Microbiology: Lab Review Chapter 3

Chapter 3 Laboratory Review Questions

A few ideas to get down before answering this lab review.

Ocular lens/Eyepiece - Lens of the eyepiece, usually 10X in our case.

Objective lens - Three lenses on the nose piece, 4X, 10X,40X, 100X. Light goes up through the objective lens into the ocular lens. 

Condenser - Gathers light from the microscope light source and concentrates it into a cone of light that illuminates the specimen with uniform intensity over the entire view field.

Rheostat - Controls the intensity of the light produced.

1) [CH-03-01] Why is the light microscope described as compound microscope?

The light microscope uses multiple lenses together to magnify a image. In the case of our microscope in the lab: one for the optical lens and one for the objective lens. These multiple lenses are where the compound: composed of one or more parts, comes from. 

2) [CH-03-02] How is the orientation of the letter "e" changed by the lenses of the microscope?

The orientation of the letter "e" is reflected over the x and y axis.

3) [CH-03-03] Explain the differences between magnification and resolving power of microscope?

Magnification is how much an image is dilated (enlarged) from its original.

Resolving power is the ability of an optical instrument or type of film to separate or distinguish small or closely adjacent images.

4) [CH-03-04] What is meant by the limit of microscope's resolution? Which objective has higher resolving power, 40X or 100X?

Limit of a microscope's resolution is the smallest distance between two objects that a microscope is still able to identify two objects as separate objects. Thus the lens with the greater magnification has a high resolving power, in this case the 100X.

5) [CH-03-05] Why do microscopists always start with the low magnification lens?

The low magnification serves as a "scanning lens" that allows people to find the object that they are looking for. Once the image is centered, higher magnification lenses can be used. 

6) [CH-03-06] Compare the function of the brightness control with the iris diaphragm?

Brightness or intensity of the light is controlled by the rheostat, while the amount of light that is allowed to strike the stage is controlled by the iris diaphragm.

7) [CH-03-07] Explain the term parfocal. Which objective produces a larger field of view, 10X or 40X? 

Parfocal focusing on the image in the center plane so that the same image can be reacquired quickly after switching lenses. Sample kept in focus since object is on same focal plane.

The 10X objective produces a larger field of view because of its low magnification.

8) [CH-03-08] Calculate the limit of resolution for yellow light (wavelength 550nm) and blue light (wavelength 450nm). Which light produces superior resolution?

The formula for resolution is D = λ/(NAcondenser + NAobjective) or ≈ D = λ/2. Where λ is wavelength.
For Yellow light resolution: D = λ/2, D = (550*10^-9)m/2 = (2.75*10^-7)m
For Blue light resolution: D = λ/2, D = (450*10^-9)m/2 = (2.25*10^-7)m

Blue light has a smaller resolution limit, making it the super resolution.

9) [CH-03-09] If all other variables remain constant, explain why light of shorter wavelengths will produce a clearer image than light of longer wavelength?

As demonstrated by the formula above, light of shorter wavelengths are able to have smaller resolution limits, and therefore better resolution. Smaller wavelengths are able to produce higher resolution images (e.g. comparison between light microscope and electron microscope).

10) [CH-03-10] Why should closing the iris diaphragm in light microscope improve your ability to see more details in the resulting image?

Closing the iris diaphragm decreases the amount of light hitting the specimen on the stage and increases the contrast between the specimen and its surroundings. Thereby, making the specimen easier to make out.