Consider the overhead projector as an educational innovation. Prior to the overhead projector, a teacher wrote notes and drew pictures on slate boards with chalk. When writing or drawing, a teacher’s back was to the students and the contents were bounded by the edges of the slate. With the overhead projector, teachers faced the students when writing or drawing on acetate sheets with markers of different colors. An endless supply of acetate sheets could extend the surface, and sheets could be prepared prior to the class and sheets from other sources (such as textbook publishers) were available, so sheets could include professionally prepared illustrations and edited text.
Overhead projectors and acetate have largely been replaced with computers connected to projectors, so teachers show text, images, and video coming from digital sources. Through three generations of technology, the experience of curriculum and instruction has been unchanged for the student as they sat and consumed (and continue to sit and consume) the words and images on the wall, albeit with greater amounts of information presented in a more efficient manner.