Shadows and eclipses result from the "rectilinear propagation of light." This means that light travels in a straight line, and when an opaque object is placed in the path of the light, it blocks the light rays and creates a shadow. Similarly, an eclipse occurs when an object such as the Moon passes between the Sun and the Earth, blocking the Sun's light and casting a shadow on the Earth. In both cases, it is the straight-line path of light that is responsible for the formation of shadows and eclipses. Therefore, the correct answer is "rectilinear propagation of light."