Spitfire canopy

I volunteer on an information desk in a military museum. A young visitor asked why Spitfires had a plastic rear canopy when it is patently obvious that a pilot would not be able to see through the armour plating behind his head and that there seems to be no equipment that would need daylight to operate. I can’t find any reference at all. The prototype Spitfire didn’t have one but the Mk 1 and subsequent marks did, apart from those fitted with the later large bubble canopy.
Any help gratefully received!