The ____________ lens magnifies the real image produced by the objective lens.
Ocular lens
Does the ocular lens produce a image?
No, it produces the illusion of an image, which we call a virtual image
The ultimate magnification of a compound microscope is the product of the individual magnifications produced by which two lenses?
The objective and ocular lenses
Which lens of the compound microscope do you look through?
The ocular lens
Resolution of the light microscope is normally limited to about ____ µm.
0.2 µm
What is resolution?
Resolution is the ability to distinguish between two closely spaced objects; it is the smallest resolvable distance between two objects
What is the limit of resolution (d)?
The distance separating the centers of two dots that can just be barely distinguished as separate
What two characteristics relate to the limit of resolution?
- The wavelength of the light being used (λ)
- The numerical aperture of the objective lens (NA)
What equation demonstrates the relationship between the limit of resolution, the wavelength of light used, and the numerical aperture?
d = 0.5λ/NA
What is the numerical aperature defined as?
The range of angles over which the system can accept or emit light
The numerical aperture is a function of two physical properties of the objective lens: _____________ and __________ __________.
- Aperture
- Working distance
What is the working distance of an objective lens?
How close to a specimen the objective lens must come for the specimen to be in focus
How is the numerical aperture quantitatively determined?
NA = n sinα
In the following equation, what does n represent?
NA = n sinα
The refractive index of the material in between the lens and the specimen, usually air
In the following equation, what does α represent?
NA = n sinα
α is one-half the objective’s opening angle
Because the physical properties of each lens are fixed and cannot be altered, the only way to improve resolution with a given lens is to increase what?
The refractive index of the material between the lens and specimen
A shorter _______ would increase resolution but hurt the eyes.
Wavelength
How does oil help with resolution of a microscope?
Oil has a higher refractive index than air, and thus it increases the resolution of an image
____________________ _____________________ is used for observing stained cells.
Brightfield microscopy
Under compound light microscopy, why does the specimen appear darker than the surroundings?
Because the specimen has absorbed some of the light
When a specimen is very small, like a microbe, it appears nearly translucent. Why is this?
Microbes are so small that they absorb very little light



