3. When observing a specimen under medium power, which adjustment knob should be used to focus the image? Explain why?
When observing a specimen under medium power, the coarse adjustment knob should be used to focus the image.
The coarse adjustment knob is used for initial focusing because it allows larger movements of the objective lens closer or farther away from the specimen, resulting in a more rapid change in focus. This knob moves the stage or the entire body of the microscope up and down, which translates to a quicker change in the distance between the objective lens and the specimen.
Medium power objectives have a relatively lower magnification than high power objectives, so their field of view is larger and allows for a greater depth of field. As a result, the specimen may already appear relatively focused under medium power when compared to high power. Therefore, the slightly larger depth of field makes focusing under medium power relatively easier.
However, fine adjustments to obtain maximum clarity and sharpness of the image are usually required before moving to higher magnifications. For accurate focusing and achieving the best possible image, the fine adjustment knob should be used while observing under higher magnifications. The fine adjustment knob moves the stage or objective lens very slightly, allowing for precise focusing and fine-tuning the image.