Final answer:
The most useful concept for predicting the path of light in a block of glass or acrylic is understanding refraction, which is the bending of light when it passes through different materials due to changes in speed.
Step-by-step explanation:
The idea that is most helpful for predicting the path of light in a block of glass or acrylic is that light bends when passing through different materials. This concept is known as refraction. When light passes from one medium to another, such as from air to glass, it changes speed due to the differences in the optical density of the materials, leading to a change in direction. For instance, when light travels from air (where it travels faster) into glass (where it travels slower), it bends towards the normal (an imaginary line perpendicular to the surface). The degree of bending is determined by the respective speeds of light in the different media and their refractive indices.
It is important to note that the phrase "the speed of light is greater in medium 1 than in medium 2" relates to how light behaves when transitioning between mediums. If medium 1 is air and medium 2 is glass, the speed of light is indeed greater in air than in glass, resulting in light bending towards the normal as it enters the glass.