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

Answer 1

so light can shine through it from below

Explanation: For transmitted-light microscopes the specimen must be thin enough for light to pass through and form a clear image; thick specimens block or scatter light and blur overlapping structures.