Question

why does the sample on a microscope slide need to be very thin?
so it can fit on the stage of the microscope
so light can shine through it from below
so it can be seen with a diverging lens
so the eyepiece lens can be focused properly

Explanation:

Microscopes use light - based techniques. For light to pass through the sample and form an image, the sample must be thin. This allows light from the light source below the stage to shine through the sample and reach the objective and eyepiece lenses for magnification and visualization.

Answer:

so light can shine through it from below