Question 1

Which of the following best explains why phase-contrast microscopy is superior to brightfield microscopy for observing live, unstained cells?

Accepted Answer

It converts small differences in refractive index into differences in brightness, allowing visualization of internal structures without staining.

Question 2

A student observes a microorganism under a microscope. The organism appears to jiggle in place but does not move in any particular direction. What is the most likely explanation for this observation?

Accepted Answer

The organism is displaying Brownian movement, caused by water molecules striking it.

Question 3

Which type of microscopy would be most appropriate for visualizing Treponema pallidum, a spirochete that is difficult to stain?

Accepted Answer

Darkfield microscopy

Question 4

In the context of microscopy, what does the term 'resolution' refer to?

Accepted Answer

The ability to distinguish two points as separate entities.

Question 5

A student prepares a wet mount of a hay infusion and observes many organisms moving in different directions, some spinning or rolling. What is the best conclusion?

Accepted Answer

The organisms are truly motile, moving independently and in various directions.

Question 6

Why is it important to avoid using the 100× oil immersion lens with a hanging-drop slide?

Accepted Answer

The oil can dissolve the petroleum jelly seal, causing the drop to fall.

Question 7

Which of the following best describes the main advantage of the hanging-drop technique over the wet-mount technique?

Accepted Answer

It provides a freely suspended drop, reducing evaporation and fluid currents, making it easier to observe true motility.

Question 8

Which of the following equations correctly represents the relationship between resolution ($$d$$), wavelength ($$\lambda$$), and numerical aperture ($$NA) in $$light microscopy?

Accepted Answer

$$d = \frac{\lambda}{2NA}$$

Question 9

A student is using a phase-contrast microscope and notices that some structures within the cell appear brighter than others. What physical property of the cell is responsible for this difference?

Accepted Answer

Differences in refractive index of cellular components

Question 10

Which of the following best distinguishes true motility from Brownian movement in a wet mount preparation?

Accepted Answer

True motility is characterized by directed movement, often in different directions, while Brownian movement is non-directional jiggling.

Question 11

During the preparation of a wet mount, why is it important to handle the coverslip by its edges and press it gently onto the drop?

Accepted Answer

To prevent contamination and avoid breaking the coverslip, ensuring an even spread of the sample.

Question 12

A student observes a sample under darkfield microscopy and sees bright, spiral-shaped organisms against a dark background. Which property of darkfield microscopy allows this visualization?

Accepted Answer

It blocks direct light and only allows light scattered by the specimen to enter the objective lens.

Question 13

Which of the following best explains why phase-contrast microscopy can reveal internal structures in live cells without staining?

Accepted Answer

It collects direct, reflected, refracted, and diffracted light to generate differences in brightness based on refractive index.

Question 14

A student is preparing a hanging-drop slide to observe Bacillus motility. Which of the following steps is unique to the hanging-drop technique compared to the wet-mount technique?

Accepted Answer

Applying petroleum jelly to the coverslip to create a seal.

Question 15

Which of the following best describes the effect of increasing the numerical aperture ($$NA) of a $$microscope objective on resolution?

Accepted Answer

Increasing $$NA$$ decreases the resolution distance $$d$$, allowing finer details to be distinguished.

Question 16

A student is asked to distinguish between reflection, refraction, and diffraction of light in microscopy. Which of the following correctly matches each process with its description?

Accepted Answer

Reflection: light bounces off a surface; Refraction: light bends as it passes through a medium; Diffraction: light spreads after passing through a small opening.

Question 17

A hay infusion incubated in the light for one week is compared to one incubated in the dark. Which of the following is the most likely result?

Accepted Answer

The light-incubated infusion will have more photosynthetic organisms such as algae.

Question 18

Which of the following best explains why it is important to adjust the iris diaphragm when switching from 10× to 40× objective during wet-mount observation?

Accepted Answer

To increase the amount of light and improve contrast at higher magnification.

Question 19

A student observes a sample under phase-contrast microscopy and notes a glowing, three-dimensional appearance of the cells. Which optical phenomena contribute to this effect?

Accepted Answer

A combination of direct, reflected, refracted, and diffracted light

Lab 2: Examination of Living Microorganisms: Microscopy Techniques and Motility

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