Final answer:
The student likely damaged the slide by using the coarse-adjustment knob while utilizing the high power objective lens, which should be avoided as it can cause the lens to crash into the specimen. Only the fine adjustment knob should be used at high magnifications.
Step-by-step explanation:
The mistake the student most likely made was using the coarse-adjustment knob with the high power objective lens. When using higher power objective lenses, especially those with 40x magnification or greater, the working distance between the lens and the slide becomes very small. It is essential to use the fine adjustment knob for these higher magnifications to avoid contact between the lens and the slide, as this could damage both the lens and the specimen. Using the coarse adjustment knob at high magnifications can lead to rapid, large-scale movement that increases the risk of the lens crashing into the slide.
As part of microscope maintenance best practices, you should start with a lower power lens and use the coarse focus knob to get the specimen into general focus, and then switch to the high power lens. From there, only minor adjustments with the fine focus knob are needed to sharpen the image. This approach leverages the parfocality of the lenses and helps to protect the microscope from damage.