Question

why should the fine focus knob typically be used with the 10x and 40x objective lenses?

the coarse focus knob is nonfunctional with the 10x and 40x objective lenses.

the slide should be close to in focus after focusing at the 4x objective. focusing with the coarse focus knob would result in too large a change in focus.

focusing using the coarse adjustment knob with the 10x or 40x objective may crack the stage.

Explanation:

Analyze the question and options

The question asks why the fine focus knob should typically be used with the \(10\text{X}\) and \(40\text{X}\) objective lenses.

Evaluate the options

• Option 1: "The coarse focus knob is nonfunctional with the \(10\text{X}\) and \(40\text{X}\) objective lenses." This is incorrect; the coarse focus knob still moves the stage, but using it at high magnification is unsafe and imprecise.
• Option 2: "The slide should be close to in focus after focusing at the \(4\text{X}\) objective. Focusing with the coarse focus knob would result in too large a change in focus." This is correct. Microscopes are designed to be parfocal, meaning once the specimen is in focus under low power (\(4\text{X}\)), it remains nearly in focus when switching to higher powers (\(10\text{X}\) and \(40\text{X}\)). Only minor adjustments with the fine focus knob are needed. Using the coarse focus knob at high power moves the stage too much, making it easy to lose the focus plane or crash the lens into the slide.
• Option 3: "Focusing using the coarse adjustment knob with the \(10\text{X}\) or \(40\text{X}\) objective may crack the stage." This is incorrect; it is the slide or the objective lens itself that is at risk of cracking, not the stage.

Apply mastered concept

Using the Focusing Mechanisms knowledge point

• Microscopes are parfocal, so switching objectives keeps the image nearly in focus.
• Coarse adjustment moves the stage too rapidly for high-power objectives.
• Fine adjustment allows precise, small-scale focal changes without risking slide damage.

Answer:

• (A) The coarse focus knob is nonfunctional with the 10X and 40X objective lenses.
• (B) The slide should be close to in focus after focusing at the 4X objective. Focusing with the coarse focus knob would result in too large a change in focus. (Correct answer)
• (C) Focusing using the coarse adjustment knob with the 10X or 40X objective may crack the stage.