Final answer:
The most likely reason the organism was not visible when switching to high power on a microscope is that the area of the slide being viewed becomes smaller, and not because the field became too bright or the image too small.
Step-by-step explanation:
When the student switched to high power, the organism was no longer visible. The most likely reason for this is because switching to high power made the area of the slide being viewed smaller. With higher magnification levels, like those used in microscopy, the working distance between the objective lens and the specimen decreases, leading to a significant reduction in the size of the field of view. This means you'll see a more zoomed-in section of the slide, but a smaller overall area. Since microscopes are designed to be parfocal, when switching from low to high power, you should not need to make major adjustments if the initial focus was set correctly; a small fine-tuning with the fine adjustment knob is all that's necessary for a clear image.