Final answer:
Light passing through a triangular aperture behaves like it does through a prism, getting dispersed into its constituent colors. The blue and yellow sections of a subtractive color wheel will appear blue and yellow, respectively, due to the absorption and reflection of specific wavelengths of light.
Step-by-step explanation:
When light passes through a triangular hole in a mask, it behaves in a similar way as it does when it passes through a prism. According to the information from Figure 5.9, as light enters one face of a triangular prism, it gets refracted and dispersed because different wavelengths of light bend by different amounts based on their properties and the material of the prism. This results in the separation of light into its constituent colors, with violet light being bent the most and red light the least. Thus, when mixed red, blue, and green light shines on the subtractive color wheel, the blue section will absorb red and green, reflecting blue light and therefore appearing blue (Part A). Likewise, the yellow section will absorb blue light and reflect red and green, which combine to form yellow light, so it will appear yellow (Part B).