Final answer:
Giotto's work in the Arena Chapel reflects his pioneering use of a 'window' effect in painting, characterized by his foreshortening, chiaroscuro, and expressive figures, setting the stage for the Renaissance 'window on the world' concept.
Step-by-step explanation:
The painting by Giotto that demonstrates his pioneering use of a "window" effect in painting is likely one found within the Arena Chapel, also known as the Capella degli Scrovegni in Padua, Italy. This chapel is the location of Giotto's renowned fresco cycle.
The specific painting that shows this window effect could be referencing the overall design and spatial illusion of the chapel's frescoes, as Giotto's work does not contain a singular painting with this description unlike later artists such as Robert Delaunay with his Windows series.
Giotto is acclaimed for his use of foreshortening, chiaroscuro techniques, and the expressive figures that break with the Byzantine tradition of flat, idealized art.