Final answer:
The fine adjustment knob on a light microscope is used to fine-tune the focus of the objective lens. However, it is best to first use the coarse focusing knob to quickly bring the sample into the general focus range.
Step-by-step explanation:
When you turn the fine adjustment knob on a light microscope, it moves the objective lens slightly up or down to bring the sample into focus.
However, it is best to adjust the focus using the coarse focusing knob first because it moves the stage or the objective lens much faster, allowing you to quickly bring your sample into the general focus range.
Once you have achieved a rough focus using the coarse adjustment knob, you can then fine-tune the focus with the fine adjustment knob. This knob moves the stage or the objective lens very slowly for precise focusing.