Which of the following is a common application of bright-field microscopy?

• A. Examining viruses directly
• B. Visualizing live, unstained motile protozoa
• C. Detecting fluorescently labeled molecules
• D. Observing stained bacterial cells

Answer: (D)

Verified step by step guidance

1. Understand the principle of bright-field microscopy: it uses visible light to illuminate the sample and produces a dark image against a bright background.
2. Recognize that bright-field microscopy is best suited for observing specimens that are naturally pigmented or have been stained to increase contrast, such as bacterial cells.
3. Note that viruses are generally too small to be seen directly with bright-field microscopy due to their size being below the resolution limit of light microscopes.
4. Understand that live, unstained motile protozoa are better visualized with phase-contrast or dark-field microscopy, which enhance contrast without staining.
5. Know that detecting fluorescently labeled molecules requires fluorescence microscopy, which uses specific wavelengths of light to excite fluorophores, not bright-field microscopy.