- The important concept to
understand the colored shadows is that light travels in straight lines. I'm gonna show that using this ruler. I'm gonna put the one end of the ruler at the center of the red bulb. It shows the light traveling
along one path to the screen. Of course, light from the red bulb hits everywhere on the screen
traveling in straight lines, however along this path,
if I put the pencil there, the pencil blocks the light traveling in a straight
line along the ruler, and makes a shadow on the wall. However, light from the blue bulb also travels in straight lines, and its straight line misses the pencil, and fills in the shadow
from the red light.