Final answer:
A blue object under white light viewed through a yellow filter appears black because the object reflects no light that can pass through the filter.
Step-by-step explanation:
When a blue object is illuminated with white light and viewed through a yellow filter, the object will appear black. This is because the blue object absorbs all colors except blue; it reflects blue light. However, the yellow filter only allows yellow light to pass through it. Since the blue object does not reflect yellow light, and there is no blue light after the yellow filter to reflect, no light will be reflected from the blue object to the viewer's eyes, and the object will appear black.