Blackboard Optics Total Internal Reflection - Physics Demonstrations - Blackboard Optics Total Internal Reflection | Physics Undergraduate Labs

Purpose

Students observe total internal reflection using plexiglass blocks and a ray projector on a magnetic blackboard. Any of the available block shapes (triangular, semicircular, rectangular) can be oriented to demonstrate TIR when the internal angle of incidence exceeds the critical angle (~42° for plexiglass-air).

$Triangular plexiglass prism on magnetic blackboard with ray entering from left, refracting into the prism, then partially reflecting and partially refracting at the hypotenuse face$

$Semicircular plexiglass block on magnetic blackboard with ray entering through the flat face and refracting at the curved surface$

Rectangular plexiglass block on magnetic blackboard with ray entering from left, reflecting internally off the top face, and exiting through the right side

Triangular plexiglass prism on magnetic blackboard with ray entering from left, undergoing two total internal reflections inside the prism before exiting to the right

Figure 1: Light ray incident at less than the critical angle, showing both partial reflection and refraction at the plexiglass-air interface. Note the relative intensities of the reflected and transmitted beams.

Figure 2: As the angle of incidence increases toward the critical angle, the reflected beam intensifies while the refracted beam weakens and bends closer to the interface.

Figure 3: Total internal reflection achieved: the incident ray is completely reflected with no refracted beam emerging from the block, demonstrating the phenomenon above the critical angle.

Figure 4: Multiple internal reflections within the prism geometry, demonstrating how total internal reflection can be used to redirect light through multiple bounces in optical systems.