,BIOD 171 Lab Exam 3 | Essential Microbiology
w/Lab | Portage Learning | Q & A | 2026 Edition
1. Define the measurement of a micrometer.
A) 10⁻³ meters
B) 10⁻⁶ meters
C) 10⁻⁹ meters
D) 10⁻¹² meters
Correct Answer: 10⁻⁶ meters
Rationale: A micrometer is one-millionth of a meter, which is 10⁻⁶ meters. This is a standard unit of
measurement in microbiology for describing the size of bacteria and other small cellular structures,
distinguishing it from a nanometer which is 10⁻⁹ meters.
2. What are the two critical factors that influence your ability to see an object under a microscope?
A) Magnification and resolution
B) Resolution and contrast
C) Contrast and illumination
D) Magnification and illumination
Correct Answer: Resolution and contrast
Rationale: Resolution and contrast are the two key factors that determine how well you can visualize a
specimen. Resolution is the ability to distinguish two adjacent objects as separate, while contrast is the
difference in light absorbance between the specimen and the background.
3. Assuming a constant light source, which part of the microscope would you adjust to limit the amount
of light entering the eyepiece?
A) Condenser
B) Objective lens
,C) Iris diaphragm
D) Stage
Correct Answer: Iris diaphragm
Rationale: The iris diaphragm is the component that controls the amount of light passing through the
sample and into the objective lens. It can be opened or closed to alter the light intensity, which is crucial
for optimizing contrast and resolution.
4. What is the total magnification of a sample viewed with a 60x objective and a 10x eyepiece?
A) 60x
B) 100x
C) 600x
D) 6000x
Correct Answer: 600x
Rationale: Total magnification is calculated by multiplying the power of the objective lens by the power
of the eyepiece. Therefore, 60 multiplied by 10 equals 600x total magnification.
5. True or False: A cell that is adherent, flat, and unstained is easily identified using bright-field
microscopy.
A) True
B) False
C) True, but only if the cell is pigmented
D) False, because such cells are too large
Correct Answer: False
, Rationale: Adherent, flat cells are almost invisible under bright-field microscopy due to limits on both
resolution and contrast. Without staining or specialized techniques like phase-contrast, these cells are
very difficult to visualize.
6. Which of the following could NOT be seen clearly by the unaided eye? Select all that apply.
A) Skin cell (1500 μm)
B) Virus (0.2 μm)
C) Bacteria (24 μm)
D) Protozoa (150 μm)
Correct Answer: Virus (0.2 μm) and Bacteria (24 μm)
Rationale: The unaided eye can resolve objects larger than approximately 100 μm. Viruses (0.2 μm) and
bacteria (24 μm) are below this threshold. Protozoa (150 μm) and skin cells (1500 μm) are large enough
to be seen.
7. What is one limitation of fixing a sample for microscopy?
A) It makes the sample too dark to see
B) The heat-fixing procedure kills the specimen
C) It takes too long to perform
D) It causes the sample to shrink
Correct Answer: The heat-fixing procedure kills the specimen
Rationale: Fixing a sample, typically through heat, irreversibly kills the specimen. This prevents any
observation of motility, can distort cell shape and structure, and limits the types of analyses that can be
performed.
8. Phase-contrast microscopy provides what benefits to imaging?
A) It allows for the visualization of live, unstained cells with greater detail
B) It uses ultraviolet light to excite fluorescent tags
w/Lab | Portage Learning | Q & A | 2026 Edition
1. Define the measurement of a micrometer.
A) 10⁻³ meters
B) 10⁻⁶ meters
C) 10⁻⁹ meters
D) 10⁻¹² meters
Correct Answer: 10⁻⁶ meters
Rationale: A micrometer is one-millionth of a meter, which is 10⁻⁶ meters. This is a standard unit of
measurement in microbiology for describing the size of bacteria and other small cellular structures,
distinguishing it from a nanometer which is 10⁻⁹ meters.
2. What are the two critical factors that influence your ability to see an object under a microscope?
A) Magnification and resolution
B) Resolution and contrast
C) Contrast and illumination
D) Magnification and illumination
Correct Answer: Resolution and contrast
Rationale: Resolution and contrast are the two key factors that determine how well you can visualize a
specimen. Resolution is the ability to distinguish two adjacent objects as separate, while contrast is the
difference in light absorbance between the specimen and the background.
3. Assuming a constant light source, which part of the microscope would you adjust to limit the amount
of light entering the eyepiece?
A) Condenser
B) Objective lens
,C) Iris diaphragm
D) Stage
Correct Answer: Iris diaphragm
Rationale: The iris diaphragm is the component that controls the amount of light passing through the
sample and into the objective lens. It can be opened or closed to alter the light intensity, which is crucial
for optimizing contrast and resolution.
4. What is the total magnification of a sample viewed with a 60x objective and a 10x eyepiece?
A) 60x
B) 100x
C) 600x
D) 6000x
Correct Answer: 600x
Rationale: Total magnification is calculated by multiplying the power of the objective lens by the power
of the eyepiece. Therefore, 60 multiplied by 10 equals 600x total magnification.
5. True or False: A cell that is adherent, flat, and unstained is easily identified using bright-field
microscopy.
A) True
B) False
C) True, but only if the cell is pigmented
D) False, because such cells are too large
Correct Answer: False
, Rationale: Adherent, flat cells are almost invisible under bright-field microscopy due to limits on both
resolution and contrast. Without staining or specialized techniques like phase-contrast, these cells are
very difficult to visualize.
6. Which of the following could NOT be seen clearly by the unaided eye? Select all that apply.
A) Skin cell (1500 μm)
B) Virus (0.2 μm)
C) Bacteria (24 μm)
D) Protozoa (150 μm)
Correct Answer: Virus (0.2 μm) and Bacteria (24 μm)
Rationale: The unaided eye can resolve objects larger than approximately 100 μm. Viruses (0.2 μm) and
bacteria (24 μm) are below this threshold. Protozoa (150 μm) and skin cells (1500 μm) are large enough
to be seen.
7. What is one limitation of fixing a sample for microscopy?
A) It makes the sample too dark to see
B) The heat-fixing procedure kills the specimen
C) It takes too long to perform
D) It causes the sample to shrink
Correct Answer: The heat-fixing procedure kills the specimen
Rationale: Fixing a sample, typically through heat, irreversibly kills the specimen. This prevents any
observation of motility, can distort cell shape and structure, and limits the types of analyses that can be
performed.
8. Phase-contrast microscopy provides what benefits to imaging?
A) It allows for the visualization of live, unstained cells with greater detail
B) It uses ultraviolet light to excite fluorescent tags
