In blackboard optics, multiple beams of light are passed through large-scale optical elements like prisms and semicircles to demonstrate total internal reflection. When light enters these elements at specific angles, it reflects entirely within the material rather than refracting out. This principle, known as total internal reflection, occurs when the angle of incidence exceeds the critical angle, which is determined by the indices of refraction of the materials involved. The setup visually demonstrates how light can be trapped inside a medium.