When using a light microscope, what must be able to pass through a specimen in order for it to be viewed?

From Internet:

The functioning of the light microscope is based on its ability to focus a beam of light through a specimen, which is very small and transparent, to produce an image.

When using a light microscope, the specimen must be transparent or translucent in order for it to be viewed. This is because light needs to pass through the specimen to reach the objective lens and form an image. If the specimen is opaque and does not allow light to pass through, it will appear dark and no details can be observed.