Light
Objects that block rays of light can create shadows. Opaque objects block all light rays and create a darker shadow, translucent objects block some rays and scatter others.  They create a lighter shadow. Transparent things will allow almost all light rays to pass straight through. They have no or very little shadow. Light can easily move through transparent things. When light is not in a vacuum, it travels slower than the speed of light. The slowest speed of light ever recorded was 39 miles per hour.