Final answer:
To focus a specimen using a low power objective lens on a light microscope, initially use the coarse focusing knob to roughly focus and then the fine focusing knob for detailed focusing. Adjust the stage using mechanical knobs, and modify brightness using the condenser and iris diaphragm as needed. Microscopes with parfocal objectives maintain focus when switching between lenses of different magnifications.
Step-by-step explanation:
To focus a specimen under a light microscope using the low power objective lens, one must follow a series of steps. First, the specimen is placed on a glass slide and clipped onto the microscope's stage. The mechanical stage knobs are used to center the specimen over the light without altering the height of the stage. The focusing begins with the coarse focusing knob, which is suitable for large adjustments when using low power objectives like the 4x or 10x lenses. Only after the image comes into general focus should the finer details be adjusted with the fine focusing knob, which is more apt for high power objectives but can also be used to sharpen the image at low magnification.
As magnification increases with higher power objectives, the length of the lens becomes longer, and the aperture decreases in size, so adjustments to illumination via the condenser adjustment knob and the iris diaphragm may be needed to achieve optimal image brightness and clarity. This process is particularly important since, as the light passes through the specimen, its intensity can decrease, affecting the overall visibility of the specimen. Microscopes with objectives arranged in a way that keeps the sample in focus when switching between them are known as parfocal microscopes.