Why does a microscope slide need to be transparent

For anything to be seen it must either emit light or reflect light. When you look at the functioning of a microscope you will notice that when you put specimen under the microscope, you need an external light source to illuminate the specimen. For this purpose a light source is placed right under the specimen holder.

For the specimen to be visible clearly the slide is made transparent so that it is backlit by the light and easily visible in the microscope.

The microscope slide needs to be transparent so it allows the light to pass through it, which causes the microscope to magnify the slide and the object on the slide. The light must come from beneath it in order for you to the object clearly.