Some comments have touched on the difficulty of cleaning plastic windows on observation hives.
Because plastic is easy to scratch (both on the inside where it gets gunked up and on the outside
where inevitably it gets scratched) I used laminated safety glass on the box I built.  Laminated
glass has the durability and ease of cleaning (scrape, then Windex or similar product, then rinse)
of glass, with the added advantage that if broken the pieces stay attached to the plastic inner
layer (like an automobile windshield).

Tempered glass, although stronger, when broken becomes a pile of "nuggets", leaving no barrier
between the bees and a classroom full of children.

For me the added sense of security was worth the additonal cost of the glass.