First is it might be because of the amount of light that passes through the lens. In this case, adjust the daiphgram to reach the right amount of light needed to properly observe the specimen. It can also be that, the specimen might be to too thick that the light cannot pass through it. In this situation, scrape the leaf to achieve the thinnest of the specimen that is enough for the light to pass through it. And she focused the specimen using a high power objective (She should only use the fine adjustment knob in high power).
SCENARIO 2:
If you turn the coarse adjustment knob while on high power, the objective could easily break your slide. You should never use the coarse adjustment when using high power. (The coarse adjustment knob should only be used with the lowest power objective lens. Once it is in focus, you will only need to use the fine focus. Using the coarse focus with higher lenses may result in crashing the lens into the slide.)
