I thought that plexiglas blocked visible light, but on googling, that turned out to be wrong, as you can see from the quote:
"In colorless form, Plexiglas sheet is as transparent as
the finest optical glass. Its total light transmission is
92%, and its measured haze averages only 1%.
The wavelengths of visible light fall within the range
of approximately 400 to 700 nanometers in the elec-
tromagnetic spectrum. Electromagnetic energy from
the sun is rich in these wavelengths, tapering off in
the ultraviolet and infrared regions, as shown by the
relative energy distribution diagram in Figure 1.
When a light ray strikes colorless Plexiglas sheet
perpendicular to its surface (0° angle of incidence),
most is transmitted, some is reflected at both the top
and bottom surfaces, and a negligible fraction is
absorbed (Figure 2)."