Final answer:
It is best to use the coarse focus knob with a low power lens first due to the working distance and parfocality. This ensures smoother transitioning to higher magnification lenses, where only fine adjustments are needed, avoiding potential damage from lens contact with the slide.
Step-by-step explanation:
When focusing a light microscope, it's best to start with the coarse focus knob using a lower power objective lens, such as the 4x or 10x, before using the fine focus knob. This is due to two concepts: working distance and parfocality. The working distance decreases as the magnification increases, meaning there's less space between the objective lens and the specimen. Microscope objectives are designed to be parfocal, which means once you've focused at a low power, higher power objectives should be nearly in focus when switched to. This makes using the fine focusing knob sufficient for making small adjustments after using the coarse focus knob initially.
Using the coarse focus knob with higher magnification lenses, like 40x or greater, is cautioned against because it can result in the lens coming into contact with the slide, risking damage to both the specimen and the objective lens.